Showing papers in "Taxon in 1973"

Aquatic and wetland plants of southwestern United States

Abstract: Part 6 is the first part of a monographic treatment of the North American Desmids dealing with the Saccodermae containing the Mesotaeniaceae (8 genera). The treatment follows the well-known lines of the North American Flora which is now in its second series. This series contains treatments published in the order in which they become available; part 1 (Tuberales) came out in 1954, part 5 (Onagraceae) in 1965. This instalment on the Saccoderm Desmids contains a particularly valuable and extensive bibliography of all the literature that deals with desmids in North America.

Taxonomy, phytogeography and evolution

Abstract: A symposium volume, containing the papers delivered by 26 authors at a symposium held by The Linnean Society of London, the Botanical Society of the British Isles and of the International Oraginsation of Plant Biosystematists, Manchester, 9-11 September 1971. The papers of the symposium were divided in five sections, the first of which, 'introduction', contained preliminary remarks by Valentine and a major paper by Stebbins on Ecological distribution of centers of major adaptive radiation in Angiosperms. The second section contained papers on "major geographical disjunctions in relation to evolution and migration"; the third deals with "Endemism", the fourth with "geographical evolution in genera and families of special interest"; the fifth section was on "special topics" [migration of weeds, history and ecology of continental European plants, floristic connections between Southeast England and North France, computational methods in the study of plant distribution]. The sixth section of the book contains A.R. Clapham's discussion "questions answered and unanswered." The symposium was of special importance because it brought together modern evolutionary studies and perhaps more traditional but still valid macrophytogeographical approaches.

Chemosystematics in the rutaceae ii. the chemosystematics of the zanthoxylum/fagara complex

Abstract: Summary The biosynthetic pathways leading to the major groups of secondary metabolites isolated from the Zanthoxylum/Fagara complex are reviewed and there is shown to be a common precursor in chorismic acid. The use of the compounds as chemotaxonomic markers is discussed and they are found to support the concept of a direct link between the Rutaceae and ranalian stock. The potential value of the secondary metabolites in chemosystematic analysis within the family is also briefly discussed. The alkaloids, coumarins, flavanoids and lignans of the taxa Zanthoxylum and Fagara are examined in detail and the distribution found to support authorities who have expressed the view that these taxa are congeneric (Zanthoxylum s.l.). The chemosystematics of Zanthoxylum s.l. indicates that the genus is primitive within the family. The secondary metabolites of Zanthoxylum s.l. are used to construct a putative phylogeny of the genus and its possible "key" position in the evolution of the family is speculated upon. It is noted that the Englerian classification of Zanthoxylum s.l., and of the Rutaceae in general, appears to be unsatisfactory.

Are categories in different phyla comparable

Abstract: Summary There are several kinds of criteria for determining whether an average family of insects is comparable to an average family of mammals. Different combinations of criteria are appropriate for different purposes and give different results. A new metataxonomic criterion is explored for all phyla of animals and plants and for various subordinate taxa. Differences in it can be caused by sampling error, different degrees of splitting, and different patterns of evolution; these causes are more or less confounded but indicate the domain of the criterion's applicability. Body weight and other factors affect the number of species in genera and families. It is not yet known to what extent boundaries between taxa correspond to real subcontinuities in properties of the organisms. We should know what characters are, or have been, important to an organism, even for angiosperms, where little research has been directed at the problem as formulated here. The paradigm of the sort of question I will consider is the following: Is an average family of mammals the same as an average family of insects, and if not how do these taxa (not, of course, the animals themselves) differ? I will point out several interpretations of the question and evaluate one interpretation quantitatively for all phyla of animals and plants. It is commonly believed that the question cannot be answered. This may be true in the sense that there may not be a unique way of weighting the various possibly relevant criteria. However, single criteria, or more or less definable combinations of criteria, may be the only ones relevant for such applications as the comparison of taxonomic rates of evolution, evolutionary patterns, analysis of hierarchial diversity (Pielou, 1967) of a community, or deciding whether more splitting or lumping would make categories in one taxon closer to those in another taxon. Clearly the criteria need not be the same for all applications, although some are more generally useful than others and some may not really be useful at all. Appropriate, although not objectively derived, weighting of several criteria can be done and is normally done implicitly in the construction of classifications. Criteria

A morphological hypothesis for the origin of heterostyly in the rubiaceae

Abstract: Dimorphic heterostyly is common in the Rubiaceae, distributed throughout the family in a way that suggests that it has evolved repeatedly. In its essentials this heterostyly suggests protandry in which some plants have normal elongation of the styles and others have precocious maturation of the stigmas at a stage of arrested elongation of the styles. It is suggested that the initiation of heterostyly in this family may lie in a modification of the common protandry. High reproductive fitness of the self-pollinating short-styled plants would begin a shift in the population from predominant outcrossing to predominant autogamy, even though the resulting homozygosity might be disadvantageous. This shift might be halted by the addition of diallelic self-incompatibility of the kind regularly found in heterostylous plants. The population would then achieve the stable polymorphism characteristic of dimorphic heterostyly. Additional morphological refinements might accrue in time. There are more species with dimorphic heterostyly in the Rubiaceae than in any other family of flowering plants (Vuilleumier, 1967), perhaps even more than in all other families of flowering plants put together. Bir Bahadur (1968) lists 416 species in 91 genera of the Rubiaceae. These species do not form any sort of phylogenetically homogeneous group that might suggest their derivation from one or a few heterostylous ancestors. Rather, they are distributed in 21 tribes (Bir Bahadur, 1968), and their congeners are often homostylous species (e.g. Hedyotis, Ornduff, 1969; Pentas, Verdcourt, 1953). These facts suggest that, unless the family as a whole is descended from a heterostylous ancestor, heterostyly must have evolved repeatedly in the Rubiaceae. This conclusion suggests an obvious question: What special features of the Rubiaceae have allowed andlor facilitated the recurrence of heterostyly in the family? Heterostyly in the Rubiaceae consists basically of a balanced polymorphism in which some plants bear flowers having the receptive stigmas above the level of the anthers ("pin"), and other plants bear flowers having the receptive stigmas below the anthers ('thrum"). This difference is often accompanied by differences in corolla tube length, filament length, pollen size, and stigmatic papilla size. However, it is best to view the dimorphism in terms of its basic characteristics when inquiring into its possible origin, because unless the whole complex of features is assumed to have originated in one macro-mutational event, it must have initially consisted of basic features which were later augmented by gradually acquired refinements. It is interesting and perhaps significant that heterostyly, shorn of refinements, really does come down to "different styles", i.e. plants with short-styled flowers and plants with long-styled flowers. It is

The text of vellozo's flora fluminensis and its effective date of publication

Abstract: Summary Flora Fluminensis was written by Fra J. M. da C. Vellozo at the end of the I8th century but it was published only later, after the author's death. In I825 three fourths of the text was printed, up to Sabbata romana, p. 1-352, but it remained stored in the Typographia Nacional for four years. The sale of this incomplete text began on a date in 1829 that is difficult to determine precisely. Perhaps because the text remained unfinished, distribution was interrupted between 1832 and 1833, but it was reinitiated in I834. By 1838 the book was almost out of print. The complete text was published in i88i, as a second edition. The effective date of Vellozo's species described in the first unfinished edition lies between September 7 and November 28, 1829.

Axor and monit: two new polythetic-divisive strategies for hierarchical classification

Abstract: Summary Two new divisive strategies for hierarchical classification are reported. Both use a form of 'directed search' to limit the number of possible polythetic splits examined. An objective method for testing the accuracy of divisions is defined. Preliminary results of tests of the two new methods against certain existing techniques have shown the former to produce superior divisions. In numerical methods of hierarchical classification the basic strategies may be partitioned in two ways: monothetic versus polythetic and agglomerative versus divisive. Except when special-purpose classifications are required (e.g. as a basis for key construction), most taxonomists have shown a marked preference for polythetic techniques, i.e. those using the properties of all the attributes taken together to define the classes. To date the only polythetic methods in general use have been agglomerative. On the other hand, polythetic-divisive methods have theoretical advantages in that all the available information is used to make the critical topmost divisions. Since even for a simple dichotomous hierarchy there are 2(n-a-I possible splits for each subdivision (Edwards and Cavalli-Sforza, 1965), however, polythetic-divisive methods are computationally out of reach for sets in excess of about 16 individuals unless some form of 'directed search' is used to limit the number of splits examined (Williams and Dale, 1965). There have been surprisingly few attempts to date aimed at producing a satisfactory polythetic-divisive technique, the only methods aoparent in the literature being the 'dissimilarity analysis' of Macnaughton-Smith et al. (1966) and the 'farpoint' clustering technique suggested by Gower (1967). We are currently investigating a number of other polythetic-divisive strategies, which will be described elsewhere. The purpose of the present communication is to report briefly on the two most promising of our methods, and to compare their performance with that of other techniques. In any divisive system the partition of a given population may be into two, three or more subpopulations. Since we have noted that a trichotomous split is often immediately obtainable from two consecutive dichotomous splits, we have as a first approach restricted our tests to the first dichotomy of the total population. Most methods extend with ease to the polychotomous situation, but then inevitably become computationally less attractive. Any partition of a population can be measured in terms of some coefficient, and to seek the optimal partition this coefficient is maximised. A num

Rellimia thomsonii, a new name for milleria (protopteridium) thomsonii lang 1926 emend. leclercq and bonamo 1971

Abstract: In a study of new material of Milleria (Protopteridium) thomsonii Lang 1926 from the Middle Devonian of Belgium (Leclercq & Bonamo, i97i) we reinstated the name Milleria Lang 1926 for the invalid name, Protopteridium Krejci i88o. This name is a nomen nudum because Krejci designated it in his publication of i88o as a provisional name ("provisorische Benennungen"). His description was not accompanied by a specific, generic or combined generic-specific description, nor was the plant figured. We also rejected as inappropriate for our taxon, the generic names Ptilophyton (Dawson, 1878; see also Kraiusel & Weyland, 193o), Hostinella (Stur, 188i) and Spiropteris (Potoni6 & Bernard, 1904) because all three were based on previously existing specimens unlike our material. We adopted the name Milleria because Lang assigned it for the first time (1926) with careful descriptions accompanied by photographs, generic and specific diagnoses