The Cellular Basis of Growth 



31 



it is related to the absorption of water and increase in size of the vacuole. 

 Differences among tissues in mature cell size are very considerable and 

 are one of the most important aspects of differentiation. In tissues where 

 division persists relatively late there is little time for cell expansion before 

 maturity is reached, and the cells remain small, as is often the case in the 

 epidermis. Where division ceases early, as in storage parenchyma, the cells 

 grow to a much greater size. In many axial structures there is a gradient 

 from without inward, the cells becoming progressively larger toward the 

 center either because of more rapid increase or earlier cessation of divi- 



200 

 180 

 160 

 140 



to 



-1 120 



li- 100 

 O 



£ 80 



2 

 D 

 z 60 



40 



20 - 



SUBEPIDERMIS 

 THIRD LAYER 

 FOURTH LAYER 



INNER EPIDERMIS 



-o OUTER EPIDERMIS 



10 20 30 



COLEOPTILE LENGTH IN MM. 



40 



50 



Fig. 3-6. Number of cells, lengthwise, in various cell layers of the oat coleoptile at six 

 stages in its growth. There is evidently no division in the outer epidermis after the 

 coleoptile begins its development, and division ceases early in the other layers, though 

 there are differences among them in the frequency of division. ( From Avery and Burk- 

 holder. ) 



sion. This is not always the case, for the epidermis may have larger cells 

 than the other tissues, as Avery and Burkholder found in the Avena 

 coleoptile. 



There is a question as to just where in the cycle of cell division growth 

 actually occurs. Abele (1936) distinguishes between Teilungswachstum, 

 growth during division itself, and Streckungswacristum, growth after divi- 

 sion has ceased. There is certainly a considerable visible increase in size 

 during prophase but not much more until telophase. Of course the dura- 

 tion of these phases must be taken into account. It is probable that non- 

 aqueous material increases at a constant rate throughout growth. Sinnott 

 ( 1945a ) found that in gourds there was no change in rate of growth of the 



