Showing papers in "Biological Reviews in 1965"

Patterns of species diversity

Abstract: Summary 1. Species diversity is most simply measured by counting species. More complicated measures, which take into account the relative abundance of the species, have been derived from information theory or from parameters of statistical distributions fitted to the census data. The information theory formulae can also be used to measure habitat diversity and differences between communities or habitats. In this way, changes in the pattern of species diversity can be compared with changes in the environment. 2. Small or remote islands and islands with uniform topography have fewer species than large or complex islands or islands nearer the source of colonization. For birds and some orders of insects it appears that the rate of colonization of new species is virtually balanced by the rate of extinction, so that the number of species has reached equilibrium. For other organisms, such as mammals, and for all organisms on the most remote islands, this equilibrium has probably not been reached and further increases in the fauna may be expected. The comparison of impoverished island faunas with the mainland faunas whence they were derived shows the effect of relaxed competition. 3. Local variations in the species diversity of small uniform habitats can usually be predicted in terms of the structure and productivity of the habitat. Habitats of similar structure on islands and mainland often have similar species diversities; the impoverishment of the island is reflected in the fact that different habitats on the island have nearly the same species, while different habitats on the mainland have more different species. This is interpreted as evidence that uniform habitats are nearly saturated with species and that new species usually colonize by occupying different habitats from present species. 4. The theory of competition and the facts of character displacement indicate that there is a limiting similarity to species which co-exist within a habitat. Species more similar than this limiting value must occupy different habitats. According to the theory, this limiting value should be less where productivity is high, where family size is low and where the seasons are relatively uniform. It should also be less for pursuing hunters than for species which search for stationary prey. 5. Total species diversities, from areas composed of many types of habitat, are usually, but not always, much greater in the tropics than in temperate regions. This is accomplished by a finer subdivision of habitats (habitat selection) more than by a marked increase in diversity within habitats. This total diversity may still be increasing and may have not reached saturation.

Structure and function of mammalian tendon.

Abstract: Summary 1. Tendons are a specialized form of connective tissue uniting muscle and bone and as such have functions essential to normal mobility. 2. The structural unit of tendon is the fibre, up to 300 µ in diameter, consisting of fibrils of collagen, encircled by the anastomosing processes of fibroblasts. Fibres are arranged in fasciculi but may pass from one to another so that successive cross-sections show slightly different appearances. Such interweaving ensures the equal distribution of muscular tension over the whole area of insertion, particularly where the movement of a joint alters the angle between tendon and bone. 3. There are two hypotheses concerning the nature of the muscle-tendon junction. First, the tension developed within a muscle fibre might be transmitted to the helical perisarcolemmal fibres of connective tissue surrounding its length and thence to the tendon, and secondly, and more probably, the tension might be transmitted directly from the end of the myofibrils to the interdigitating fibrils of the tendon. The interdependence of muscle and tendon is further illustrated by their longitudinal growth since the changing length of the muscle belly relative to the distance between the bony attachments determines the rate of growth and relative length of tendon. 4. The orientation of fibrous tissue in the tendon supports the hypothesis of a mechanical influence upon tendon growth, but there is no strict relationship between muscle strength and tendon thickness and various muscles differ in the ratio of their total fascicular cross-sectional area to tendon thickness. It would appear that this difference develops post-natally and, since a red postural muscle has a relatively thick tendon, it has been suggested that the duration as well as the level of transmitted tension might influence the growth of collagen. 5. Tendon thickness may increase proportionately with muscle cross-sectional area in conditions which cause the muscle to hypertrophy, but when the muscle is de-nervated or excised in the young animal growth of tendon thickness occurs in such a manner as to suggest that the growth of collagen is determined by the history of the total tension transmitted. 6. The tendon has a tensile strength which is probably four times as great as the maximum tension that it has to transmit in vivo and an even greater margin of safety is present in penniform muscles which transmit less maximum isometric tetanic tension per unit fascicular cross-sectional area than do fusiform muscles. Although the wave form seen on the surface of a tendon when at rest is eliminated by less than 10 % of the maximum tension which its muscle is liable to transmit, it is possible that the normal range of tensions transmitted in vivo might fall within that part of the stress-strain curve where the tendon is still easily extensible.

The ecology of the freshwater phytoplankton

Abstract: Introduction . . . Light and temperature . Weather . . . . Water movements and flotation Inorganic nutrients . . ( I ) Nitrogen and phosphorus (2) Silicon . . . (3) Calcium and magnesium (4) Potassium . . . (5) Sulphate and chloride . (6) Iron and manganese . (7) Other trace metals , CONTENTS . 231 VI. . 232 . 238 . 239 VII. . 242 VIII. . 242 . 250 IX. . 253 x. . 254 XI. XII. * 255 . 255 . 257 XIII. General ionic environment . . 258 258

Vitamin a, carotenoids and cell function

Abstract: Summary I. Both deficiency and excess of vitamin A produce many diverse pathological changes. Lipoprotein membranes have recently been found to be concerned in a number of the actions of excess of the vitamin. This article is devoted principally to a discussion of the interactions and possible functions of vitamin A within membranes, both in hypervitaminosis and under physiological conditions. 2. Excess of vitamin A. (a) As a result of its amphipathic molecular structure, retinol is highly surface active. (b) The initial action of excess of retinol on erythrocytes is an expansion of the cell membrane; this is followed by haemolysis unless the cells are kept cold, or inhibitors, such as vitamin E, are present. (c) Addition of retinol to fibroblasts growing in vitro causes degranulation and swelling of the endoplasmic reticulum, swelling of Golgi vacuoles and mitochondria, and the formation of cytolysomes. (d) Isolated mitochondria from certain tissues also swell in the presence of retinol. This swelling is apparently not dependent on respiration, but is inhibited by vitamin E. (e) Lysosomal enzymes are released by excess of retinol, both in vivo and in vitro. A number of the effects of excess vitamin A on connective tissues may be ascribed to this action; these effects are inhibited by hydrocortisone, which stabilizes lysosomes, but are not significantly inhibited by vitamin E. (f) Closely related, but physiologically inactive, derivatives of retinol are relatively inactive towards membranes; membranes might therefore also be concerned in the physiological functions of retinol. (g) Vitamin A has recently been shown to have marked effects on the membranes of certain bacteria and viruses. 3. Deficiency of vitamin A. (a) The diminished synthesis of mucopolysaccharide observed in deficiency may result from an interference with the synthesis of ‘active sulphate’ but, in view of conflicting observations, this cannot be considered as proven. (b) Suggestions have been made that lipoprotein membranes may be concerned in the actions of vitamin A on both mucopolysaccharide and steroid biosynthesis. (c) The action of the vitamin in biological oxidations is far from clear. Recently it has been found that deficiency increases, and excess of vitamin A decreases, the oxidation of succinate by homogenates of rat liver. (d) Organ culture experiments indicate that vitamin A acts directly on mouse prostate glands to prevent the squamous changes that are characteristic of deficiency. 4. A conjugated chain of alternating single and double bonds is a chemical feature common to vitamin A and the carotenoids. (a) Such a chain is characterized by a high electron mobility; this has been studied both theoretically and experimentally in relation to electron transfer in photosynthesis, to ion transport, and to super-conduction and semi-conduction in biological systems. (b) The role of vitamin A in vision may be an evolutionary development of the function of the carotenoids in photosynthesis. In the eye, retinal is intimately concerned with the structure and function of lipoprotein membranes. We have suggested that changes in electron mobility may be functionally associated with the isomerization of 11-CM retinal by light. (c) Vitamin A, or carotenoids, may be concerned in the receptor systems of olfaction and taste. (d) Photosynthesis in chloroplasts has many structural, chemical and functional features in common with respiration in mitochondria. Carotenoids are found in ox-heart mitochondria but the presence of vitamin A is uncertain. (e) We have suggested that vitamin A or a metabolite retaining the conjugated chain may be present under physiological conditions in the lipids of biological membranes and that, by virtue of its mobile electrons, it may function in mitochondrial electron transport. 5. Active forms of vitamin A. (a) Recent work indicates that retinoic acid, or a metabolite of retinoic acid, may be the active form. It has not been established conclusively, however, that retinoic acid is normally formed in vivo from retinol. (b) It is conceivable that, if vitamin A is concerned in electron transfer, the vitamin may penetrate lipoprotein membranes as retinol and that it may subsequently be converted to a functional derivative. In this way, the membranes of animal cells may receive the carotenoid skeleton which, unlike plant and bacterial cells, they cannot synthesize for themselves. We are most grateful to Dame Honor Fell, F.R.S., Sir Rudolph Peters, F.R.S., and Miss Audrey M. Glauert for reading the manuscript of this article and for their helpful suggestions. We thank Mr R. A. Parker and Mr I. Armond for preparing the plates and figures, and Mrs M. Wright for her careful typing.

Fine structure of fungi as revealed by electron microscopy.

Abstract: Summary 1. The cells of yeasts and filamentous fungi are surrounded by a cell wall. The cytoplast is bounded by a plasmalemma and contains nuclei, mitochondria, food granules and vacuoles. 2. Cell walls of vegetative fungal cells usually possess a microfibrillar ‘skeleton’, architecturally similar to that of plants, but composed of chitin or hemicellulose. True cellulose predominates only in the Oomycetes. Spore walls are often differently constructed from those of vegetative cells of the same species. 3. The cell contents (plasmalemma, endoplasm and endoplasmic reticulum, nucleus, mitochondria, vacuoles and miscellaneous granular inclusions), as seen in sections and in centrifuged material, are in general similar to those of plants. Some minor differences are indicated. The structure of the flagella of motile cells of the Lower Fungi is fundamentally similar to that of the flagella of lower plants and animals. 4. The electron microscope has contributed to our knowledge of growth and development of fungal structures. It has extended previous knowledge of the process of budding in yeasts, of the development of haustoria and of the formation and germination of fungal spores. 5. Poor nutrition, anaerobic conditions and sublethal irradiation cause abnormalities in the structure of mitochondria. Toxic amounts of irradiation prevent normal nuclear division in budding yeast cells. 6. Electron microscope studies have aided taxonomy by demonstrating fundamental structural differences between Fungi and Actinomycetes and between certain subdivisions of the fungi.

The relationship between blue-green algae and bacteria.

Abstract: Summary Recent studies of bacteria and blue-green algae in the electron microscope have shown striking similarities in the basic cellular architecture of the two groups. This evidence has been presented in detail above and may be summarized as follows: (I) The cell envelope of blue-green algae bears a superficial resemblance to the cell wall of bacteria, particularly to the Gram-positive organisms. Like bacteria, blue-green algae also possess a mucopeptide component as an important part of the cell wall. However, the composition of this mucopeptide component resembles more closely that of Gram-negative bacteria, though more species of blue-green algae need examination before the relationship can be established with any certainty. (2) The older distinction between the chromatophores of photosynthetic bacteria and the lamellae of blue-green algae has been shown to be superficial. In both forms the essential feature of the photosynthetic apparatus is a pair of membranes enclosing a space; this may be large in bacterial chromatophores or small in ‘true’ lamellae. This basic structure is seen in higher plants where the lamellar systems are enclosed in a membrane-bounded structure, the chloroplast. No such membrane isolates the photosynthetic machinery of bacteria and blue-green algae. (3) No membrane isolates the nuclear material of blue-green algae or bacteria. The nucleoplasm of these two groups is structurally similar and show the same morphological changes with variations of fixative treatment. (4) Both bacteria and blue-green algae lack mitochondria, true vacuoles and an endoplasmic reticulum. Membranous structures (mesosomes) are widespread in bacteria and recently similar structures have been seen in blue-green algae. (5) The nucleoplasm presumably divides by an amitotic process, as no stages of mitosis or meiosis have been observed in either group. The cell division in blue-green algae shows a superficial resemblance to that of Gram-positive bacteria. Recent observations on the cytoplasmic inclusions of the two groups have been presented above, but no pattern has yet emerged. The spores of the two groups have also been discussed and the little detailed observation available supports earlier suggestions of some important differences between the two groups. Bacteria and blue-green algae also resemble each other and differ from other organisms in their method of ornithine biosynthesis, their apparent absence of sterols and their sensitivity to certain antibiotics. The evidence here summarized supports the suggestion that bacteria and blue-green algae should be isolated in a group distinct from all other organisms. Two speculative aspects of the problem have been discussed: their evolutionary relationship with other organisms and with each other. With regard to the latter, it seems probable that bacteria and blue-green algae arose from a common ancestor, since it is unlikely that so many features common to the cells of both groups arose independently more than once.

Statistical methods in systematics.

Abstract: Summary From the biological viewpoint the tasks of systematics may be subdivided into analyses of infraspecific variation (both intra- and interpopulation studies), the separation of genetic from environmental effects on the phenotype, the definition of species (and possibly subspecies), the definition of supraspecific taxa, the measurement of similarity among taxa, life-history stages or organs, the measurement of evolutionary rates, the evaluation of biogeographical relationships and information-handling problems. In all types of statistical analysis in systematics the correct choice of characters to be measured is of great importance. The sources of error and bias in measurement are listed here and ways are discussed for overcoming their effects. Ratios, frequently employed in systematics, have certain numerical disadvantages. Frequency distributions of characters provide insight into biological processes affecting the characters and they test assumptions about the distributions implied in the various statistical analyses. Simple description of single samples is carried out by statistics of location and dispersion, such as the mean and the variance. Interpopulation variation usually involves two-dimensional comparison of geographically differing populations, but other comparisons are discussed as well. The analysis of variance is generally applicable. Care must be taken in choosing an appropriate sampling distribution for testing significance of differences among means. The frequently employed e-tests and the so-called ‘Dice-grams’ are not suitable. Graphical methods for representing geographical variation are still in need of considerable improvement. The definition of subspecies and the so-called percentage rules are critically reviewed. Covariation between characters is studied by means of correlation and regression techniques. In correlation, individuals are randomly chosen with respect to two variables, while regression deals with the dependence of one (randomly chosen) variable upon the other, whose values are assumed to be fixed. Regression can be used in systematics descriptively (to show the relations of one variable upon the other), correctively (to permit samples differing in an important causal variable to be compared for other variables), and as an explanatory device (to relate character variation to climatic or other ecological factors). Multivariate analyses permit simultaneous statistical treatment of several characters. They are laborious but the ever-growing speed and capacity of digital computers will increase their application. Resolution of covariation is carried out by analyses of covariance, partial correlations and factor analysis. It aims at understanding the interrelationships of characters among themselves. Partitioning of covariation into components representing environmental and genetic forces of various kinds helps to understand evolutionary factors in a given study. The methods of numerical taxonomy for quantifying similarities among taxa and describing taxonomic structure are briefly reviewed. Similar methods may aid in biogeographic problems and in the measurement of rates of evolution. The discrimination of groups representing different taxa or sexes can be accomplished by means of discriminant functions. Profound changes in the cataloguing and data-handling aspects of systematics are resulting from the development of electronic data processing. Many statistical computations in systematics can also be appreciably accelerated by processing them on a computer. This article was prepared during my tenure of a Fulbright professorship at Tel Aviv University in Israel. I am greatly indebted to Professor H. Mendelssohn, Chairman of the Zoology Department of that institution, for his cordial hospitality and encouragement. The following persons have read various drafts of this manuscript and their comments and criticisms are very much appreciated: Anthony J. Boyce (Oxford University), Paul R. Ehrlich, Larry G. Mason (Stanford University), K. Reuven Gabriel (Hebrew University, Jerusalem), and F. James Rohlf (University of California, Santa Barbara).

Organic production, turnover and mineral cycling in woodlands

Abstract: Summary 1. A distinction is made between the production of tree trunks (economic) and the production of all kinds of primary organic matter (biological) by woodland plants capable of photosynthesis. 2. Yield tables, quality class and site index are considered to be reasonably satisfactory indicators of trunk wood production, but their value as indicators of biological production has not been demonstrated. 3. Some factors, namely geographical, weather, edaphic, genetical, management and biotic, are considered in relation to their influence on economic production. 4. Attention is drawn to the relative paucity of data on biological production. 5. The available information of woodland biomass suggests that the greatest biomass of the world's forests exceeds 500 times 103 kg. of oven-dry material per hectare. 6. The annual accumulation of plant material in woodland ecosystems can be very rapid, up to about 20 kg./ha. when growth is active. The bulk of the organic matter is in tree trunks but where decomposition is slow large amounts of organic matter are present as litter on the forest floor. 7. Biomass change is considered an unsuitable measure of biological production since it is affected greatly by tree harvesting. 8. Net primary production is regarded as a better measure of biological production. Some of the more serious sources of error in determining net primary production are emphasized. Generally net primary production is underestimated because of failure to take into account the removal of photosynthate by animals and because of infrequent sampling so that the production of plant parts such as inflorescences is missed. 9. Woodlands are shown to be one of the most productive terrestrial communities, giving a range of annual production as 103 kg./ha., from 5 for fir forests of the northern taiga in the U.S.S.R. to 33 in tropical rain forests. The capacity of forests to produce large amounts of organic matter annually is attributed to the large root and leaf mass which permit relatively full use to be made of the soil and incident solar energy. 10. A large part of the organic matter produced by woodland plants is broken down by the secondary producers. Associated with the annual turnover of organic matter in woodlands, nutrient elements circulate through woodland ecosystems. The annual retention of nutrient elements within the biomass is much less than what is returned to the soil by litter fall. Taking a hard beech forest in New Zealand as an example, the woodland plants have an annual uptake as kg. per hectare of potassium, calcium, magnesium, phosphorus and nitrogen of 34, 84, 12, 3-3 and 40 respectively, but about 30, 74, 11, 2–6 and 37 are returned annually in the litter fall. 11. The annual cycle of mineral elements is regarded as a polycyclic system affected by various input and output factors. The loss of nutrients through the harvesting of tree boles, for instance, is relatively small compared to the addition of nutrients in precipitation. 12. Soil changes resulting from the variable balance in the factors affecting mineral cycling are considered, particularly with respect to general assumptions made about soil deterioration under monocultures of coniferous tree species. From the point of view of the maintenance of soil fertility, general condemnations of conifers and universal recommendations for hardwoods are regarded as unsound, since great variations in their effects on the soil occur in both conifers and broad-leaved trees as groups. 13. The need for more comprehensive studies, embracing a whole or several forest rotations, is stressed for the evaluation of woodland production, organic turnover, mineral cycling and soil change. Such evaluations are becoming more important as multiple use of woodlands increases. 14. It is suggested that at present the greatest advances in woodland production ecology will be made by studies of primary production in forests known regionally to be the most productive. This may permit meaningful relationships between site factors and production to be recognized. 15. With more complete production and turnover data, it seems likely that existing woodland classification schemes will be improved, since it becomes possible to emphasize both the dynamic nature of woodlands and the long-term changes taking place in environmental conditions.

Adaptation of mice to cold

Abstract: Summary Since 1953 laboratory mice have been bred continuously in an environment kept at – 3° C. Control stocks are kept at 21° C. All mice have cotton wool bedding. The effects of the cold environment are reviewed, and related to observations on other species exposed to cold. Physiological adaptation The principal physiological adaptation, of a small mammal exposed to cold, is increased heat production. Inbred mice, fully adapted to – 3° C., expend up to 4–5 kcal./100 g. body weight/hr., or about four times that of controls at 21°C. This is probably a maximum, maintained only outside the nest and for short periods. Colon temperature is unchanged; skin temperature, though high (31 °C. at the surface), is lower than at 21° C. (34° C.). Virgin female inbred mice at – 3° C. eat about twice as much food, per unit body weight, as controls at 21° C. The difference during late pregnancy is, however, much less; and, relative to body weight, consumption during the first 10 days of lactation is lower at – 3° C. than at 21° C. There are fewer young at - 3° C. The lactating females may eat the maximum they can digest. Food is probably utilized more efficiently at - 3° C. than at 21° C. Lowered external temperature reduces activity. Mice in a cold environment economize in general exploration of their environment; they also do no ‘functionless’ gnawing of friable materials. After increased heat production, the most important adaptive response is nest-building. When accustomed to a cold environment, mice build better nests than in a warm one. But, given unfamiliar nest material, in a novel situation, they build a nest less quickly at – 3° C. than at 21° C.-another example of economizing in energy. On exposure to cold, an increase in body weight would be adaptive. Some wild mammals increase their weight and body fat in winter. But adult laboratory mice, after transfer from 21 to - 3° C., lose weight, largely owing to loss of fat. In this they resemble laboratory rats transferred to a cold environment. Similarly, inbred mice reared at - 3° C. are usually lighter, at all ages, than controls at 21° C. Appendages, especially tails, are usually shorter in a cold thanin a warm environment; in mice the caudal vertebrae are shorter. The extent to which this reduces heat loss is not known. A genetically mixed stock of mice, selected for fertility at - 3° C., developed longer tails during eighteen generations in the cold. Laboratory rats, after a few weeks at about +4° C., alone, without nests, have heavier liver and kidneys, intestine, heart and adrenal glands, than controls. Exposure in groups, or to seasonal cold, evokes different responses. In cold-adapted inbred mice of the first or second generations reared at -3′ C., stomach, intestine, liver, and heart are heavier than those of controls. Insulation by hair is slightly increased at - 3° C., though the heat-conserving effect of this is small; the weight of the cropped skin is reduced in inbred mice, but it was unchanged in a genetically mixed stock after twelve generations in a cold environment. There is a higher proportion of body water, and lower collagen, at -3° C. Males, but not females, have a lower relative calcium content.

Stomatal responses to illumination

Abstract: Summary 1 Stomata have been found to respond to small changes in carbon dioxide concentration within the leaf, and movements due to illumination, temperature, leaf-water content, or metabolic inhibitors can be explained, at least in part, by their affecting the internal carbon dioxide concentration This is confirmed by the fact that effects of such factors can be reversed experimentally by flushing the leaf with air of an appropriate carbon dioxide content We conclude that changes in carbon dioxide concentration set in motion reactions affecting guard cell turgor relations (see 8 below) 2 The photosynthetic production of carbohydrates, or of intermediates in their synthesis such as glycollic acid, does not seem to have any major effect on guard cell turgor, which can readily change in the absence of photosynthesis in darkness in response to different carbon dioxide concentrations Factors which affect photo-synthetic production ordinarily affect the internal carbon dioxide concentration and it is the latter which is more directly involved in changing guard cell turgor relations 3 In addition to the effects of environmental factors on carbon dioxide concentration, light (blue), temperature and leaf water content may also affect guard cells independently of carbon dioxide Whether such effects are due to hydrolysis of starch or another polysaccharide, or to changes in permeability or quite another mechanism, is not known 4 As well as being effected by the environment, stomatal movements are under the control of endogenous rhythms in light and darkness Rhythms can produce opening in darkness and partial closure in light, and hence can modify or overrule the response to external factors However, the phase of the rhythm does come under the control of the environment through a low-intensity light reaction sensitive to red and far-red wavelengths This light reaction is quite distinct from those directly concerned in the production of stomatal opening 5 Stomatal behaviour in cacti and other succulents is at first sight practically the opposite of that in other plants, in that they open at night and close during the day We suggest that this type of behaviour may, however, be a relatively simple development from the normal pattern 6 Stimuli affecting the stomata can be transmitted within a leaf, or from one part of a plant to another The mechanism of the transmission is not known, but it could be brought about by a chemical substance which is translocated 7 There is evidence that the processes of stomatal opening and closing are different in nature, and that one is not simply a reversal of the other It is probable that an active (oxygen-requiring) mechanism in involved in stomatal opening Stomata come under the control of so many factors, both external and internal, that several processes are likely to contribute to the turgor changes which bring about their movements 8 The mechanism of carbon dioxide control over stomata is not known

Chemical embryology of the crustacea

Abstract: Summary 1. The dry weight of a neonate crustacean is sometimes lower than the initial dry weight of the egg, owing to utilization of respiratory substrates, as in Daphnia, where the dry weight decreases by 16–25 %. In Artemia any loss is compensated by an uptake of salts so that there is no change in dry weight during the course of embryonic development. The embryos of Ligia receive nutrients secreted into the maternal brood pouch, and the dry weight of the neonate is greater than the initial dry weight of the egg. 2. Early embryos of Simocephalus, Artemia and Balanus consume less oxygen per unit dry weight than older embryos. The increase in oxygen consumption from early to late embryos is approximately six to eightfold. 3. Oxygen consumption decreases when the rate of embryonic development of Artemia is reduced by increasing osmotic pressures in the external medium. 4. Glycogen is formed rather than utilized in the embryos of Simocephalus and Artemia. In the latter the main carbohydrate reserve is trehalose, which is transformed to glycogen and glycerol. The relative formation of these two substances is influenced by external osmotic pressures. At high external osmotic pressures more glycerol is formed, and this aids osmotic rupture of the tough outer shell of the egg. 5. Trehalose seems to be important in relation to the dormant state of Artemia embryos. The embryos which develop rapidly in the maternal brood pouch contain much less trehalose than those which become encysted and dormant. 6. The proportion of the initial lipid content of a crustacean egg that is utilized during development is very variable. In Homarus 60% disappears by time the neonate emerges, in Ligia 32%, and in Artemia the decrease is very slight, or there may even be a slight increase. 7. Haemoglobin in the eggs of Cladocera does not appear to act as a respiratory pigment, but does serve to accelerate development in poorly aerated water. It is suggested that the main function is to act as a supply of stable protein. 8. The haemochromogen found in the gut of late embryos of Daphnia appears to be formed in part at least from haemoglobin, which diminishes in concentration as development proceeds. 9. Breakdown of haemoglobin during embryonic development of Daphnia is not accompanied by formation of bile pigments. 10. Biliverdin is synthesized in the embryonic eyes of Polyphemus but not elsewhere in the embryo. 11. The carotenoproteins in the eggs of many different crustaceans break down towards the end of embryonic development, liberating free carotenoid. It is suggested that these proteins are stabilized and held in reserve by linkage to a carotenoid until a particular stage of embryonic development has been reached. 12. Metabolic pathways in the formation of astaxanthin from β-carotene may involve monoketo and diketo carotenes, or by an alternative route monohydroxy and dihydroxy carotenes.

The ontogeny of the vascular cambium in the stem of seed plants

Abstract: Summary 1. A striking characteristic of the vascular cambium is its plasticity. It may arise in any position in any tissue except the epidermis. It may function in a variety of ways, giving rise to xylem or phloem alone, or to both tissues. If both xylem and phloem are formed from the same cambium, they may be derived from opposite faces of the meristem or be combined as bundles or be mixed less regularly. One cambium may be succeeded by another and the method of functioning of successive cambia may vary. A number of these possibilities may be combined in one plant either simultaneously or at different stages of growth. 2. The mode of operation of the cambium in monocotyledons is so different from that of most dicotyledons, that it is unlikely that they can be homologous. It is probable that a cambium has been acquired independently in several lines of mono-cotyledonous evolution, although the virtual identity of the cambium wherever it occurs in monocotyledons is remarkable. Some dicotyledonous cambia are almost equally distinctive. To distinguish these from the usual bi-directional cambium, the term uni-directional cambium is applied to them in this account. A uni-directional cambium may be a primitive feature of the families Chenopodiaceae, Amarantaceae, and Nyctaginaceae, but essentially identical cambia have been acquired in the Styli-diaceae, the Phytolaccaceae and in several independent lines within the Compositae. 3. Another feature of the cambium is the pressure of competition between its cells. This is especially evident during the early growth of a stem, when increase of circumference is relatively great. Many cells are produced by pseudo-transverse divisions within the initial layer. These cells increase in size by intrusive growth, and many are lost. Survival has been shown to be influenced by a wide variety of factors. 4. A large number of investigations has established a pattern of variation in size of xylem elements within the tree. This reflects a similar pattern of size variation in the cambium. In broad outline this pattern is found in most trees which have been investigated, although considerably modified in timbers derived from storeyed cambia. Groups of woody plants showing other modifications of the pattern have been described. 5. The transformation of fusiform initials into ray initials and vice versa is another example of the plasticity of the cambium. The maintenance of the proportion of ray to fusiform initials characteristic of a species indicates that cambial activity is a well-co-ordinated and orderly process. The importance of a correct proportion of ray tissue among the longitudinal elements of the xylem is evident.

Environmental and biochemical control of stomatal movement in leaves

Abstract: Summary The chloroplasts of guard cells are largely responsible for the response of stomata to light. The opening movement is probably associated with the synthesis of ATP by non-cyclic photophosphorylation, and the obligatory fixation of carbon dioxide to produce glycolic acid. The oxidative metabolism of glycolic acid is related to ATP synthesis as well as to the possible formation of carbohydrate. In the absence of oxygen in the gas phase, stomata open poorly, and the synthesis of glycolic acid is also inhibited. α-Hydroxysulphonates, which are inhibitors of glycolate oxidase, prevent stomatal opening in a competitive manner. High concentrations of carbon dioxide close stomata and inhibit the formation of glycolic acid, and the effect of high carbon dioxide on closing can be reversed by providing the tissue with glycolic acid. When leaf hydration is not limiting, raising the temperature in the environment causes an increase in the steady state of the stomatal aperture. There is little effect of temperature on the rate of closing in darkness, hence opening is the active process. A large number of chemical inhibitors were found to prevent opening, either by interfering with metabolic reactions in the guard cells concerned with the ‘pump’ responsible for the increased turgidity, or by acting on the membranes that serve as a ‘check valve’. Sodium azide is a substance which acts by both mechanisms, but the maximal effect occurs at different concentrations, and experiments with this inhibitor have shown that the opening and closing reactions operate independently.

MICRO-ORGANISMS and SOIL STRUCTURE

Abstract: Summary 1. Experimental analysis of the influence of micro-organisms on soil structure has been confined to aggregation, i.e. the formation of small clusters of soil particles which are resistant to the disintegrating action of water. 2. Aggregation is influenced by the mineral constituents of the soil, notably clay, but organic matter is considered to be most important. 3. Decomposition of organic matter regularly leads to increased aggregation in non-sterile soils. Under sterile conditions (i.e. in the absence of micro-organisms) no increase in aggregation is observed, so that it is evident that micro-organisms are implicated in this process. 4. Investigations of the role of micro-organisms have been concerned with two possible effects: (1) the action of filamentous organisms (particularly fungi) in mechanically entangling soil particles to form stable aggregates, and (2) the action of products of micro-organisms (of all groups) which may cement soil particles together. 5. In most soils the mycelium of filamentous organisms (fungi and actinomycetes) is only present for relatively short periods so that aggregation produced by them tends to be ephemeral. In sandy soils, however, mycelium appears to be more persistent and hence of greater significance. Since fungi are commonly associated with the early stages of decomposition of plant remains, it is suggested that they may also be important in situations where plant residues are regularly being added to the soil, e.g. grassland and forest. 6. Of the many compounds produced by micro-organisms, most attention has been given to extracellular polysaccharides. These are produced by many soil organisms and can be shown to be effective aggregating agents. Compounds of similar character have been demonstrated in natural soils but satisfactory methods for their quantitative estimation have not been developed. Consequently their contribution to aggregation cannot be accurately evaluated. Evidence so far presented, does, however, indicate that in many cultivated soils they make a considerable contribution. 7. In old grassland and forest soils, extraction of polysaccharide by periodate oxidation appears to have little or no effect on aggregation and in these cases aggregation would seem to be due to other components of the soil organic matter. Although this may well be so, it is also suggested that the distribution pattern of organic materials in the soil may be an important contributory factor. Certain materials, e.g. lipids, though not effective as aggregating agents when mixed with the soil, increase stability when sprayed on the surface of unstable aggregates. The significance of such effects has not so far been critically examined.

The enzymic basis of specific antibacterial action by structural analogues.

Abstract: Summary 1 The mechanism of action of structural analogues is examined to discover the extent to which their properties make them effective antibacterial agents in vivo 2 Two main types of structural analogue may be recognized First, there is the type which competes with a natural metabolite for a recognition site on the surface of a protein Secondly, there are a few examples–notably cycloserine, and possibly penicillin–which inhibit non-competitively by reacting covalently with an enzyme to inhibit its action In general, inhibitors which react covalently are more effective in vivo than those which inhibit competitively 3 Analogues which inhibit competitively act in three main ways: they may compete at the active centre of a single enzyme, or exert a ‘false feedback’ inhibition of the endogenous supply of an essential metabolite, or repress the formation of enzymes necessary for the biosynthesis of an essential metabolite 4 Although structural analogues which inhibit competitively in one of these three ways can cause temporary inhibition in the growth of bacterial cells, their bacteriocidal effect is low, since resistant bacterial populations readily emerge 5 Resistance to structural analogues which inhibit competitively at the active centre of a single enzyme may occur physiologically by the accumulation of the precursor to the inhibited enzyme 6 Resistance to structural analogues which inhibit competitively may also occur by mutation In the case of analogues acting at the active centres of enzymes, mutation may lead to resistance by lowering the affinity of the relevant enzyme for the analogue Resistance to structural analogues acting as ‘false feedback’ inhibitors occurs when a mutation causes the synthesis of an enzyme with a lowered affinity for the structural analogue as a feedback inhibitor Similarly, organisms resistant to the action of analogues as repressors are probably the result of a mutation which lowers the affinity of the ‘genetic repressor’ for the analogue in its role as the ‘low molecular weight repressor’ 7 Inhibitors which react covalently with an enzyme have certain advantages not found with analogues which act competitively, and these are discussed in relation to the mode of action of the cholinesterase inhibitor, di-iso-propylfluorophosphate One advantage of this type of inhibitor is that reversal by the normal enzyme substrate does not readily occur Secondly, this type of inhibitor is active at appreciably lower concentrations than simple competitive inhibitors 8 The antibacterial action of cycloserine and penicillin are examined to see to what extent they are examples of structural analogues which react covalently with essential enzymes 9 The fact that analogues which react covalently with enzymes are more effective inhibitors than analogues which act competitively, suggests that some form of covalent reaction may often be involved in the action of antibiotic substances

The localization of biochemical activities in the cells of higher plants.

Abstract: Summary 1. Since knowledge of the detailed morphology of the cell is essential for the study of intracellular localization of enzymic activities, a summary is presented of the results of electron microscopic examination of the fine structure of plant cells. Most of the data refer to meristematic cells, and relatively little is known of the structure of those fully differentiated tissues, such as roots, stems and fruits, which are commonly subjected to fractionation procedures. 2. Cell fractionation involves disintegrating the selected tissue, separating the organelles, and purifying and characterizing them. A survey of the techniques which have been used for disintegration and separation reveals that they are all relatively crude compared with the complexity of the cell structures and that as a result each of the isolated fractions is likely to contain a number of types of organelles, with one predominating, and that damage to the structures is difficult to avoid. Characterization of the fractions has often been restricted to a definition of the conditions'of sedimentation, but electron microscopic and chemical examination are essential for definitive studies; in their absence any assignment of an enzyme to a particular organelle must be provisional. 3. Chloroplasts and ribosomes can be isolated relatively intact and almost un-contaminated. Methods have been developed for preparing biochemically active nuclei, mitochondria, cell walls, fragmented endoplasmic reticulum, starch grains and proteoplasts. Less satisfactory procedures are in use for isolating chromoplasts and proplastids, native fat, and vacuoles. So far dictyosomes and spherosomes cannot be separated from other subcellular particles. 4. The histochemical determination of precise intracellular localization of enzymes requires the resolution of the electron microscope, and the recent adaptations of the methods of classical histochemistry for use with the electron microscope are briefly outlined. So far, none of these techniques is in routine use, but it is probable that the combination of improved fractionation procedures with electron microscopic histochemistry will provide definitive information on the intracellular sites of enzyme activity. 5. Data on the intracellular distribution of enzymes and enzyme systems involved in nucleic acid, amino acid, protein, lipid and carbohydrate metabolism have been assembled. Ribonucleic acid synthesis occurs in the nuclei, and also in chloroplasts, deoxyribonucleic acid synthesis is apparently restricted to the nucleus. Amino acid incorporation into protein occurs in ribosomes, nuclei, chloroplasts and proteoplasts, and possibly in mitochondria; activating enzymes are usually detectable in the same subcellular fractions. Amino acid, lipid and carbohydrate metabolism involve enzymes present in a number of fractions, but in many cases the composition of the fractions is undefined and so the subcellular localization is uncertain. 6. Photosynthetic fixation of carbon dioxide is restricted to the chloroplast, but the activity of isolated chloroplasts is less than 5 % of that of the intact tissue. This low activity is due to loss of enzymes and cofactors during isolation and to dilution of the remaining catalysts, but even slight damage to the chloroplast structure may also reduce the efficiency of photosynthesis by disorganizing the co-ordinated electron transport and carbon fixation systems. 7. It is technically difficult to determine the precise distribution of enzymes located partially in mitochondria. The procedures in use do not yield mitochondria free from contamination, and mitochondria or mitochondrial fragments may be sedimented with heavier or lighter particles. If mitochondria are ruptured, then the soluble intra-mitochondrial enzymes will be found in the soluble fraction of the cell.