Differentiation 



209 



normally and has a relatively small meristem. Normal leaves are not pro- 

 duced until the meristem reaches a diameter of 80 to 90 /*. 



Schaffalitzky de Muckadell (1954) has reviewed the literature on ju- 

 venile stages. 



Juvenile traits often resemble those of plant types presumably an- 

 cestral for the stock in question (Sahni, 1925, and others). This seems 

 evident in many of the examples cited. Most Leguminosae other than 

 Acacia have leaves and not phyllodes, and most Myrtaceae other than 

 Eucalyptus have dorsiventral leaves. These facts suggest that the seedling 

 repeats or recapitulates ancestral traits, much as the animal embryo has 



LEAVES AT NODES 1-5 





A-8 



A-24 A-20 



A-9 



A-24XA-8 

 Fi 



Fig. 8-17. Changes in leaf shape in cotton at five successive nodes above the cotyle- 

 dons, in four varieties of cotton and an Fi. ( From Dorothy Hammond. ) 



been thought to do. There is much doubt in many cases, however, as to 

 what the course of evolution actually has been and so much variation in 

 early ontogeny in many plants that it is impossible to establish the doc- 

 trine of recapitulation as an invariably useful guide to phylogeny. 



Progressive Developmental Changes. More common than these con- 

 spicuous cases of differentiated juvenile stages are those where there is 

 not a sharp distinction between juvenile and later forms but a gradual 

 change from the younger part of the plant to older ones. Many examples 

 of this are reported in the literature. Goebel (1896) described eight suc- 

 cessive leaf types in the climbing aroid Anadendrum medium which 

 occurred at different levels and showed an increasing degree of com- 



