32 



Growth 



ovary, as measured by gain in dry weight, at the time when cell division 

 ceased and cell expansion began (Fig. 3-7). 



Cell Size and Organ Size. There are several important implications of 

 the problem of cell size for morphogenesis. One is that of the relation be- 

 tween the size of an organ or body and the size of the cells that compose 

 it. Is a body large because its cells are large or because they are more 

 numerous? This problem was discussed by Gregor Kraus in 1869 in con- 

 nection with his work on structural changes during etiolation, but Sachs 

 (1893) and his student Amelung (1893) seem to have been the first to 

 attack it directly. Sachs called attention to the fact that the size of a cell 



Time in Days 



Fig. 3-7. Relation of cell division and cell enlargement to growth. Logarithm of ovary 

 volume plotted against time for three races of Cucurbita differing in fruit size. The 

 period between the vertical bars is that during which cell division ceases. To the left 

 of it, growth is by cell division; to the right, by cell enlargement. Despite this change, 

 the rate of growth at this period remains constant. ( From Sinnvtt. ) 



must be closely related to its physiological activity and that cells of a par- 

 ticular tissue should thus be expected to be of about the same size. If this 

 is so, size in a plant would be related to the number rather than the size 

 of its cells. His measurements supported this conclusion. Amelung made 

 a much larger series of measurements and found the same general result, 

 although he observed a good many cases where cell sizes differed consid- 

 erably between comparable plants or tissues. 



The problem is not quite as simple, however, as these early workers 

 thought. It is true that most of the size differences between plants, espe- 

 cially those in the indeterminate axial structures, result from differences 

 in cell number. In determinate organs, on the other hand, notably in bulky 



