212 The Phenomena of Morphogenesis 



terest. Attention has already been called (p. 38) to Benedict's work on 

 progressive reduction in size of structural units (cells and vein islets) in 

 vegetatively propagated clones as they grow older, presumably the result 

 of loss of vigor. This conclusion is still open to doubt, however. 



Topophysis. The changes so far discussed have been either juvenile ones 

 or those distributed through most of the life cycle. In many plants, how- 

 ever, the contrast in differentiation does not come until the onset of repro- 

 ductive maturity. Diels ( 1906) observed that environmental factors which 

 promote flowering also hasten the transition from juvenile to adult foliage. 



z 

 o 



CO 



1X1 

 LU 



NODES NUMBERED FROM BASE 



T 



~l 1 1 1 1 1 « ■ » TTT1 



2 3 4 5 6 7 8 n-8 n-7 n-6 n. 



Fig. 8-20. Change in leaf shape at successive internodes from base to tip in Ipomoea 

 caerulea. (From Ashby.) 



The reproductive stage is marked by characteristic changes at the meri- 

 stem, especially in the shape of its terminal dome. In the vegetative phase 

 this is typically low and rounded, but when flower buds are to be differ- 

 entiated it assumes a much steeper and more elongate form. The produc- 

 tion of reproductive organs marks for most plants a radical reorganization 

 of their developmental processes (p. 184) and is often accompanied by 

 changes so profound that they are irreversible. This is of especial morpho- 

 genetic significance. 



In most cases these changes have little effect on the character of the 

 vegetative organs, but such cases are sometimes found. In the conifer 



