166 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



The juvenile stages often show relationships different in character 

 from those of the adult and conform in their configuration with plants 

 which are not xerophilous, particularly in having well-developed leaves, 

 although this is not always the case. The development of the leaves 

 in the seedlings of Cacti, Casuarina, Ruscus aculeatus, R. Hypoglossum, 

 and other plants, is not essentially different from that in the adult, but 

 differences in this respect are seen even on nearly allied forms ; thus 

 the seedling of Ruscus androgynus possesses large well-developed foliage- 

 leaves which are not produced on the older plants where the leaves are 

 reduced to small scales. It has been pointed out above that the absence 



el 

 f 



FIG. 101. Ruscus aculeatus Shoot. On the 

 leaf-like phylloclades, cl, flowers are developed, 

 /;y~axillant leaf. Lehrb. 



FIG. 102. Acacia. Seedling. 1-4 primary leaves as in other 

 species of Acacia ; 5 and 6 show transitions to phyllodes ; 7-9 

 phyllodes ; n nectaries. Lehrb. 



of xerophilous characters in such juvenile forms is connected with the 

 growth of the seedling plants under the protection of others, and that 

 the development of such seedlings only takes place as a rule when 

 sufficient moisture is present, whilst the further development of the plant 

 is associated with claims of another kind. The following is a series of 

 examples : 



i. The best known and most frequently quoted are the species of 

 Acacia which produce phyllodes. The phyllodes arise by the broadening 

 in a vertical direction of the leaf-stalk, sometimes also of the leaf-midrib, 

 whilst the lamina aborts. Seedling plants (Fig. 102) however have without 

 exception, so far as they have been examined, leaves which are like those 

 of the species possessing a bipinnate lamina and a normal leaf-stalk. As 

 successive leaves are formed the leaf-stalk gradually broadens whilst the 



