Differentiation 213 



Dacrydium ciipressinam, for example, needle-like foliage (resembling 

 that of seedlings in many species with scaly leaves ) occurs not only in the 

 seedling but throughout the early life of the tree. Only when it begins to 

 bear reproductive organs, at the age of 20 years or more, does the foliage 

 assume the scale-like character which then continues throughout the rest 

 of its life. This may be interpreted as the persistence of a juvenile con- 

 dition until the period of reproduction. 



A more conspicuous example and one which has been widely studied 

 is that of the English ivy, Hedera helix ( Fig. 8-21 ) . The vegetative phase 

 of this plant is a vine with five-lobed leaves climbing by adventitious roots 



Fig. 8-21. Hedera helix. Flowering shoot with ovate, entire leaves, and a single leaf 

 of the vegetative "juvenile" region. (From Goebel.) 



and flattened against its support and often is the only form of this species 

 to be seen. After some time and under conditions favorable for reproduc- 

 tion, however, flower-bearing shoots arise and will grow for many years. 

 Their tropistic behavior is changed, for they no longer climb but grow 

 directly outward toward the light and away from their support. Their 

 structure is also much altered, for the leaves are now oval rather than 

 lobed and are spirally arranged. The lobed climbing form may be re- 

 garded as a persistent juvenile condition. 



The reorganization of the pattern of differentiation in the transition 

 from seedling to adult or from the vegetative to the reproductive phase 



