HOMOLOGY OF TENDRILS 185 



When seed of this Acacia is sown, the young plant 

 always has after its cotyledons, small but normal 

 bipinnate leaves. Gradually as the seedling grows the 

 leaves that arise have in succession, smaller pinnae 

 and broader petioles, till the usual flat ' leaf ' appears, 

 after that no more pinnate leaves are formed. The 

 seeds germinate so readily that some should be sown 

 and this change noted. It is an instance of a pheno- 

 menon common enough among animals, of an indivi- 

 dual going through in its own life the same sort 

 of changes as have occurred in the evolution of the 

 species. 



Homology of tendrils 



In chapter x, section 4, we learnt that many 

 plants climb, fastening on to other sturdier trees and 

 shrubs by means of thin sensitive organs called tendrils, 

 and it was said that these tendrils are usually to be 

 considered as modified forms of ordinary organs such 

 as branches or leaves. 



Now examine the tendrils and flowering parts of the 

 ordinary ANTIGONON. The tendril is three-pronged at 

 the top. On the lower parts of the plant it arises like 

 a branch in the axil of a leaf, in the upper parts, it is 

 obviously a continuation of the axis of an inflorescence 

 (i.e. of an ordinary branch), and is homologous, there- 

 fore, with the end of a branch, or inflorescence-axis. 



In the case of the PASSIFLORA, the Passion-flower, 

 .and many other plants, the whole of the axillary branch 

 is modified as a tendril. In VITIS the Vine, etc., some 

 of the tendrils stand opposite to the leaves, and in the 

 axil of those leaves there is no bud. Hence the tendril 



