2 PART I. THE MORPHOLOGY OF PLANTS. 



functions of the various members of the body, except in so far 

 as the form or other morphological characters of any member may 

 be affected by special adaptation to the performance of some 

 particular function. 



For instance, foliage-leaves and sporophylls are specially homologous ; but 

 their special homology is incomplete on account of the differences in form, due 

 to special adaptation to their functions, which they respectively present. In 

 this case essentially similar members come to differ widely : in other cases, 

 essentially dissimilar members come to resemble each other. For instance, 

 tendrils are climbing-organs, and are all much alike ; but they may be of very 

 different morphological value ; either modified branches, or leaves, or parts of 

 leaves. Tendrils, therefore, are not all homologous ; but they are all analogous, 

 that is, whatever their morphological value may be, they are modified in the 

 same direction for the performance of the same function. Similarly the 

 flattened stem-segments of Opuntia, and the phylloclades of Euscus, are the 

 analogues of foliage-leaves ; and again, the subterranean shoots of Psilotum, 

 and the submerged leaves of Salvinia, being absorbent organs, are the analo- 

 gues of roots and of each other. 



From the purely morphological standpoint of development and 

 mutual relation, the various parts of a plant are regarded as 

 members of the body ; whilst from the physiological standpoint 

 of function, they are considered as organs of the body, each being 

 specially adapted for the performance of some particular kind of 

 work. The more clearly the members of the body are marked off 

 from each other, and the greater the variety of them, the higher 

 is the degree of morphological differentiation which the body pre- 

 sents ; similarly, the more special the adaptation of the organs, 

 and the greater the variety of them, the more complete is the 

 physiological differentiation, or the division of the physiological 

 labour, of the body. 



A remarkable fact in the general morphology of plants, is that 

 they are more or less polymorphic : that is to say, a plant assumes, 

 as a rule, at least two different forms in the course of its life- 

 history. Most commonly it presents but two forms which, while 

 they may differ more or less widely in form and structure, differ 

 essentially in that the one, termed the sporophyte, bears asexual 

 reproductive organs which produce certain asexual reproductive 

 cells, termed spores, each of which is capable by itself of giving 

 rise to a new organism ; whilst the other, the gametophyte, bears 

 sexual reproductive organs, which, as a rule, produce sexual repro- 

 ductive cells, termed gametes, which, though each is by itself in- 

 capable of giving rise to a new organism, yet by their fusion in 



