elaboration of mechanism, orientations, and anatomy as "leaves";
attachment-ramuli exaggerate their absorbing function as they
penetrate massed decaying material, now for the first time asso-
ciated with minute heterotrophic organisms as bacteria, constituting
the first soil. Internal and effete cell-units storing waste polysac-
charide of photosynthesis are utilized as mechanically supporting
fibres; others from a water-storing function attain a conductive
significance as tracheides; intercellular spaces are elaborated in
connexion with a transpiration mechanism which now becomes the
only means of obtaining food-ions of inorganic nature. Most re-
markably and constantly the asexual spore-tetrads, following meiosis
in an asexual generation (as in Diclyola and Florideae) are utilized
as air-borne spores; while the sexual gametes retain their older
plankton mechanism of sexual fusion in an aqueous medium, so
far as this may be available From such beginnings arise the Bryo-
phyta (Mosses) in which fertilization in situ is associated with the
more or less complete parasitic decadence of the spore-producing
generation, and the Pteridophyta (Ferns), in which great perfection
of the free asexual land-plant is associated with a sexual phase re-
duced to a mere protonema stage with precociously effective sexual
organs, correlated with a minimum period in which the necessary
water may be available; and sori of tetraspores are adapted to a
sporangial mechanism'which will dehisce in the air.
No plant-phylum which had not previously attained to a two- phase cycle has made good on the land ; since following the attain- ment of fertilization in situ the asexual spores of the complementary generation or " person " were required for a dispersal function. Higher types of land-vegetation follow the Pteridophyte progression, passing on to the evolution of the seed-habit as Spermatophyta; in so doing expressing the successful method of evading problems of the utilization of free external water for the plankton process of fertilization. Much residual algal life of simple category persists as heterotrophic races of fungi, in which the problem of aquatic cross-fertilization is largely solved by eliminating it altogether, or retaining mechanism in the merest vestigial expressions. Sugges- tions as to the time involved in such evolutionary progression have been emphasized by data for the decay of radio-active minerals, as affording a time-chart by means of which geological epochs may be approximately estimated. The datum of 300 million years for the Carboniferous and Devonian, in which forest-trees of coniferous habit are known to have existed, as also the Rhynia group of the Lower Devonian which may express extreme types of Pteridophyta or limiting cases of Bryophyta, appears but of small value in the evolution of such high-grade land organisms as timber-trees from mere marine algae. A general estimate of 2,000 million years for the first stages of transmigration may not appear excessive; and behind this stretches the indefinite range of the evolution of the algal series, to the more remote epochs of the plankton-phase of the evolution of the cell in all its manifold possibilities and controlling functions, from the material of sea water alone. Yet in these re- spects there can be little doubt that the autotrophic plant, as the sole response of what is termed life to the biological factors of ancient seas, is more likely to be a sure guide to the history of the more modern world than any biologically unsupported and equally fragmentary testimony of the rocks. From the standpoint of con- ventional views of " descent " the story of evolution now becomes the history of biological and physiological progression to higher horizons as determined by changes in the condition of the external environment, of which residual plant-groups, each as absolutely cut off behind as non-progressive in other respects, remain as " Land- marks of Limitation " to point the way the progression has passed, as wholly isolated genetically as if the independent creations of an older philosophy, yet all meeting in the phase of the common initial medium of the sea.
See Bower, Origin of a Land Flora (1908) ; Doflein, Protozoenkunde (1916); Pascher, Archiv fur Prolistenkunde 36 (1917); Kidston and Lang, "Rhynia," " Hornea," Trans. Roy. Soc. Edin. (1920); Church, " Building of an Autotrophic Flagellate," " Thlassiophyta and the Subaerial Transmigration," Oxford Botanical Memoirs, I (1919); "Somatic Organization of the Phaeophyceae," ibid. JO, (1920); Arber, Devonian Floras (1921). (A. H. CH.)
IX. Anatomy and Palaeobotany. Progress in anatomical, and in palaeontological Botany essentially go hand in hand. The discoveries of well-petrified new forms of fossil plants, which are often difficult of identification, lead to more critical examina- tion of the structure of recent plants, and thus bring to light interesting features in the latter. Yet both the methods pursued and the type of material available for the botanist and palaeo- botanist tend to differ. Knowledge of the anatomy of the fossil Angiosperms, for instance, has been naturally restricted owing to the scarcity of material other than of Tertiary age; while the isolated fragments of Tertiary wood have not attracted any particular attention in recent years, largely owing to the diffi- culty of mastering the overwhelming mass of living species with which they have to be compared. The origin and phylogenetic
source of the Angiospermic families is thus still wrapt in mystery in spite of various theoretical conceptions. The only secure fact is that in geological time corresponding to the Wealden in Great Britain, and approximately to the Neocomian of the world, no reliable material of Angiosperms of any sort has yet been discovered. Claims to have identified Angiosperms in these or earlier rocks are not substantiated, and originated from such errors as incorrect diagnosis of ferns possessing reticulated venation like Dictyophyllum (see exposures by Berry, 1911, and Slopes, 1915).
The earliest authentic Angiosperms are found in the Lower Green- sand or Aptian of Great Britain. The flora of this epoch was mark- edly distinct from that of the Wealden, which is of the Jurassic type; that of the Lower Greensand, on the contrary, was a rich, mixed flora, including many varieties of coniferous woods, the famous Bennettites Gibsonianus described by Carruthers, and other Ben- ncttitalian plants, and, in particular, several Angiospermic woods. The systematic position of these Angiosperms is scarcely delermin- able, owing to the fact that modern plant anatomists have not yet codified the significance of woody structures in the living genera in spite of the extensive beginning made by Moll and Jannsonius. The petrified features of the ancient genera Cantia, Woburnia, Sabulia, Aptiana and Hythia are in no way " primitive " or pseudo-Angio- spermic, but exhibit typical features of highly organized Angio- spermic timbers. Hence the origin of the Angiosperms remains obscure, and a problem to be solved only by the discovery of the anatomical features of Angiosperms of an even earlier age.
The American school headed by Jeffrey, although contributing little to the description of new fossil Angiosperms, has worked on the problem of their descent on the basis of a series of well-defined theories. Jeffrey's main thesis is that the herbaceous forms are less primitive than the woody, and " the degenerate herb is derived from ancestral forms characterized by woody stems." While Jeffrey's conclusions and deductions are not universally accepted, workers of his school have contributed handsomely to the apcumula- tion of data from living forms, and his text-book of anatomy at- tempts to bring out guiding principles, chief among which is the " Doctrine of Conservative Organs," springing from Scott's ob- servations on the Cycadales, and the " Doctrine of Reversion." In Britain no comprehensive theoretical work on general anatomy has appeared recently. The anatomy of seedlings has been pursued (Hill and Thomas), but, unfortunately, has no corresponding de- velopment in palaeontological works owing to their tenuous rarity.
From rocks of Palaeozoic age onwards, well-petrified Gymno- sperms are constantly being discovered, and the study of their structures has necessitated the reexamination of all the modern genera. A steady output of memoirs dealing with the anatomy of living and fossil gymnosperms has been maintained (see in particular the works of Seward, Groom, Slopes, Gothan, Thomson, Hollick and Jeffrey, Coulter and Chamberlain, and others). While the English school have in the main added wherever possible new data on the recognized accepted lines of the generic grouping, Americans under Jeffrey have actively maintained the heterodox view that the Araucarincae are less primitive than the Abietineae, basing most of their generalizations on the minutia of tracheid structure, which appeared clearer and more dogmatic guides so long as com- paratively little was known of the infinite variety of the Mesozoic forms, but which have become self-contradictory as generic or even specific diagnostic features when such a wealth of material as is now available has been examined.
The primitive Palaeozoic gymnosperms are gradually becoming very well known from the relative frequency with which their stems, leaves and other parts are found petrified. The most notable recent addition to the.group is the exceptional little flora of plants from the very base of the Carboniferous of Kentucky (see Scott and Jeffrey) which bears considerable likeness to the primitive Saalfeld flora described long since by Unger.
Anatomical work on the large group of the Bennettitales (which became entirely extinct before the Tertiary period) has yielded re- sults of morphological and phylogenetic importance, and enriched general concepts of fructification and seed structure. Such work, initiated in 1870 by Carruthers, with his acute determination of the type fossil, remained for long in abeyance owing to the dearth of material in Britain, but was continued on the Continent in the last decade by Lignier and recently in England by Slopes. Studies on this group were actively pursued in America by Wieland, the main results of which are collected in his Iwo magnificenl volumes Amer- ican Fossil Cycads, from which ihe diagrammalic resloralion of the peculiar fructifications and many interesting points of vegetative anatomy have proved a mine of information for theorizcrs. The view lhat the Bennettitales were ancestral angiosperms was, tem- porarily at any rate, held by a number of leading botanists and received definite expression by Arber and Parkin. This view, how- ever, is not built on a sufficiently secure anatomical foundation. The vascular peculiarities and vegetative appearance of the Ben- nellilales are so closely allied to those of Cycads still living that their complex gymnospermic fructifications are best looked upon as no
