The Cellular Basis of Growth 



29 



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TOMATO HAIRS 



1 - PLANT 7 DAYS OLD 



2 - 28 



3- " MATURE 



CELL VOLUME 

 Fig. 3-5. Logarithmic graph of relation of nuclear volume to cell volume in cells of 

 stem hairs of tomato. Nucleus increases about two-thirds as fast as cell. ( From Trom- 

 betta. ) 



numbers of cells at intervals has been developed by Brown and his stu- 

 dents (p. 41). Erickson (1956) has analyzed mathematically the rate of 

 division in certain root tips. 



In growing gourd fruits Sinnott (1942) determined cell number by 

 dividing tissue volume by cell volume and found that increase in cell 

 number takes place at approximately the same rate in epidermis, outer 

 wall, inner wall, and placental region, regardless of the marked differences 

 in cell size in the four. Cell division in the epidermis takes place at the 

 rate necessary to maintain a constant cell size, and this tissue may thus 

 serve in a sense as a pacemaker for division in the whole ovary primor- 

 dium. Jahn (1941) has made a detailed study of the localization and 

 degree of cell division and cell expansion in the epidermis of growing in- 

 ternodes of Vicia faba. 



CELL SIZE 



The size of plant cells is obviously an important element in growth, 

 differentiation, and other morphogenetic problems. Cells are relatively 

 small objects, presumably because the ratio of surface to volume, and thus 

 the ease of exchange of material between a cell and its environment, is 

 inversely proportional to its size. Cells with high metabolic rates tend to 

 be very small, and large cells are relatively inactive. Rapidly dividing 

 cells, for example, are much smaller than those of storage parenchyma. 

 There is a wide range in cell size among various tissues of a plant. In meri- 

 stematic regions they are often as small as 1,000 cu. microns or less but in 

 pulp of watermelon may be almost a million times this volume. Stras- 



