Showing papers in "New Phytologist in 1975"

Comparison of the effects of a localised supply of phosphate, nitrate, ammonium and potassium on the growth of the seminal root system, and the shoot, in barley

Abstract: SUMMARY Barley plants were grown for 21 days in sand culture, continuously irrigated with nutrient solution. The rooting depth was divided into three compartments, one above another, such that different zones of the root system could be supplied with very low or high concentrations of a single inorganic nutrient, all other nutrients being maintained at a high concentration. Exposure of parts of the main seminal roots (axes) to high concentrations of phosphate caused a localized promotion of the initiation and subsequent extension of both first and second order laterals, compared with zones receiving very low concentrations of phosphate. This resulted in considerable modification to root form, but with only a small depression in shoot growth, compared with control plants receiving an ample supply to all parts of the root system. The extension of seminal axes was little affected by the concentration of phosphate to which they were exposed. Similar responses to those described for phosphate occurred with variation in concentration of nitrate or ammonium, but with potassium a localized supply promoted the growth of laterals to approximately the same extent as controls throughout the entire root system. The experiments show that adequate external concentrations of nitrogen and phosphorus are required by any part of the root system for optimal growth of laterals, but not axes. Possible mechanisms which compensate shoot growth when nutrients are supplied to only part of the root system, and agronomic implications, are discussed.

Criteria of intactness and the photosynthetic activity of spinach chloroplast preparations

Abstract: SUMMARY Chloroplast preparations capable of rapid rates of photosynthesis contain large numbers of intact (Class A) chloroplasts and smaller numbers of ruptured (Class C) chloroplasts. It is suggested that there is also a third or intermediate class in which the envelope has re-sealed following rupture and release of stromal protein. The number of re-sealed chloroplasts seems to be lowest in preparations containing the largest proportion of Class A chloroplasts. Assays for intactness based on the ability of Class C chloroplasts to utilize ferricyanide as a Hill oxidant are quick and convenient but do not distinguish between Class A and re-sealed chloroplasts. If it is assumed that there is an inverse linear relationship between percentage intactness and ability to react with ferricyanide, such assays over-estimate the proportion of intact chloroplasts and, therefore, underestimate their photosynthetic activity and protein content when these are expressed on a chlorophyll basis.

TRANSPORT OF INDOLEACETIC ACID IN PLANT CELLS IN RELATION TO pH AND ELECTRICAL POTENTIAL GRADIENTS, AND ITS SIGNIFICANCE FOR POLAR IAA TRANSPORT

Abstract: SUMMARY The distribution of IAA between the vacuole and the bathing solution in Hydrodictyon africanum is consistent with passive entry of undissociated IAA and passive efflux of both undissociated IAA and of IAA-, with PIAA (permeability coefficient)* about io-3 cms-1 and PIAA- about -6 -1A o-6 cm s . The involvement of IAA- in the efflux results from the inside-negative P.D. between the medium and the vacuole. The cytoplasm is at a higher pH than either the vacuole or the bathing solution used in most experiments; this is maintained by active H+ efflux at the plasmalemma, and active influx at the tonoplast. In this situation the efflux of IAA- is further promoted by the increased concentration of IAA- in the relatively alkaline cytoplasm. Thus the apparent active efflux of IAA from these cells can be explained in terms of passive driving forces of concentration and electrical potential acting on IAA and IAA-, with the distribution of these two species dictated ultimately by PIAA, PIAA-) the pH of the various compartments and the electrical potential difference between them. If PIAA/PIAA- were larger at the apical than at the basal end of coleoptile cells, such an effect could explain polar IAA transport, with metabolic energy being used only to maintain the relative permeabilities to the two species, the pH gradient and the electrical gradient.

Phosphate uptake zones of mycorrhizal and non-mycorrhizal onions

Abstract: SUMMARY Mycorrhizal and non-mycorrhizal onion seedlings were grown in individual soil chambers in which roots were confined to one side of a barrier. External hyphae of Glomus fasciculatus arising from mycorrhizal roots grew into an adjacent volume of soil. 32p was injected into soil at I-cm intervals up to a distance of 8 cm from the confined roots. Relatively high levels of radioactivity were subsequently detected in root segments of mycorrhizal plants at all distances from tracer injection. High levels of radioactivity were detected also in leaf segments of mycorrhizal plants. Radioactivity of root or leaf segments of non-mycorrhizal plants did not exceed background level at any distance from tracer injection. 32P did not move more than 7.5 mm from the point of injection as indicated by gross autoradiography. Absorption of phosphate and its translocation to the host by hyphae of G. fasciculatus can extend the phosphate uptake zone of mycorrhizal onions to at least 7 cm from the root surface.

Experimental ecological genetics in plantago

Abstract: SUMMARY Induction of roots and shoots from leaves of Plantago lanceolata L. using IAA is a very effective technique which could be extended to large scale cloning for vegetative propagation and assay procedures in genecological studies on this and other species. In P. lanceolata, there were no differences between populations from contrasting habitats with regard to proportion of leaves producing roots and shoots. There was a high correlation between zinc tolerance measured on rooted leaves and on equivalent rooted tillers.

Ultrastructure and recrystallization of plant epicuticular waxes

Abstract: SUMMARY A wick feed method was used to recrystallize a range of different plant epicuticular waxes from solutions in organic solvents. Using the scanning electron microscope the ultrastructure of the recrystallized wax from each species was compared with that of the wax on the corresponding intact plant surface and in many cases was found to be similar. It is concluded that the crystal structure of epicuticular waxes is greatly influenced by their inherent chemical and physical properties rather than by properties of the underlying cuticular membrane or by any mechanism of extrusion or transport of the wax to the plant surface.

A theoretical analysis of the contribution of algal cells to the attenuation of light within natural waters i. general treatment of suspensions of pigmented cells

Abstract: SUMMARY A theoretical treatment of light attenuation within suspensions of phytoplankton, which takes account of the heterogeneous distribution of pigment in the system, is described. As a measure of the effect of canopy structure on light attenuation, a new parameter, the penetration coefficient (Pλ) is introduced, this being defined as the ratio of the downward monochromatic radiation flux at a given depth in the suspension to that within an equivalent solution of the algal pigments. A set of rules is established, applicable to cells of any shape or orientation, which characterizes the relationship between the penetration coefficient and the parameters of canopy structure (cell number, size, pigment composition etc.). A standardized procedure for calculating the spectral distribution of, and the vertical attenuation coefficients for, photo-synthetically active radiation within model phytoplankton suspensions, is outlined.

A theoretical analysis of the contribution of algal cells to the attenuation of light within natural waters ii. spherical cells

Abstract: SUMMARY A theoretical treatment of light attenuation within natural phytoplankton suspensions, developed in a previous paper, has now been applied to model suspensions of green and blue-green algal cells and colonies, in order to obtain a quantitative assessment of the effects of changes in the phytoplankton canopy structure on light attenuation within the system. It is shown that for green algae suspended in water in which effectively all the blue light is removed by dissolved yellow substances (gelbstoff) the light attenuation properties of the suspension are rather similar to those of an equivalent solution of algal pigments: this is because the individual cells have relatively low absorption values in the spectral region (545–655 nm) in which most of the transmitted light occurs. With blue-green algae, which have a strong absorption in this region, there can be marked differences between the light attenuation properties of suspensions and of equivalent pigment solutions. The suspensions transmit substantially more photosynthetically active radiation than the solutions: the effect increases with cell/colony size, with algal concentration and with depth, and can amount to a difference of several-fold. Values of vertical attenuation coefficients, and increments in attenuation coefficients per unit algal concentration, for the photosynthetic waveband have been calculated, and their dependence on pigment composition and cell/colony size is demonstrated. Calculations have also been carried out to show the relationship between the phytoplankton canopy structure, and the euphotic depth and maximum sustainable standing crop, of the suspension.

Movement of ethylene from roots to shoots, a factor in the responses of tomato plants to waterlogged soil conditions

Abstract: SUMMARY Epinastic growth by petioles of tomato (Lycopersicon esculentum Mill., cv. Moneymaker) is stimulated by waterlogging the soil and the responding leaves and other shoot parts contain higher concentrations of ethylene than those of non-waterlogged plants. Waterlogged soil also contains increased amounts of ethylene in solution and the possibility that ethylene from this source can move to the shoots of waterlogged plants has been tested by applying the gas to the roots of plants growing in water culture. When concentrations of 2 ppm and above are supplied in this way, petiole epinasty develops in association with increases in ethylene extracted from the responding petioles. When 100 ppm is applied to the roots, increases in ethylene in the petioles precede the development of epinastic growth. Use of [14C] labelled ethylene indicates that the gas can move rapidly, unchanged from roots to shoots. The movement of ethylene and epinastic growth are both reduced if plants are previously steam-girdled near the base of the stem, suggesting that much ethylene moves by a route other than the transpiration stream. Ethylene supplied to the roots is shown to promote adventitious rooting and downward rolling of the young leaf laminae and to modify the orientation of shoot growth in the tomato mutant‘diageotropica’. It is proposed that increases in soil ethylene and movement of the gas to the shoot system are factors contributing to the development of epinasty and other responses of the shoots to waterlogging the soil.

The influence of light on stomatal density in the tomato

Abstract: SUMMARY Stomatal densities (stomatal number per unit leaf area) were determined on tomato plants grown at ‘high’ (100 W m−2) and ‘low’ (20 W m−2) light in a growth cabinet. The upper epidermis of a fully expanded leaf in low light had fewer than one stoma mm−2, whilst that in high light had thirty stomata mm−2. On the lower epidermis there were about eighty and 100 stomata mm−2 in low and high light respectively. Similar differences were observed on plants growing in natural light. Stomatal initiation is most active early in the development of the leaf and effects of light on initiation were greatest at this stage, being observable within 3 days of a change in light level. Since light influenced the rate and duration of all stages of leaf expansion, the final stomatal densities achieved under natural conditions varied with the light history of the leaf. Stomatal density reached a peak before the leaf had expanded to 10% of its final area. Stomatal initiation and stomatal density both decreased from then until leaf expansion ceased. Fully-expanded leaves of plants grown continuously in the same environment had similar stomatal densities irrespective of their position on the plant. A quantitative relationship is described between stomatal density and leaf area during the growth of a leaf.

Some aspects of tissue water relations in three populations of Eucalyptus viminalis Labill

Abstract: SUMMARY Using the pressure bomb technique, measurements have been made of the tissue water relations of seedlings of three populations of Eucalyptus viminalis, and of adult shoots of trees from two contrasting sites, to facilitate an understanding of the physiological basis of intra-specific variation in drought resistance. For both juvenile and adult material, the shoot water potential of the most drought resistant population declined most rapidly with decreasing water content. Estimates of the fraction of water in the cell walls at full turgor were also greatest for this population, averaging 0.36 for juvenile and 0.40 for adult material. Calculated values for the bulk modulus of elasticity of the walls were very variable, but tended to be higher, in adult than juvenile tissues. The results suggest that in dry habitats there has been selection for plants which, during temporary periods of drying, can maintain a water content above the critical level for cell damage. Differences between two very closely situated field populations emphasize the need to consider the role of edaphic as well as climatic factors in selection for drought resistance.

I. numerical analysis

Abstract: SUMMARY Results are presented from Principal Components Analysis of three data matrices (pollen concentration per cc, percentages of total pollen, and analyses for eight or nine chemical elements) of c thirty samples from each of two profiles of lake sediment of Late-Devensian age (15000 to 10000 years BP) from the Lake District and NW Scotland The groupings of samples by their scores on the first two components of these Principal Components Analyses have been shown to produce similar divisions (zonations) to those originally produced subjectively by inspection of the percentage pollen diagrams Though neither profile includes the classical Late-Weichselian oscillation of pollen zones I, II and III, involving expansion of percentages of birch pollen within biogenic sediment between two minerogenic layers, numerical analysis has revealed in the data from both sites a pattern involving a threefold division with revertence, and another pattern of twofold division The subjective zonation of the percentage diagrams clearly involved a process of mental integration of these two patterns A plot of the scores of the samples on the first two components of the Principal Components Analyses groups the samples into clusters corresponding with the zones determined by inspection; similar results are obtained for each of the three Principal Components Analyses This is interpreted as indicative of a close relationship between pollen and chemical data which can most readily be explained on the hypothesis that the boundaries of pollen zones represent the results of environmental changes which simultaneously affected soils and vegetation

Inhibition of seed germination under leaf canopies in arenaria serpyllifolia, veronica arvensis and cerastum holosteoides*

Abstract: SUMMARY The germination of imbibed seeds of Arenaria serpyllifolia L., Veronica arvensis L. and Cerastium holosteoides Fr. is inhibited under two layers of Tilia×europaea L. leaves. The ecological significance of this phenomenon is discussed.

Stimulation of photosynthetic starch formation by sequestration of cytoplasmic orthophosphate

Abstract: SUMMARY Photosynthetic starch formation in leaf discs of spinach beet and related species can be increased more than ten fold by the presence of exogenous mannose at 10−2 M. In these species, carbon from mannose is not incorporated into starch to a significant extent and the stimulation is believed to be an indirect effect resulting from the sequestration of cytoplasmic ortho-phosphate as mannose phosphate. In species such as tobacco, there is substantial incorporation of carbon from mannose into starch and into soluble compounds other than mannose-6-phos-phate.

Ion transport and endodermal suberization in the roots of Zea mays

Abstract: SUMMARY Progressive suberization and thickening of the endodermis along secondary seminal axes of Zea mays had little effect on the radial movement of phosphate into the vascular tissue. In all parts of the root, inhibitors of respiration, and low temperature, reduced uptake and xylem translocation of phosphate by more than 90%. With calcium, however, there appeared to be two components of the translocated fraction; one was sensitive to metabolic inhibition and was most prominent in young tissue near to the root tip, while the other was insensitive to metabolic inhibition. In older zones near the base of the root, where there was little calcium movement into the stele, only the second insensitive component of calcium transport was found. A marked maximum in calcium translocation, but not in the translocation of phosphate, was found 12 cm from the root tip. This was not associated with a marked increase in total absorption but was due to a high proportion of the absorbed calcium which moved into the stele. This region is one where lateral roots are initiated in the pericycle and where the structure of the endodermis may change transiently. At the extreme base of the roots the development of a suberized hypodermis appeared to restrict very severely the translocation of both phosphate and calcium. Iron and manganese were also absorbed and translocated by all regions of the root examined; the proportion translocated was much greater for manganese than for iron.

Wind‐borne seed and fruit movement

Abstract: SUMMARY Seeds and fruits of widely differing sizes and shapes and which employ different flight styles in wind-borne movement during dispersal are discussed qualitatively in general aerodynamic terms, and grouped according to aerodynamic rather than botanical features. Most of these groups are considered separately, though not in any great detail, and the salient features of the differing motions belonging to each are described. The response of species, in particular to variations in vertical wind velocities, is mentioned as an important factor affecting-deposition and dispersal range, and how because of this response some forced deposition may be possible with the introduction of suitably shaped obstacles to wind flow.

Polyphosphate in eucalypt mycorrhizas: a cytochemical demonstration

Abstract: SUMMARY Small, more or less spherical metachromatic granules have been detected within the Hartig net and inner sheath hyphae of Eucalyptus fastigata mycorrhizas, and shown by various cytochemical tests to contain polyphosphates. These granules are believed to be the cytological equivalent of the biochemically characterized phosphate storage pool which other workers have defined on the basis of radiotracer studies.

The evolution of morphological novelty exemplified in the growth patterns of some gesneriaceae

Abstract: SUMMARY The growth pattern of certain South African species of Streptocarpus sub-genus Streptocarpus has been investigated, using in particular the species S. fanniniae. It is shown that the vegetative plant body consists of repetitive units, the first being formed by the accrescent cotyledon. These units are here called phyllomorphs: each has a stalk (the petiolode) showing a mixture of axis-like and petiole-like features, and a lamina. The development of the phyllomorph is largely controlled by a triad of meristems situated near junction of petiolode and lamina. The basal meristem provides new growth at the base of the lamina; the petiolode-meristem contributes to the growth of the midrib as the lamina enlarges and to elongation of the petiolode; the groove-meristem initiates the growth of new phyllomorphs and of the inflorescences. Recognition of the phyllomorph as a repetitive structural unit permits satisfactory description and comparison of anomalous growth patterns of various species of Streptocarpus and of certain other genera of Old World Gesneriaceae. Its evolution and diversification is briefly discussed.

Electron microscopical studies of the microflora of roots of sand dune grasses

Abstract: Summary Scanning and transmission electron microscopy were used to study microflora of roots of three species of sand dune grasses. Observations were made on the mucigel on intact epidermal cells and the colonization of sloughing superficial root cells by bacteria and actinomycetes. Thin sections of the same root samples showed that bacteria penetrated epidermal cells either by perforating the cell wall or by exploiting the middle lamella between adjacent cells as an avenue of entry. Cortical cells of all three species contained vesicular-arbuscular endophytes. Several phases of the mycorrhizal association could be identified.

Zinc and copper uptake by agrostis stolonifera, tolerant to both zinc and copper

Abstract: SUMMARY Zinc and copper uptake by Agrostis stolonifera, tolerant to both zinc and copper, showed that different zinc levels had no effect on copper uptake and, conversely, copper had no effect on zinc uptake. There was, however, a strong interaction in the toxic effects of zinc and copper as measured by root elongation. The mechanisms of zinc and copper tolerance are therefore independent even when they act in the same individual.

The activity of invertase and its relevance to the accumulation of storage polysaccharides in leaves infected by biotrophic fungi

Abstract: SUMMARY Infection of Senecio squalidus by Albugo tragopogonis and of Tussilago farfara and Poa pratensis by Puccinia poarum results in an increase in the activity of an acid invertase localized at the sites of infection. It is concluded that sucrose from the host is first hydrolysed and then absorbed by the parasite. Invertase also plays a key role in the provision of substrate for the typical accumulation of starch around pustules of biotrophic fungi on host species which store this polysaccharide. Compartmentation of invertase and the enzymes concerned with synthesis of sucrose and fructans permits these carbohydrates to be accumulated within pustules on host species which normally store fructans.

THE UPTAKE OF COPPER AND ITS EFFECT UPON RESPIRATORY PROCESSES OF ROOTS OF COPPER‐TOLERANT AND NON‐Tolerant CLONES OF AGROSTIS STOLONIFERA

Abstract: SUMMARY The uptake of copper by non-tolerant and tolerant clones of Agrostis stolonifera has been studied. In both clones the roots accumulated more copper than the leaves and the roots of the tolerant clone accumulated more copper than the non–tolerant one. When supplied at concentrations above 10 μM in culture solutions copper moved up into the leaves of the non-tolerant clone more readily than the tolerant one. Uptake of oxygen by roots was only affected when they were pretreated with copper, the non-tolerant ones being more sensitive to inhibition than tolerant ones. Over the concentration range tested copper had no effect upon l-malate-dehydrogenase (MDH) activity in root extracts of either clone. Extracts prepared from non–tolerant roots pretreated with copper were found to contain less MDH activity than those prepared from tolerant ones. It is suggested that this was due to an inhibition of protein synthesis in the roots of the non-tolerant clone, which did not occur in the tolerant one.

Pollen diagrams from the shetland and faroe islands

Abstract: SUMMARY Two pollen diagrams are presented, one from Murraster, Shetland Mainland, and one from Hoydalar, Streymoy, Faroe Islands. Both localities are former lakes, now filled up, and in both places the sedimentation started about 10,000 years ago. In the Faroese diagram herbs are dominant throughout, while it can be seen that Juniperus and Salix have been of considerable significance and Betula nana has had a wide distribution in early Flandrian time. In the Shetland diagram large–leaved Betula and Corylus are also present, even though herbs were also dominant.

Polyphosphate granules in three different kinds of tree mycorrhiza

Abstract: SUMMARY Using cytochemical techniques, granules containing polyphosphate have been detected in Hartig net and sheath hyphae of Pinus ectomycorrhizas; in sheath, intercellular and intracellular hyphae of Arbutus ectendomycorrhizas; and in hyphae and vesicles of Liquidambar endo-mycorrhizas.

Breeding systems in cotula iii. dioecious populations

Abstract: Summary Some male and female plants of dioecious populations of seven Cotula species are strictly unisexual, or constant; other plants of the same populations are inconstant, with a low proportion of florets of the opposite sex (minority-sex florets) which function normally. The distribution of minority-sex florets among heads of male and female plants is shown by rank-frequency curves. The average inconstancies of males and females are quantified as the proportions of genes transmitted via gametes of the minority-sex florets, estimated by the proportions of minority-sex florets. The average inconstancy of males is 9.9 × 10−4; that of females is 4.3 × 10−4. The Primary Sex Index, 0.9993, is the average constancy (1—inconstancy) of all individuals. This provides a measure of the degree of separation of male and female gamete functions in the two sexes, on a scale from 0.5 (sexual monomorphism) to 1.0 (complete separation of the sexes). The sex Asymmetry Index, male inconstancy–female inconstancy/male inconstancy + female inconstancy, + 0.39, measures the relative magnitude of inconstancies of the two sexes on a scale from –1.0 (androdioecy) to + 1.0 (gynodioecy). Male and female heads differ in several secondary characters. The progeny of experimental crosses show that females are heterogametic and males are homogametic for sex-determining chromosomes. Males and females occur in equal numbers in the progeny of male × female crosses and in natural populations. The distribution and experimental manipulation of sex inconstancies provide considerable information on the origin and genetic maintenance of dioecy. The evolution of dioecy in Cotula has been accomplished by divergence in the sexes of the timing of the switch from male floret to female floret formation during head development, rather than by the introduction of male- and female-sterility genes. No genes essential for female reproduction are restricted to the Y chromosome. A polyploid series in the dioecious populations (2n= 52 to 2n=c.312) indicates an ‘active Y’ mechanism, and may have been facilitated by the regeneration of sexual dimorphism from inconstant females with new chromosome numbers.

Stimulation of sex organ formation in phytophthora by antagonistic species of trichoderma

Abstract: SUMMARY Two methods are described for the induction of sex organ formation in heterothallic Phytophthora species in response to Trichoderma, a volatile method and a direct inoculation (‘contact’) method. Tested for their ability to induce sexual reproduction in Phytophthora, isolates of Trichoderma koningii, T. piluliferum, T. polysporum and T. viride were found to be active, and isolates of T. hamatum, T. harzianum, T. longibrachiatum and T. pseudokoningii inactive. Response in Phytophthora was, without exception, confined to the A2 compatibility type. Response of different Phytophthora species, however, was more variable. All forty-one isolates of P. cam-bivora and P. cinnamomi tested responded to Trichoderma viride. Isolates of Phytophthora cryptogea, P. drechsleri, and P. nicotianae were apparently polymorphic for response. The black pepper form of P. palmivora responded, whereas P. palmivora forms 1 and 2 did not. A minimum induction period of c. 24 h exposure to volatiles of active Trichoderma was apparently necessary for consistent oogonial formation to occur. Oogonial formation occurred on a range of sterol-containing media but not in their absence and over a wide range of conditions of temperature and light. Antagonism of Phytophthora cultures by active Trichoderma isolates was readily apparent both in the volatile and contact methods. The ability of Trichoderma isolates to induce a sexual response in Phytophthora was correlated with their ability to produce volatile antibiotics. Possible implications of the ‘Trichoderma effect’ for the taxonomy, reproductive physiology and genetics of Phytophthora spp. are considered.

Nitrogen fixation in the root nodules of vicia faba l. in relation to the assimilation of carbon ii. the dark fixation of carbon dioxide

Abstract: SUMMARY High activity of phosphoenolypyruvate carboxylase, malate dehydrogenase and malic enzyme has been found in extracts from broad bean nodules Tracer studies of the metabolism of `4C02 fixed by detached nodules showed that the amino acid fraction sequestered most radioactivity and that no radioactivity was associated with glucose and fructose, the main free sugars in ethanolic extracts of the nodules The pattern of labelling supports an important role for glutamate, aspartate and asparagine in the metabolism of the fixed carbon, but relatively little radioactivity was associated with glutamine Most of the radioactivity in the acidic fraction of the nodules was in malic and itaconic acids Although the latter was not detected immediately on detachment of the nodules, it was present in amounts equal to malic acid 30 min after detachment

A regional sub-division of dinoflagellate cysts around the british isles

Abstract: SUMMARY A cluster analysis of the dinoflagellate cyst assemblage of sixty-eight intertidal sediment samples from the British Isles has been used to divide the coastline into eleven regions. The resulting pattern of regions is discussed in relation to hydrography and the distribution of dinoflagellates in the plankton. The subdivision of dinoflagellate regions shows a close relationship to the occurrence of chaetognath indicators of water masses around the British Isles. This suggests that cyst propagation is localized and that each water mass has its own characteristic dinoflagellate population.

Correlation between the breeding system and recombination index in five species of senecio

Abstract: SUMMARY The species Senecio aetnensis Jan., S. joppensis Dinsm., S. squalidus L., S. viscosus L. and S. vulgaris L. have been compared for automatic selfing ability, capitulum conspicuousness, pollen yield, chromosome number and chiasma frequency. It has been established that the species S. aetnensis, S. joppensis and S. squalidus which have low automatic selfing ability have large, showy capitula, high pollen output and low recombination indices. The species S. viscosus and S. vulgaris which have a high selfing ability have relatively inconspicuous capitula, low pollen yields and a high recombination index. The differences in recombination index values are mainly due to the fact that the selfing species are polyploids but it may be significant that the highest average chiasma frequencies per bivalent were found in some S. vulgaris plants. The adaptive significance of a high recombination index in inbreeding taxa is discussed and it is suggested that a high chiasma frequency and/or polyploidy may have a preadaptive value in facilitating the change from outbreeding to inbreeding in some species.

The climatic control of the altitudinal distribution of sedum rose a (l.) scop. and s. telephium l.

Abstract: SUMMARY When plants of Sedum rosea (L.) Scop, and S. telephium L. ssp. fabaria Syme are grown in competition at different altitudes it is found that the growth of the species is affected differentially. This effect is such that at low altitudes S. telephium is a larger plant than S. rosea, while the reverse is the case at high altitudes. The observed differences are caused by a marked sensitivity of the growth of S. telephium to differences in altitude, while S. rosea is very insensitive. The most probable cause of these responses would appear to be differences of climate. The influence of altitude in northern England on air temperature, saturation deficit and irradiance has been investigated in order to establish the magnitude of the differences which may cause these responses.