150 INTERPOLATION OF A CONTINUOUS 



At first a general similarity prevails in the successive 

 shoots, amounting in most cases almost to identity — the 

 instances of diversity of leaves on the same tree being quite 

 exceptional — but in by far the majority of plants, the 

 floAver-shoots, on which alone reproductive organs are pro- 

 duced, differ widely from the leaf-shoots, though the argu- 

 ments on which the science of Vegetable Morphology is 

 based prove clearly an essential community of nature be- 

 tween them. In all normal development these floral leaves 

 or bracts immediately precede the evolution of the repro- 

 ductive organs, but the number of successive leaf-buds of 

 the common form seems to be to some degree dependent on 

 external conditions — many plants vegetating vigorously in 

 situations where they never flow^er. 



All that has now been said of such gemmation in plants 

 wdll apply also to zoophytes among animals, if w^e merely 

 substitute " pol}^e-bud" for " leaf-bud," and " ovigerous 

 capsule" for '' flower-bud." 



That the different leaf-shoots in the plant, and the polype 

 shoots in the zoophyte, are so far distinct individuals that 

 they possess a certain independent life of their own, is now 

 so generally admitted, that it is needless to spend time in 

 adducing arguments in its support — the well-kno^vn practice 

 of propagating plants by cuttings must occur to every one. 

 The majority of plants, indeed, are equally entitled with 

 zoophytes to be termed compound organisms.* 



§ 4. From the gemmae in such cases so frequently re- 

 maining attached to each other and to the parent stock, it is 

 proposed — for want of a better expression — to distinguish 



* It lias been already sliown that the conipoTind stnieture does not 

 necessarily imply the derivation of its component units from each other 

 by gemmation, for it may also depend on their aggregation by a common 

 derivation from the same stock, as in the Catenated Sal/pee, the pile of 

 Medusae, derived from a " Hydra Tuba," or the caudal zooids of some 

 Cestoidea. See again in Ch. IX. 



