The sacrifice of mobility consequent on a fixed position is seen in all but the simplest plants : it accords well with the require ments of a self-nourishing green organism, but it imposes limits upon certain of the essential functions of competitive life. The nourishment of the plant must be brought to it by external agen cies, such as diffusion, and absorbed from the medium, molecule by molecule. A raptorial habit, as in animals, is then unnecessary, and is even precluded by the fixed habit. On the other hand, the propagative cells must either themselves be motile or must be easily spread by external forces : otherwise any extension of field would be impossible. Throughout descent these two facts have dominated the morphological evolution of plants.
the larger the size of the organism and the more complicated its form, other things being equal, the better the nourishment of the individual, and the greater will be its propagative capacity. In simple organ isms increase in size may be general throughout the plant-body. But in all larger plants growth is localized. Sometimes it is inter calary at some point between apex and base ; but usually it is distal or apical. Where tissue-formation is thus localized the cells involved in it retain their youth as embryonic cells : this is espe cially marked at the distal tip, which has thus the character of a growing point, or punctum vegetationis. Here not only is pro vision made for continued growth of a central axis, but also for the initiation of successive branchings, which often mature as lateral appendages upon it. The result in the higher plants is the leafy shoot. These elaborations of the vegetative system may be seen illustrated either in the successive stages of an individual life, or by comparison of distinct organisms, such as a seaweed, a liverwort, a fern or a flowering plant. With varying detail these all share that distinctive feature of the more elaborated plant body, its continued embryology. In all the higher animals the embryonic phase is a transient step towards full development, resulting in limbs definite in number and position : but plants have the power of forming an indefinite number of parts, in a succes sion theoretically without term or limit. Every bud of a forest tree or of a herb contains its own growing point deeply embedded among the successive leaves it has produced, and holds the poten tiality of unlimited further development. This continued embry
ology is the leading feature in the organization of the enlarging plant-body.
The vegetative system of the more highly organized plants consists of parts which have been classed as stem, leaf, root, emergence and hair. So long as the more highly organized plants alone are studied these categories appear fairly distinct. But when such comparison is applied to include forms lower in the scale, the less complete differentiation of their parts raises many questions touching the evolutionary origin of the several categories themselves. Here the fossil evidence takes its place : for conclusions based upon structure, position, individual development or comparison are liable to be checked by the dis covery of some ancient form that may raise a doubt, or present even a firm negative. Thus the conception of a leaf as a lateral appendage upon a stem that is fixed at its base by roots, involves three categories of parts well marked in the higher vascular plants. How does this accord with the existence of the Devonian fossil Hornea, which is a vascular plant without leaves or roots? It is clear that those categories are not absolute, but are results of evolutionary history still incomplete in Hornea.
Many difficult questions as to the category of parts are apt to arise when we attempt their classification in plants at large. Mere comparison of external form or even of internal structure will not suffice : decisions must be based upon the origin and the place which the parts take relatively to other parts at the time when they first appear. Following this method, those parts of the individual, or of different individuals, species or genera, have been distinguished as homologous which bear the same relation to the whole plant-body, whatever their form or function or external conditions may be. On the other hand, parts may resemble one another in form and in function, though they may differ in their relation to the whole plant-body. Such parts have been de scribed as analogous one with another. For instance, the flat green organs of the butcher's broom are by origin axillary buds, not leaves. Though their main function is like that of leaves, nutritional, they are only analogous to leaves.