126 SCIENCE PROGRESS. 



that the separate individuals may differ amongst themselves 

 both in form and in function, just as in a mass of roots the 

 lateral fibres differ to some extent from the positively 

 geotropic primary root ; or again, in a shoot, such as that of 

 Equisetum, the lateral branches which perform the task of 

 assimilation do not precisely resemble the axis from which 

 they spring. 



It is true that division of labour does not necessarily 

 involve metamorphosis, but it may certainly be regarded as 

 a condition of its manifestation. Hence, not morphology 

 but physiological requirement is the main factor, teleologi- 

 cally speaking, in effecting structural differentiation, and the 

 particular organ which will undergo the modification will be 

 that which is most susceptible to a change in the desirable 

 direction. This consideration serves to account for the 

 apparently capricious character which modifications some- 

 times exhibit. Why should the leaf stalk, for example, 

 become flattened out and replace the blade in Oxalis 

 bupleuriioha ? Assuredly because trie latter structure was 

 incapable of fulfilling the conditions imposed by the environ- 

 ment. An analogous case is that of Genista sagittalis, a 

 plant which inhabits arid districts, and thus requires to limit 

 the area of its transpiring leaf surface. On the other hand, 

 however, its stem has become furnished with broad, leathery 

 wings, and these serve to increase the assimilating surface. 

 It is obvious that in this case also the leaves were unable 

 to satisfy the requirements of the plant's surroundings, 

 and hence the wino^s arose as their substitutes. And the 

 experience of plant cultivation still more clearly demonstrates 

 the conclusion that the various organs of different plants 

 may exhibit the power of adaptation in any given direction 

 in widely differing degree. 



K. Goebel. 



