The Cellular Basis of Growth 33 



structures such as tubers and fruits, the greater size is due to an increase 

 in both the number and the size of their cells. Lehmann (1926) found a 

 positive correlation between the size of a potato tuber and that of its cells. 

 Since the increase in cell size was by no means proportional to that in 

 tuber size, it was evident that large tubers have more as well as larger 

 cells. The same relations are found in tomatoes (Houghtaling, 1935). A 

 more detailed study of this problem, in large-fruited and small-fruited 

 races of gourds, was made by Sinnott (1939; Fig. 3-4). Here cell size in- 

 creases in the young ovary but much less rapidly than organ size, showing 

 that cell division is taking place. During this period there is more increase 

 in cell size in the larger races. The size at which the cells divide steadily 

 increases. At about the time of flowering, however, division in most of the 

 young fruit ceases, first in the central region and then progressively out- 

 ward, so that nearly all later fruit growth is by cell expansion. In large- 

 fruited races the period of cell division and that of cell expansion are both 

 longer than in small-fruited ones, so that the greater size of the former is 

 due to both more and larger cells. This general developmental pattern 

 was found by Riley and Morrow ( 1942) in Iris ovaries and fruits, by W. H. 

 Smith (1950) in apple fruits, by Ashby and Wangermann in leaves (p. 

 210), and by others. In avocado fruits, however, cell division in the fruit 

 wall continues to some extent until maturity ( Schroeder, 1953b ) . In gen- 

 eral, larger fruit size results from an extension, so to speak, of all parts of 

 the developmental history. 



In several genera Ullrich (1953) studied the relation between epider- 

 mal cell size and leaf size in the series of successive leaves up the stem. 

 He found that the size of the cells decreases steadily whereas that of the 

 leaves increases for several nodes and then decreases. Thus there is no 

 close relationship, at least in this tissue, between cell size and organ size. 

 Under unfavorable conditions, however, both tend to decrease together. 

 A somewhat similar variation in the correlation between cell and organ 

 size has been reported in wheat (Nilson, Johnson, and Gardner, 1957). 



It is noteworthy that tissues differ considerably in the relation of the 

 size of their cells to that of the organ of which they are a part. This rela- 

 tion is usually closest in storage parenchyma and least in the epidermis. In 

 general, as Sachs pointed out, cells that are physiologically important, like 

 most of those in the leaf blade, are relatively constant in size and show 

 little relation to the size of the organ. In mosses, unlike higher plants, cell 

 size and leaf size are usually rather closely proportional to each other, 

 cell number being much more constant. 



Size relationships sometimes extend below the level of the cell. The 

 ratio of cell size to nuclear size has already been mentioned here. Both 

 Budde ( 1923 ) and Schratz ( 1927 ) found a rather close correlation be- 

 tween the total surface area of the plastids and the volume of the cell. 



