120 ELEMENTS OF BIOLOGY. 



and take no part in reproduction. That they do not strike 

 us in the same light as do the " polypites " of the Hydroid 

 colony arises mereiy from the fact that they are devoid of 

 the animal "functions of relation." In reality, however, 

 they lead a life which is just as independent of the whole, 

 whilst the life of the latter is in no way commensurate with 

 the existence of the leaves. 



Similarly, the tree ordinarily consists simply of a collec- 

 tion of leaves, or nutritive factors, which have no power of 

 producing the sexual elements. At certain times, however, 

 the tree produces special buds the flowers in which the 

 generative elements are produced, and by the agency of 

 which the perpetuation of the species is insured. 



We may, then, regard ordinary plants as colonies consist- 

 ing theoretically of a "trophosome" and "gonosome," each 

 of which is made up of an indefinite number of zooids. The 

 zooids of the trophosome or leaves are all like one 

 another, and are devoted to the nutrition of the colony. 

 The zooids of the "gonosome" or flowers are also 

 usually all alike, but do not resemble the leaves, though the 

 two can be shown to be morphologically identical. They 

 take no part in the nutrition of the colony, but are simply 

 devoted to the production of new individuals. The inter- 

 esting and important point about this comparison is the 

 clearness with which it brings out the fact that gemmation 

 and fission are merely to be regarded as forms of growth. 

 No one thinks of looking upon the leaves or flowers of a 

 tree as independent or separate beings ; and yet in reality 

 they have just as much claim to this title as have the zooids 

 of the Hydroid colony. On the contrary, every one recog- 

 nises that a tree is the result of a process of growth ; and 

 every one would equally recognise that this is the case with 

 the Hydroids, if the polypites of the latter were endowed with 

 as little power of spontaneous motion and as little sensation 

 as the leaves of a plant. 



In plants, as in animals, the only genuine form of .repro- 



