IO6 ELEMENTS OF BIOLOGY. 



of budding, remain connected to one another, and have 

 no power of producing the essential elements of reproduc- 

 tion. Further, each zooid has to contribute to the nourish- 

 ment of the colony as a whole, at the same time that its life 

 is, to a limited extent, independent of that of the other 

 members of the growth. Lastly, the life of the colony is in 

 no way dependent upon the life of its individual factors, but 

 the polypites may be destroyed or may die, and the general 

 stem may yet retain its vitality, and may recommence the 

 process of budding. Similarly, the leaves of a tree are pro- 

 duced by a process of continuous gemmation, remain per- 

 manently connected, and have no power of sexual repro- 

 duction. They are nutritive factors of a common growth, 

 to the maintenance and development of which they minis- 

 ter ; and the existence of the tree is in no way limited by 

 the life of any individual leaf. 



This comparison, however, may be carried still further 

 without breaking down. The ordinary leaf-buds of the 

 tree are in no way connected with reproduction ; and 

 whilst the tree may increase considerably, as an individual, 

 by the constant formation of fresh buds, it has no power 

 of perpetuating its species so long as it merely produces 

 leaves. At certain periods, however, the tree produces 

 special buds or flowers, in which are developed the essen- 

 tial elements of reproduction, by the union of which a seed 

 is produced, from which, under suitable conditions, a young 

 tree will spring. Not only is this the case, but we have the 

 remarkable fact that the flowers or reproductive buds of the 

 tree are morphologically identical with the leaf-buds or nutri- 

 tive buds ; whilst the difference of function causes such a 

 difference of structure that the morphological unity of the 

 two can only with some difficulty be recognised. Similarly, 

 in Hydractinia, the ordinary zooids of the colony have no 

 reproductive organs ; and though there is theoretically no 

 limit to the size which the organism may reach by gemma- 

 tion, its buds are not detached, and the species would die 



