34 Growth 



Mobius (1920), however, observed no relation, in 215 species, between 

 chloroplast size and that of cells or organs. Irmak (1956) confirmed this. 

 There are other complications in the problem of cell size, some of which 

 are of morphogenetic significance. One involves the dwarfing of plant 

 structures. This has been discussed in a number of early papers, among 

 others by Gauchery (1899), Sierp (1913), Oehm (1924), Sinoto (1925), 

 and Abbe (1936). It is generally agreed that where dwarfing is the result 

 of unfavorable environmental conditions cell size is reduced, though not 

 equally in all structures. A scanty water supply chiefly affects the second 

 phase of cell enlargement in which considerable quantities of water nor- 

 mally are absorbed. The problem is complicated by the fact that dwarfing 

 is often the result of genetic as well as environmental influences. 



o 30 



- 20 



_■ 10 



38 DAYS 



•Ok * "j"* 



Stf^^f^^^^ 





,xx xx "x * X 



, .•*•• xxx" r " " x .-d*arf 



HjflS't"'* x - tall 



° , 20 30 40 50 60 70 80 90 100 110 120 130 140 



NUMBER OF CELLS FROM APEX 



Fig. 3-8. Relation of cell size to plant height. Length of successive cells along the 

 terminal meristematic region of a dwarf race (above) and a tall one (below) of to- 

 mato. Cells of the dwarf are somewhat longer because they attain maturity, and thus 

 stop dividing, at an earlier stage. The tall plants have many more cells. (From Bind- 

 loss. ) 



Genetic Factors. The relation of genetic factors to cell and body size is 

 complex and will be discussed more fully in a later chapter (Chap. 19). 

 Genetic analyses of size differences in plants have been made repeatedly 

 but the histological effects of gene and chromosome differences are widely 

 various. Most genetically large plants are so because of more rather than 

 larger cells. Thus in the tall races of Lycopersicon and Zinnia studied by 

 Bindloss (1942) there are many more cells, lengthwise, than in dwarf 

 races, though the cells are somewhat shorter (Fig. 3-8). In other cases, 

 however, cell size is involved. Thus the difference between large-leaved 

 sugar beets and small-leaved vegetable beets is due chiefly to the greater 

 cell size of the former. What is inherited here is evidently the amount of 

 postmitotic expansion, for the meristem cells are the same size in both. 

 Von Maltzahn ( 1957 ) found that the difference in size of vegetative struc- 

 tures between large and small races of Cucurbita was related to both cell 

 size and cell number. 



