208 The Phenomena of Morphogenesis 



are pinnately compound but the adult ones are reduced to phyllodes 

 (Fig. 8-16). In pine, the seedling leaves are not in fascicles but are borne 

 singly. The young plant of Phyllocladus has needle-like leaves, common 

 in most other conifers, but the adult plant bears phylloclads only. Seed- 

 lings of cacti have leaves but these are absent in adult plants. Many more 

 such examples could be cited (Jackson, 1899). 



There are some cases in which the internal structure is markedly dif- 

 ferent in young plant and adult, usually being simpler in character in the 



Fig. 8-16. Juvenile leaves of Acacia seedling (pinnately compound) contrasted with 

 the flattened phyllodes that constitute the adult foliage. (After Velenovsky.) 



former. Thus in ferns which have a complex vascular system in the ma- 

 ture plant, the young sporeling possesses a relatively simple protostele or 

 siphonostele. Species with many-bundled leaf traces usually have only 

 three in the seedling. Secondary tissues are also less complex in young 

 plants. Schramm ( 1912) finds that juvenile leaves generally resemble adult 

 shade leaves in structure. There are differences, particularly as to vena- 

 tion, between the early, deeply pinnatifid leaves of Lacunaria and the 

 simple mature type ( Foster, 1951 ) . Robbelen ( 1957 ) finds that in chloro- 

 phyll-defective mutants the juvenile form of leaf is retained later than 



