280 OUTLINES OF EVOLUTIONARY BIOLOGY 



life-history of some of the Australian acacias, or wattles. These 

 are leguminous plants and many of them have the pinnately 

 subdivided leaves so characteristic of the order. Others, however, 

 have the pinnate leaves in the adult entirely replaced by narrow 

 " phyllodes," which closely resemble simple undivided leaves 

 like those of some of the eucalypts, but are really only flattened 

 leaf-stalks or petioles. The development of such phyllodes is no 

 doubt to be regarded as an adaptation to the heat arid drought of 



the Australian climate, 

 which delicately sub- 

 divided leaves of the 

 ordinary leguminous 

 type are ill-suited to 

 withstand, and the re- 

 semblance of the phyl- 

 lodes to true foliage 

 leaves of the undivided 

 type and especially to 

 those of the eucalypts 

 affords a very good 

 illustration of the 

 phenomenon of conver- 

 gence. If, now, we 

 examine the seedlings 

 of a phyllode-bearing 

 acacia (Fig. 132) we 

 shall find that imme- 

 diately after the coty- 

 ledons or seed - leaves 

 they bear typical pin- 

 nate leaves, and then 

 leaves of intermediate form, thus clearly recapitulating the 

 ancestral condition from which the phyllode-bearing forms were 

 derived. A very similar state of things occurs in the common 

 European gorse or furze, another leguminous plant, in which 

 the leaves of the adult have become modified to form spines, 

 while those of the seedling still retain the ancestral ternate 

 condition, somewhat resembling those of the clover. 



The cotyledons or true seed-leaves of flowering plants, on 

 the other hand, afford excellent examples of caenogenetic characters 

 developed in relation to the special requirements of the embryo. 



FlG. 132. Seedling of a phyllode-bearing 

 Acacia (Acacia pycnantha). (From Stras- 

 burger.) (The cotyledons have already 

 been shed.) 



