106 The Phenomena of Morphogenesis 



example, Sinnott (1921) has shown that there is a positive correlation 

 between size of leaf and size of entire plant up to a certain plant size. 

 Beyond this the size of additional leaves is no greater even if leaf 

 number and plant size may increase considerably ( Fig. 5-2 ) . Size of pod 

 and of seed show a similar relationship to plant size. These facts suggest 

 that organ size may depend on the size of the embryonic mass or the 

 shoot meristem and that this may increase up to a certain point only, 

 beyond which increase in total plant size involves only the addition of 

 more units (internodes, leaves, and others). 



In cucurbits and many other types, although organs on the same plant 

 tend to be correlated in size (forms with large fruits also having large 

 leaves, thick stems, and long internodes), there is a certain amount of 

 flexibility in these relationships, depending on genetic constitution. Thus 



.95 



10 



30 50 



Dry weight of shoot (g ) 



70 



90 



Fig. 5-2. Relation of size of leaf to size of shoot in progressively larger bean plants. 

 For a while, leaf and shoot increase together, but after a certain point, shoot size 

 increases without further increase of individual leaves. {From Sinnott.) 



if a pumpkin type, which has all its parts large, is crossed with an egg 

 gourd, where they are all small, the Fo generation contains plants that 

 show some differences in the relative size of their parts, but there are 

 none that have the large fruit size of the pumpkin and the small vine 

 type of the egg gourd. The general physiological correlation of parts 

 within the same plant makes it impossible for sizes of individual organs 

 to segregate independently in inheritance. 



The size of the meristematic region bears some relation to that of 

 plant and organ size. Crane and Finch ( 1930 ) have shown that the size 

 of buds has an effect in determining the size of shoots that grow from 

 them. In a comparative study of large-fruited and small-fruited races in 

 Cucurbita pepo, von Maltzahn (1957) found that, although the dome- 

 like undifferentiated meristem is essentially the same size in all types, 



