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Leaf proteins of the 10 herbaceous species of the genus Da!ura were separated by electrophoresis in starch gels and stained to reveal peroxidases. A. total of 19 different sites of peroxidase bands was observed for the species as a whole. Developmental studies showed that the number of bands increased with age of the leaf during seedling growth as well as in more mature plants. Among species the number of band sites for one particular leaf (AL-4) varied from 4 to 9. Each species pattern was unique, and no single peroxidase band was common to all the species. Based on considerations of interspecific cross compatibility and chromosomal rearrangements the c.assical taxonomic division of the genus into three sections was further subdivided to give five groups. The degree of phylogenetic relationship for species within vs. between these groups was assessed for the peroxidase patterns by use of statistical methods based on a hypergeometric distribution model. The probability that the observed degree of band matching was due to chance was less among species within a group than between groups, thus indicating a closer genetic association which is in agreement with the relationships arrived at by more conventional methods. 1MtULTIPLE MOLECULAR forms of enzymes, termed isozymes by Markert and Moller (1959), are known to be produced in a wide variety of higher plants (Shannon, 1968), are readily demonstrable by gel electrophoresis, and are considered to be products of genes which segregate in a Mendelian manner (Schwartz, 1960; Shaw, 1964; Serov, 1968; Scandalios, 1969; Marshall and Allard, 1969; Sing and Brewer, 1969). They are therefore useful as genetic markers at the molecular level. In developmental studies, the changing pattern of the electrophoretic bands of isozymes is interpreted as evidence of different gene activations at the various ontogenetic stages sampled (Scandalios, 1965; Scandalios, Liu, and Longo, 1968; Gupta and Stebbins, 1969). These may be associated with physiological events during differentiation and maturation (Racusen and Foote, 1966; Siegel and Galston, 1967; Kensetter and Keitt, 1967; Makinen, 1968; Bhatia and Nilson, 1969). In studies of plant evolution and systematics, the demonstration of similarities and differences in isozyme band patterns has been used as evidence of the degree of phylogenetic relationships among species, e.g., in Nicotiana (Hart and Bhatia, 1967; Sheen, 1970; Smith et al., 1 Received for publication 3 March 1971. This paper is dedicated to Miss Sophie Satina in recognition and honor of her 91st year. The authors express their thanks to Miss Alexandra Jahn and Mr. Nicholas Combatti for technical assistance and to Mr. Keith Thompson for advice and assistance with statistical computations. Research carried out at Brookhaven National Laboratory under the auspices of the U. S. Atomic Energy Commission. 1970), Phaseolus (West and Garber, 1967a), Solanum (Desborough and Peloquin, 1967), Fabaceae (Thurman et al., 1967), Triticum (Johnson and Hall, 1966), and Oryza (Chu, 1967). Other studies have been aimed at investigating the genetic relationship between a hybrid and its parental species (Schwartz, 1960; West and Garber, 1967b; Bhatia, Buiatti, and Smith, 1967; Marshall and Allard, 1969; Sing and Brewer, 1969; Conklin and Smith, 1969; Sheen, 1970; Smith et al., 1970). This study was undertaken to determine whether or not differences in peroxidase isozymes occur among the herbaceous species of the genus Datura, if they could be used to mark phylogenetic relationships, and how these patterns might change during ontogenetic development of plants within a species. This genus was chosen mainly because it is so well known cytogenetically. Speciation in Datura has not involved changes in chromosome number (n = 12 in all species), but gene differences have accumulated and are accompanied by structural rearrangements of the chromosomes. The taxonomy of the ten herbaceous species of Datura, which are characterized by distinct differences in morphology and habit of growth, was reviewed most recently by Satina and Avery (1959). Based on systematic studies that had been published previously by others, the species were classified into three sections. The distinctness of each species has been maintained by geographical isolation and differences in ecological preference, which limit hybridization under

From its characteristics and the circumstances of its origin, practically all competent observers are convinced thatSpartina Townsendii arose from hybridization betweenS. alterniflora andS. stricta.

The Code of Botanical Nomenclature (Lanjouw et al. 1961) determines the sequence of ranks, but does not define the single taxa. When plant evolution is used as the main classifying principle, the taxonomic units and their arrangement should demonstrate phylogeny within the plant kingdom. The division (the phylum) is the highest rank of taxa and the primary step in the classification of the plant kingdom. It is uniform in origin, shows no evolutionary relations to the other divisions. It is distinguished by its biochemistry (pigments, storage products, etc.), has a specific cell structure (EM structure of plastids, cell compartments) and in some cases a specific morphology. One or more parallel evolutionary lines may occur within the division, leading from simple unicells to an elaborated plant body. These evolutionary lines represent a further rank of taxa, the class. The class is thus a line of the morphological progress. The single steps in algal advance in a class represent a further rank of taxa, the order. The order is defined morphologically by the level of plant body construction. It is a degree of morphological differentiation and a step in the phylogenetical progress. The lower ranks of taxa below the order are hardly difficult to define. The system of these units is more or less artificial. Phylogenetical the relationships are indistinct and therefore subject to individual meanings of the authors.

SUMMARYEvolutionary studies in the cottons began with the demonstration by Harland that true cottons could be divided into four genetically distinct species, each carrying an internally balanced genotype that was disrupted in crosses with any other. Harland's genetic classification has stood the test of time, and contributes substantially to our understanding of speciation in other genera, as for example, Phaseolus. In addition, it provides a genetic basis for the observations described by Anderson as ‘introgression’.Studies of cottons found growing wild, and of their relationship to cultivars in the same areas, led to the hypothesis that all true cottons are cultivars, and those now found wild are escapes. This hypothesis, though it led to purposeful exploration of the distribution and relationships of the cottons, was found to be inadequate. A new approach to the relationships of the cottons became possible following Thoday's demonstration of ‘disruptive selection’ in Drosophila. The disruptive situation has been shown to obtain as between cultivars and their wild relatives in Sorghum and in a number of other crop plants.Consideration of the time scale of agricultural evolution makes it clear that the recent origin theory is untenable, and a review of the status of the ‘wild’ cottons shows that they are adequately accounted for as weedy relatives of the cultivars, descended from common wild ancestors under disruptive selection.It is concluded that crop plant evolution has gone on very largely within the balanced gene pools which Harland regarded as genetic species.

One new and several re-interpreted species of Lower Cretaceous age are included in Pyrobolospora, a new organ-genus of large spores bearing neck structures; in Triletes (section Aphanozonati) are placed two other species of megaspores, one emended and one new. These are a small part of a distinctive Wealden assemblage of plant microfossils, which represents a critical period in plant evolution and which should play its part in stratigraphy. Certain difficulties in nomenclature and morphology are discussed.

Land-plant type spores occur in argillaceous limestones and platform graptolitic mudstones of Wenlock-Ludlow age in Sweden. Analysis of the stratal sequence demonstrates that occurrence of abundant spores is a function of depositional ecology, particularly water depth and shoreline proximity. This ecological analysis and the seeming absence of any correlation between the first appearances of abundant spores and megafossils of land-plant type raise the possibility that land plants evolved far earlier than the megafossil record suggests.

The precursors of angiosperms are still unknown. All we know is that angiosperms appeared in Lower Cretaceous strata (see Fig. 2-2, page 19), became abundant and varied by Late Cretaceous time and evolved by Eocene time many modern genera. Reports of pre-Cretaceous angiosperms are usually based on fragmentary evidence such as a leaf, a pollen grain, or a bit of secondary wood. None of these specimens can be unequivocally called an angiosperm rather than a gymnosperm, and paleobotanists generally remain skeptical of them. Morphologists, on the other hand, are inclined to view the complexity of the flower and the embryo sac as well as the wide variety of early angiosperms in the Cretaceous Period, as evidence that the group must have had a long period of evolution prior to Cretaceous time. Axelrod suggested that the first angiosperms arose on highlands far back from the coastal plains and so were not preserved as fossils. But unless someone makes the fortunate discovery of an unequivocal pre-Cretaceous angiosperm we must use the available evidence to learn what we can of the first angiosperms and their subsequent evolution.

THE letter from Mr. James B. Johnston in NATURE of August 5 touches on many important points, which cannot be fully dealt with in a letter of reasonable length.

Extreme habitats have a fascination of their own. They are occupied by only a few very characteristic species, and they are easy to recognize ecologically and spatially. In extreme environments a single physical factor is usually dominating and other factors subsidiary. This makes it easy to understand what is going on, and to define the intensity of the factor. Extreme habitats are so distinctive that it is also easy to see where they begin and end. Because of this they have important advantages for the study of evolution.

THE remarkable state of preservation of many Palaeozoic plants, and some few Mesozoic forms, has raised palaeobotany to a position of considerable importance in certain fields of botanical investigation. Endless synonyms, and specific determinations of more than doubtful value, have not unnaturally prejudiced botanists against the study of plant fossils. The scientific treatment of the mineralised tissues of extinct forms has, however, been productive of exceedingly important data towards the better understanding of the lines of plant evolution. Synthetic types and intermediate forms of plant structure are already fairly abundant, arid the various suggestive facts revealed by a study of their remains are gradually assuming a more definite shape.

The Birth and Spread of Plants By Dr. J. C. Willis. (Reprinted from Boissiera, Vol. 8.) Pp. xi + 561. (Geneva: Conservatoire et Jardin botaniques de la Ville, 1949.) n.p.

The Bryophytes are of special interest to the student of plant evolution as being the first group to adopt the land habit. This change from the aquatic habit of the ancestral algae to the land habit of the bryophytes may be looked upon as one of the most critical changes of environment taking place during the evolution of the plant kingdom. It meant, first of all, that the plants must solve the problem of living without a continuous supply of free water, and, furthermore, of resisting, at least in a dormant condition, the periods of drouth which must necessarily occur. As we should expect, the liverworts, with the exception of certain of the higher genera such as Frullania, have advanced a comparatively short distance in the solution of the problem, being mostly still confined to wet habitats. But when we come to the mosses, we find a group distributed over a diversity of habitats, and living successfully even under such severely adverse conditions as exist on tree trunks and on rocks, where they are exposed to the drouths of summer and to the freezing temperatures of winter. Any contribution to our knowledge of the ability of mosses to withstand these conditions is welcome.

PLANT geography has been a neglected branch of our science. This is strange; for not only is it a fascinating study in itself, but both historically and philosophically it is a part of geography. Alexander von Humboldt, who with Ritter founded modem geography, is widely known as the father of plant geography; De Candolle, who did classic work in the nineteenth century on the origins of cultivated plants, was considered a plant geographer; and some of the recent great advances in our knowledge of the origin, distribution, adaptation, and use of plants have been accomplished by a group of Russian scientists who have worked under the appellation of "plant geographer." There has been a tendency to label plant geographers "botanists." But this is no more reasonable than labeling geomorphologists "geologists." Just as in the geomorphologist we have a scientist with interests lying between the fields of geography and geology and acquainted with both, so in the botanist-geographer we have a similar combination. What is generally unrecognized by geographers is that the botanist's study of plant geography differs less in the material used than in the point of view. A botanist uses plant geography principally as a means of gaining understanding of the problems of plant diffusion, speciation, and speed, direction, and types of plant evolution and adaptation. In its specialized form this botanical plant geography is well represented by Stanley Cain's recent excellent book "Foundations of Plant Geography."' Less specialized in form, and hence more useful to geographers, is the recent translation of Wulff's great work "Historical Plant Geography."2 These works are methodologically extremely interesting for the way in which distributional problems are handled. But for the botanical plant geographer the material with which he works is less interesting in itself than as a means of seeking out and exemplifying botanical laws. Conspicuously absent from the botanical plant geographer's work is consideration of the economic plants of the earth in their regional setting. Here is a proper and fruitful field for the geographer. It is this economic plant geography that I wish to discuss. For reasons both of space and of per

MANY readers of NATURE must have been browsing with delight in the goodly volume on “Darwin and Modern Science” which Prof. Seward, of Cambridge, has taken such admirable pains to collect. Of all its many chapters few are more significant than that on the palaeontological plant record by Dr. D. H. Scott, because there, perhaps for the first time, the evidence of the fossils with regard to the influence of natural selection has been fairly tackled by competent hands.

The students who have been working for many years to unravel the history of the vegetable kingdom, have given us a very clear and convincing body of proof that our terrestrial plants have developed from aquatic ancestors. It is doubtful if any single event in plant evolution is of greater importance than the emergence from ponds, rivers and lakes, and the development of characteristics which make life possible on dry land. We are familiar with the necessity of water for fertilization in mosses and in the prothallia of ferns, and the fact is very impressive when we realize that none of the ancestors of these plants were ever free of dependence upon water or films of water for this critical act in their life histories. The emergence of plants from the water to the land may have occurred more than once and in more than one group. However this may be, we have numerous cases in which members of purely terrestrial groups have returned to the water. All of our aquatic flowering plants show their terrestrial origin both in their relationships and in the fact that they expose their flowers above water, where fertilization is able to take place by the complex process of pollination which, more than any other performance or structure, was the thing that made terrestrial existence possible. Although ferns still depend upon water as the medium for the transfer of sperm to egg, this act may be per

The story of the archegoniate land plants as part of the story of life itself must start with the beginning of life. We can but guess at the environmental influences and selection pressures that operated upon the early living organisms of life's most ancient days. None the less, for some reason cells with true nuclei eventually arose, and the organ of motility that is the 'nine-plus-two' flagellum (Manton, 1965) was evolved. The uniformity of structure of this latter organ which is possessed by a very great diversity of nucleate organisms is a strong argument for the common origin of the latter. Only the red seaweeds of the major groups of nucleate organisms seem on good grounds to be relicts of the premotile caryotic staget (Ross, 1961, Manton, 1965). [Some authorities would include also the higher fungi-see, for example, Denison & Carroll (1966)-but it seems to me more likely that they are secondarily non-flagellate and related to some at least of the lower fungi which have a flagellate stage, and through them to some group of the Chromophyta (Christensen, 1962).] Likewise, we do not know what the factors were that caused the early and considerable differentiation of the flagellate organisms, but we do know that it occurred and that several different lines of plant evolution thus became established, which differed (and still differ) in such ancient and therefore fundamental (Jeffrey, 1964, 1966) characters as detailed flagellar structure, photosynthetic pigments, and cell-envelope and food-reserve cytochemistry, amongst others. Only one of these lines has led to archegoniate plants, that of the green plants of which the algal members have long been termed the 'Chlorophyta'. We do not find, however, amongst living algal Chlorophyta any plants recalling the archegoniate plants in their somatic (and gametic) construction. I have partially considered