progress in our knowledge of distinctively morphological anatomy, i.e. the study of tissues from the standpoint of evolution. The Russian plant-anatomist, Russow, may be said to have founded the consideration of plant tissues from the point of view of descent (Vergleichende Untersuchungen über die Leitbundelkryptogamen, St Petersburg, 1872; and Betrachtungen über Leitbundel und Grundgewebe, Dorpat, 1875). He was ably followed by Strasburger (Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pflanzen, Jena, 1891), Haberlandt and others. The explicit adoption of this point of view has had the effect of clearing up and rendering definite the older morphological doctrines, which for the most part had no fixed criterion by which they could be tested.
Since about 1895 this branch has been most actively pursued in England, where the work of Boodle and of Gwynne-Vaughan (especially on Ferns) has been the most important, leading to a coherent theory of the evolution of the vascular system in these plants (Tansley, Evolution of the Filicinean Vascular System, Cambridge, 1908); and in America, where Jeffrey has published important papers on the morphology of the vascular tissues of the various groups of Pteridophytes and Phanerogams and has sought to express his conclusions in a general morphological theory with appropriate terminology. As a result of this activity Van Tieghem's so-called “Stelar theory” has been revised and modified in the light of more extended and detailed anatomical and developmental knowledge. Schoute's Die Stelar-Theorie (Groningen, 1902), gives an important critical account of this subject.
Fourthly, attention must be called to the great development of what is called “Systematic Anatomy,” i.e. the study of the anatomical features characteristic of the smaller groups of flowering plants, i.e. the orders, families, genera and species. Radlkofer (1883) was the first to call attention to the great importance of this method in systematic botany, as providing fresh characters on which to base a natural classification. Solereder's great work, Systematische Anatomie der Dicotyledonen (Stuttgart, 1898-1908; Eng. trans., Systematic Anatomy of Dicotyledons, Oxford, 1998), brings together so many of the facts as are at present known in an orderly arrangement. Theoretically this branch of the subject should connect with and form the completion of “morphological anatomy,” but the field has not yet been sufficiently explored to allow of the necessary synthesis. The true relation of “systematic” to “ecological” anatomy (see below) also awaits proper elucidation.
Fifthly, we have to record the foundation of the modern study of “physiological anatomy” (i.e. the study of the specific functions of the various tissues) by Schwendener (Das mechanische Princip im Bau der Monocotylen, 1874, and other works), followed by numerous pupils and others, among whom Haberlandt (Physiologische Pflanzen-Anatomie, Leipzig, 1st ed., 1884, 4th ed., 1909, and other works) is pre-eminent. The pursuit of this study has not only thrown valuable light on the economy of the plant as a whole, but forms an indispensable condition of the advance of morphological anatomy. A great deal of work still remains to be done in this department, which at the present time affords one of the most promising fields of anatomical investigation.
Finally we may mention “ecological anatomy,” i.e. the study of anatomical features directly related to the habitat. A very considerable body of knowledge relating to this subject already exists, but further work on experimental lines is urgently required to enable us to understand the actual economy of plants growing under different conditions of life and the true relation of the hereditary anatomical characters which form the subject matter of “systematic anatomy” to those which vary according to the conditions in which the individual plant is placed. On these lines the future of anatomical study presents almost inexhaustible possibilities. (A. G. T.)
Physiology of Plants
The so-called vegetable physiology of a generation ago was in arrear of animal, and particularly of human, physiology, the study of the latter being followed by many more observers, and from its relative degree of advancement being the more capable of rapid development. It was fully recognized by its followers that the dominating influence in the structure and working of the body was the protoplasm, and the division of labour which it exhibited, with the accompanying or resulting differentiation into various tissues, was the special subject of investigation. Many who followed the study of vegetable structure did not at that time give an equal prominence to this view. The early histological researches of botanists led them to the recognition of the vegetable cell, and the leading writers in the middle of the 19th century pointed out the probable identity of Von Mohl's “protoplasm” with the “sarcode” of zoologists. They laid great stress on the nitrogenous nature of protoplasm, and noted that it preceded the formation of the cell-membrane. But by the ordinary student of thirty years later their work was to some extent overlooked, and the cell-wall assumed a prominence to which it was not entitled. The study of the differentiation of protoplasm was at that time seldom undertaken, and no particular attention was paid either to fixing it, to enable staining methods to be accurately applied to it, or to studying the action of chemical reagents upon it. It is only comparatively recently that the methods of histological investigation used by animal physiologists have been carefully and systematically applied to the study of the vegetable organisms. They have, however, been attended with wonderful results, and have revolutionized the whole study of vegetable structure. They have emphasized the statements of Von Mohl, Cohn, and other writers alluded to, that the protoplasm is here also the dominant factor of the body, and that all the peculiarities of the cell-wall can only be interpreted in the light of the needs of the living substance.
The Nature of the Organization of the Plant, and the Relations of the Cell-Membrane and the Protoplasm.—This view of the structure of the plant and this method of investigation lead us to a greatly modified conception of its organization, and afford more completely an explanation of the peculiarities of form found in the vegetable kingdom.
The study of simple organisms, many of which consist of nothing but a little mass of protoplasm, exhibiting a very rudimentary degree of differentiation, so far as our methods enable us to determine any at all, shows that the duties of existence can be discharged in the absence of any cell-wall. Those organisms which possess the latter are a little higher in the scale of life than those which remain unclothed by it, but a comparison of the behaviour of the two quickly enables us to say that the membrane is of but secondary importance, and that for those which possess it, it is nothing more than a protective covering for the living substance. Its physical properties, permeability by water, extensibility and elasticity, receive their interpretation in the needs of the latter. We come, accordingly, to regard it as practically an exoskeleton, and its functions as distinctly subordinate to those of the protoplasm which it clothes. If we pass a little higher up the scale of life we meet with forms consisting of two or more cells, each of which contains a similar minute mass of living substance. A study of them shows that each is practically independent of the others; in fact, the connexion between them is so slight that they can separate and each become free without the slightest disadvantage to another. So long as they are connected together mechanically they have apparently the power of influencing one another in various ways, and of passing liquid or gaseous materials from one to another. The conjoined organism is, in fact, a colony or association of the protoplasmic units, though each unit retains its independence. When we pass, again, from these to examine more bulky, and consequently more complex, plants, we find that the differences which can be observed between them and the simple lowly forms are capable of being referred to the increased number of the protoplasmic units and the consequent enlarged bulk of the mass or colony. Every plant is thus found to be composed of a number of these protoplasmic units, or, as they may preferably be termed,
