more than a specialization on " prophetic " lines by a cohort which became extinct, comparable to another specialization in another family in an earlier epoch, viz. the " seed-like " structures of the tree forms of the Lepitlodendrons, which though truly Lycopodia- ceous, developed " seeds." Another branch of the great Mesozoic group, the Williamsoniaceae, may prove to be better accredited forerunners of some angiospermic cohorts. The small form William- soniella, described by Thomas from the Jurassic, has an interesting and suggestive morphology. Anatomical work on the genus, how- ever, still awaits the discovery of suitably preserved material.
Interest in the Cycads proper, stimulated in the preceding decade, has been continued. Their fructifications and general anatomy have been searchingly examined for primitive features (Matte, Stopes, Chamberlain, Seward, Worsdell), but still remarkably little is known of the geological history or early structure of the Cycads proper.
Anatomical work on the Pteridophytes has made steady progress, as is evidenced by the considerable enlargement of the first volume of the new edition of Scott's classic " Studies." Data of interest to anatomists have been chiefly based on Coal Measure species, filling in supplementary details in previously well-established schemes. The most important contribution in this connexion has been the thorough handling of the Osmundaceae, made possible by dis- coveries of well-petrified Mesozoic and Tertiary species (see in particular the series of Memoirs by Kidston and Gwynne-Vaughan). From an anatomical study of a series of species, the authors were able to present a phylogenetic sequence in the evolution of the family from Palaeozoic times, and held that the group arose from solid protostelic ancestors. Their view has been substantiated by the recent discovery of a true protostelic Osmundaceous form. The Botryopterideae, with their intricate series of stelar variations con- tinued to receive detailed consideration (see Bertrand, Scott, Za- lessky, Gordon and others) in the last decade, and are now very thoroughly known. A fern of wide geological distribution and of peculiar anatomical structure was at last made clear by the work of Kidston and Gwynne-Vaughan on Tempskya. Access to the original paper published in Russia is difficult and reference should also be made to Stopes, 1915. The work of Schoute on the branching of the Pteropsida should be read in connexion with Tempskya which had an extraordinary, massive tree-fern-like trunk, really built up of a weft of fine solenostelic stems, petioles and roots. Although ana- tomical work (by Tansley, Lang, Seward and others) has been done on various genera of recent ferns, good petrified material from the Mesozoic or Tertiary is either very scarce or has no.t received that anatomical attention which makes it possible to demonstrate the phylogenetic series connected with the higher families of recent ferns, such as Kidston and Gwynne-Vaughan have presented for the Osmundaceae. Yet where anatomical studies have been under- taken on recent genera, as by Lang on the Ophioglossaceae, they are largely influenced by the general principles of palaeobotany and the theme (as a sort of leit-motif) of the solid ancestral stele, runs through such work.
Of the lower Pteridophytes, our knowledge of those impressive members of the Lycopodiaceae, the Lepidodendroid series, was al- ready essentially established, and in the last decade has merely received detailed amplification.
In the more primitive groups, however, great advances have recently been made through the active interest taken in the De- vonian flora by Halle of Norway, who described the morphology and part of the structural details of a number of noteworthy species. This was soon followed by Kidston and Lang, to whom were en- trusted the plant materials of the now famous Rhynia cherts. The Psilophyton of Dawson, so long relegated to insignificance and by many considered to represent merely imperfect fragments, has thus suddenly become of great interest. In the Scottish cherts are well- petrified genera whose anatomy shows very much the type of structure postulated so long before by Dawson. Sporangites of Halle, Rhynia, Hornea, Asteroxylon of Kidston and Lang represent the earliest known land plants, and though varying in details, all show a remarkable simplicity of structure and arrangement of their aerial stems and terminal sporangia. These plants are stimulating discussion on the origin of land plants, the evolution of the Pterido- phytes from the lower groups, and the morphology of the various organs in higher plants, and their anatomy is of deep morphological and phylogenetic significance.
Anatomical work on the Bryophytes still has to confine itself to living forms because fossils are almost non-existent. Various fea- tures of the last-described group of new fossils, however, are highly suggestive for those who consider the phylogeny of the mosses. That isolated family, the Characeae, is well represented in many deposits, and has been seriously taken in hand by Reid and Groves with full examination of the anatomy of many beautiful fossil forms.
In palaeontological as in modern anatomy, observation is gener- ally concentrated on the vascular tissues, and particularly on the arrangement and character of the wood. This leads not infrequently to difficulties for the palaeobotanist, who sometimes finds in his specimens other portions of plants which are difficult to diagnose owing to the practical neglect of the comparative study of such organs by recent botanists. For instance, especially since the " clearing method " was popularized by Nathorst, the study of the " mummified " cuticles from fossils of all kinds has brought
into prominence the general ignorance concerning the diagnostic value and main characters of the stomata and epidermal cells among recent plants. The palaeobotanist, therefore, has had to investigate groups of living forms to effect his own comparisons. This in small genera of gymnosperms has not been crushingly burdensome, and has resulted in considerable additions to our knowledge of the details of recent forms (see Berry, Halle, Thomas, Bancroft, Antev and others). But for workers among the more extensive families of recent angiosperms, comparisons have become overwhelmingly arduous. Recognition of the " mummified " or semi-preserved seeds of angiosperms found in Tertiary deposits has become a work of the highest specialization in the hands of Mr. and Mrs. Clement Reid, who have greatly extended our knowledge of recent deposits, both in England and on the European Continent.
In the above paragraphs the more botanical and phylogenetic aspects of plant anatomy have been considered. But the palaeo- botanist has other functions, and he finds wider fields of application for an anatomical knowledge of fossil species. The value of the anatomical structures of the leaves, stems and so on of fossils of given localities and epochs in determining the nature of the climates of the past has long been recognized, and in the last decade this subject has chiefly been pursued by Berry in America, who has extensively surveyed the Upper Cretaceous and Tertiary climates and dis- tribution of species. In the southern hemisphere, that " terra incognita," the Antarctic, has been a little illuminated by the col- lections (see Seward and others) brought back by the Scott expedi- tions. And Gothan has worked on the woods from the Arctic.
A specialized application of anatomical knowledge has developed in connexion with a detailed study of coal. Many of the earlier workers (Dawson, Williamson, Huxley and others) were interested in the spores and small structures to be seen in coal and in recent years section cutting has been improved by Lomax, who has dem- onstrated many beautiful structures in coal sections. More exact consideration of the relation of the different parts of the plants to the character and accumulation of different types of coal in conjunction with their chemical analyses, ash content and so on has recently been made by Stopes, Stopes and Wheeler, followed by Lessing, Tideswell and others. The detailed chemistry of the different cell units composing the plant body is now gradually being correlated with the qualities of the different fractions of a band of coal. The resistant properties of certain cells such as spores and cuticles (yielding fuels of peculiar types on a big scale such as the " Cuticle Coal " of Russia) are being followed up on a minuter scale in the fine zones in an ordinary band of British coal, which have been shown to contain correlated differences, both in the anatomical nature of their plant content, their physical and chemical properties, the nature of their ash and so on.
The bibliographies in the following books will give most of the references, except those of monographs published quite recently: E. W. Berry, Lower Cretaceous Deposits of Maryland (1913), and Upper Cretaceous and Eocene Floras of South Carolina and Georgia (Washington, 1914); J. M. Coulter and C. J. Chamberlain, Mor- phology of Gymnosperms (Chicago, 1910) ; E. C. Jeffrey, The Anatomy of Woody Plants (Chicago, 1917); D. H. Scott, Studies in Fossil Botany, ed. 3 (London, 1920); A. C. Seward, Fossil Plants, vol. in. (Cambridge, 1917); M. C. Stopes, Catalogue of the Cretaceous Plants in the British Museum, vol. ii. (London, 1915); M. C. Stopes and R. V. Wheeler, The Constitution of Coal (London, 1918); G. R. Wie- land, American Fossil Cycads, vol. ii. (Washington, 1916). (M. C. S.)
X. Cytology. Great advances have been made in the study of cytology, but considerable divergence of opinion still exists with regard to many details of nuclear phenomena (see also the separate article CYTOLOGY).
The discrepancies and contradictions present in the accounts of mitosis are due to the fact that no animal or plant has been found in which all of the phases connected with nuclear division can be elucidated. Each form has individual cytological char- acters, and its nuclei may show some stages with exceeding clearness, whilst others may be disguised or apparently even eliminated. The true version will only be attained by wider com- parative investigations.
(1) Somatic divisions (fig. i). Most cytologists agree that, as a rule, during telophase each somatic chromosome splits into longi- tudinal halves (threads), and these halves tend to separate, forming more or less of a reticulum, according to the degree of interkinetal rest assumed by the nucleus. The prophase stages are interpreted in two ways : (a) that the pairing of threads in the prophase is the reassociation of the chromosome halves which separated during the preceding telophase, that these gradually come together to form the univalent chromosome, and separate as daughter chromosomes on the ensuing spindle; (V) that the pairing of threads and the splitting of the univalent chromosomes into daughter halves are purely prophasic phenomena and bear no relation to the splitting of the chromosomes in telophase. (Fraser and Snell 1911, Miiller 1921, Gr<5goire 1912, 1913, Sharp 1913, Digby 1919.)
(2) First Meiotic division. Controversy rages over the mode of
