Page:EB1911 - Volume 21.djvu/805

Rh and various organs of plants are, for the most part, different forms of a small number of members of the body, which have been distinguished as follows, without any reference to function. The thallus (thallome) is a plant-body which is not differentiated into the members root, stem and leaf; it is the morphologically simplest body, such as is of common occurrence in the lower plants (e.g. Thallophyta). In a differentiated body the stem (caulome) is an axis capable of bearing leaves and (directly or indirectly) the proper reproductive organs. The leaf (phyllome) is an appendicular member only borne by a stem, but differing from it more or less obviously in form and development, though co-ordinate with it in complexity of structure. The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant. The hair (trichome) is a superficial appendage of simple structure, which may be borne by any of the other members. The emergence is also an appendicular member of more complex structure than the hair (e.g. the prickles of the rose). Further, it has been found convenient to designate the leaf-bearing stem as a whole by the term shoot, so that the body may, as Sachs suggested, be primarily analysed into shoot and root.

At the present time some objection is being taken to this purely morphological conception of the body and its parts as being too abstract. It is urged that the various parts are, as a matter of fact, organs; and that it is therefore inadmissible to ignore their functions, as is done in the foregoing definitions. To this it may be replied that pure morphology and organography are not alternatives, but are two complementary and equally necessary modes of considering the composition of the plant-body. Moreover, the abstract terms “stem,” “leaf,” “root,” &c., are absolutely indispensable; and are continually used in this sense by the most ardent organographers. It has not yet been suggested that they should be replaced by organographical terms; were this accomplished, descriptive botany would become impossible.

It is also urged against these definitions that they are not of universal applicability; that there are exceptional structures which cannot be brought within the limits of any one of them. But admitting the validity of this criticism, and even going so far as to question the possibility of ever devising absolutely inclusive and, at the same time, exclusive definitions, no sufficient reason is adduced for giving up all attempt at morphological analysis.

Homology.—All members belonging to the same morphological category are said to be homologous, however diverse their functions. Thus, in a phanerogam, the sepals, petals, stamens and foliage-leaves all come under the category leaf, though some are parts of the perianth, others are spore-bearing organs (sporophylls), and others carry on nutritive processes. The homology of members was based, in the first instance, upon similarity of development and upon similar relations to the other parts of the body, that is, upon ontogeny. But since the general adoption of the theory of evolution, similarity of descent, that is of phylogeny, has come to form an essential part of this conception; in other words, in order that their homology may be established the parts compared must be proved to be homogenetic.

The introduction of the phylogenetic factor has very much increased the difficulty of determining homologies; for the data necessary for tracing phylogeny can only be obtained by the study of a series of allied, presumably ancestral, forms. One of the chief difficulties met with in this line of research, which is one of the more striking developments of modern morphology, is that of distinguishing between organs which are “reduced,” and those which are really “primitive.” The object of the phylogenetic study of any organ is to trace it back to its primitive form. But, as will be pointed out later, organs are often found to have undergone “degeneration” or “reduction,” and such reduced or degenerate structures may easily be mistaken for primitive structures, and so the investigator may be misled.

The effect of the phylogenetic factor in homology may be illustrated in the following cases. The leaves of the true mosses

and those of the club-mosses (Lycopodium, Selaginella) being somewhat alike in general appearance and in ontogeny, might be, and indeed have been, regarded as homologous on that ground. However, they belong respectively to two different forms in the life-history of the plants; the leaves of the mosses are borne by the gametophyte, those of the club-mosses by the sporophyte. In accordance with the prevalent antithetic view of the alternation of generations in these plants (see ), the forms distinguished as sporophyte and gametophyte are not homogenetic; consequently their leaves are not homologous, but are only functionally similar (homoplastic; see infra).

Another effect is that different degrees of homology have to be recognized, just as there are different degrees of relationship or affinity between individual plants. When two organs can be traced along the same line of descent to one primitive form, that is when they are found to be monophyletic, their homology is complete; when, however, they are traceable to two primitive forms, though these forms belong to the same morphological series, they are polyphyletic and therefore only incompletely homologous. For instance, all the leaves of the Bryophyta are generally homologous inasmuch as they are all developments of the gametophyte. But there is reason to believe that they have been differentiated quite independently in various groups, such as the Marchantiaceae, the Jungermanniaceae, and the mosses proper; consequently their phylogeny is not the same, they are polyphyletic, and therefore they are not completely homologous, but are parallel developments.

Analogy.—Considering the parts of the body in relation to their functions, that is as organs, they are found to present peculiarities of form and structure which are correlated with the functions that they have to discharge; in other words, the organ shows adaptation to its functions. All organs performing the same function and showing similar adaptations are said to be analogous or homoplastic, whatever their morphological nature may be; hence organs are sometimes both homologous and analogous, sometimes only analogous. The tendrils of a vetch and of a cucumber are analogous, and also homologous because they both belong to the category leaf; but they are only analogous to the tendrils of the vine and of the passion-flower, which belong to the category stem.

Metamorphosis.—It has already been pointed out that each kind of member of the body may present a variety of forms. For example, a stem may be a tree-trunk, or a twining stem, or a tendril, or a thorn, or a creeping rhizome, or a tuber; a leaf may be a green foliage-leaf, or a scale protecting a bud, or a tendril, or a pitcher, or a floral leaf, either sepal, petal, stamen or carpel (sporophyll); a root may be a fibrous root, or a swollen tap-root like that of the beet or the turnip. All these various forms are organs discharging some special function, and are examples of what Wolff called “modification,” and Goethe “metamorphosis.” It may be inquired what meaning is to be attached to these expressions, and what are the conditions and the nature of the changes assumed by them. The leaf of the higher plants will be taken as the illustrative case because it is the most “plastic” of the members, the one, that is, which presents the greatest variety of adaptations, and because it has been most thoroughly studied.

In this, as in all morphological inquiries, two lines of investigation have to be followed, the phylogenetic and the onto genetic. Beginning with its phylogeny, it appears, so far as present knowledge goes, that the differentiation of the shoot of the sporophyte into stem and leaf first occurred in the Pteridophyta; and, in accordance with the views of Bower (Origin of a Land-Flora), the primitive leaf was a reproductive leaf, a sporophyll, from which the foliage-leaf was derived by progressive sterilization. From the nature of the case, this view is not, and could not be, based upon actual observation, nor is it universally accepted; however, it seems to correspond more closely than any other to the facts of comparative morphology. It was formerly assumed, and the view is still held, that the foliage-leaf was the primitive form from which all others were derived, mainly on