Correlation 



111 



stems of different size the pith is relatively larger in the large stems and 

 the cortex relatively smaller (Sinnott, 1936rt; Fig. 5-5). 



Sometimes these size relationships are found to extend below the level 

 of the organ. The relation of cell size to nuclear size has already been 

 discussed (p. 27). Klieneberger (1918) measured this relationship in 

 a large number of plants, and the subject has been reviewed by Trom- 

 betta (1942). Both Budde (1923) and Schratz (1927) found a rather 

 close correlation between the total surface area of the plastids and the 

 volume of the cell. 



These relationships between structures have important evolutionary 

 implications which cannot be discussed here. The increasing size of the 

 leaf during the development of the pteropsid stock seems to have been 

 correlated with the change from a protostelic to a siphonostelic stem 

 structure (Wetmore, 1943). The association of the trilacunar leaf trace 



Fig. 5-6. Diagram of a trilacunar node, 

 showing relation between stipules and lat- 

 eral leaf traces. ( From Sinnott and Bailey. ) 



with the presence of stipules (Sinnott and Bailey, 1914; Fig. 5-6) is 

 another instance. This has been emphasized by the observation of 

 Sensarma (1957) that when only one lateral trace branches only the 

 stipule on that side develops. Another case is the relation of absolute 

 size of the axis to its vascular development (p. 359). Among animals there 

 are many examples of evolutionary allometry where increasing size of 

 the organism results in a proportionally greater increase of certain organs. 

 Correlations between Dimensions. Correlations between part and 

 whole or part and part evidently involve coordinating mechanisms that 

 bind these parts into an integrated organism. The same sort of control 

 is shown, though in a somewhat different manner, in the correlation be- 

 tween the various dimensions of an organ or other determinate structure. 

 Here one is concerned with the very essence of form itself, with the way 

 in which growth is distributed in one direction relative to that in an- 

 other. This relative growth, like that between parts, is under definite con- 



