28 



Growth 



the nucleus tends to keep pace with the surface of the cell and thus 

 perhaps with the volume of the cytoplasm if the thickness of the cyto- 

 plasmic layer is constant (Trombetta, 1939; Fig. 3-5). This suggests a 

 relationship which in earlier stages may have a bearing on cell division. 

 An important element in growth and development is the rate at which 

 cell division takes place. This is essentially a physiological problem and 



60 80 100 



OVARY DIAMETER mm 



OVARY 



20 



DIAMETER mm 



Fig. 3-4. Relation of cell diameter to fruit diameter in Cucurbita pepo. TA, small- 

 fruited race. CF, large pumpkin. In early development, cell size increases less rapidly 

 than fruit size, showing that division is occurring. Later growth is by cell enlargement. 

 Solid circles, cell diameter (in microns) at last division. Final cell diameter at end of 

 each line. Lowest curve, epidermis; next higher, outer wall; next, middle wall; next, 

 inner wall; uppermost, placental region. (From Sinnott.) 



involves various internal and external factors, some of which will later be 

 discussed. There are certain techniques by which it can be measured, 

 however, which are of importance for the student of morphogenesis. 



Root tips are especially favorable material for this. Brumfield (1942) 

 recorded the rate of division in the apical meristems of small roots by 

 photographing the surface cells at measured intervals of time, and this 

 has since been done by others ( p. 78 ) . A method for measuring rate of in- 

 crease in cell number by macerating the root meristem and counting the 



