38 



Growth 



ported by Transeau ( 1916 ) for the seasonal distribution of various species 

 of Spirogyra. These differ markedly in the size of their cells, those of the 

 largest being about 150 times the volume of the smallest. The small-celled 

 forms are the first to appear in the spring, when temperatures are low, 

 and the larger-celled ones come on progressively as the season grows 

 warmer. This presumably is because of the higher metabolic rate of the 

 smaller cells which results from their greater ratio of surface to volume. 



Progressive physiological changes (p. 210) seem also to be involved. In 

 successively higher leaves on the stem of Ipomoea, Ashby and Wanger- 

 mann (1950; Fig. 3-10) found that the cells became smaller and suggest 



1000 



s 



_ »o 

 . .2 



* T. 



ui3 



100 — 



10 



o- o 



__ -to 



1 



0-01 



0-1 1-0 



Leaf area (cm. 2 , logarithmic scale) 



10 



100 cm 



Fig. 3-10. Relation between area of epidermal cells and of leaf lamina in developing 

 leaves of Ipomoea caerulea, plotted logarithmically. Solid circles, second leaves; 

 crosses, fifth leaves; empty circles, eighth leaves. Early growth is chiefly by cell di- 

 vision, since cell size increases little. Later growth is by cell enlargement since cells 

 and lamina grow at the same rate. Compare with Fig. 3-4. Cell size becomes smaller 

 in successive leaves. (From Ashby and Wangermann.) 



that this is symptomatic of a process of aging in the apical meristem. This 

 problem of possible senescence in plants has other implications for cell 

 size. Benedict ( 1915 ) presented evidence that in a number of vegetatively 

 propagated plants, notably Vitis, cell size tends progressively to decrease 

 with the age of the clone, a fact interpreted by him as the result of 

 senescence. He believed that the "running out" of certain varieties was 

 due to this cause, but it has now been shown that in many cases such a 

 change is due to virus infection. Benedict's results have had some con- 

 firmation, notably by Tellefsen ( 1922) and Bergamaschi ( 1926) in studies 

 of cuttings from trees of different ages. Ensign (1921), however, found 



