110 



The Phenomena of Morphogenesis 



the lamina and the cross-sectional area of the petiolar xylem are related 

 allometrically, but the xylem grows only about 0.6 as fast as the lamina 

 (D. J. B. White, 1954). Alexandrov, Alexandrova, and Timofeev (1927) 

 observed that in Bryonia the number of vessels in any given part of the 

 stem is correlated with the dimensions of the leaves in that region. These 

 various facts suggest that physiological factors are here involved and 

 that the amount of water transpired from the blade is important in de- 

 termining the conducting capacity of the petiole. This hypothesis will 

 be discussed later (p. 332). In the light of other evidence, it is doubtful 

 whether such a "functional stimulus" is actually operative. 



Fig. 5-5. Relation of pith diameter to diameter of shoot in young stems of Pinus 

 strobus of different sizes, showing greater relative size of pith in larger stems. ( From 

 Sinnott. ) 



Somewhat similarly, the diameter of a growing cucurbit fruit and of 

 the stalk that bears it are closely correlated in early growth, the fruit 

 increasing more rapidly. Stalk growth ceases earlier than fruit growth, 

 however (Sinnott, 1955; Fig. 5-4). 



Some correlations between parts are due to the similar effect of a 

 gene or group of genes on a series of morphologically related organs. An- 

 derson and de Winton ( 1935 ) studied the effect of a number of mutant 

 genes, in Primula sinensis, on the morphology - of the leaf, bract, sepal, 

 and petal. In several cases they had a very similar influence on develop- 

 ment (producing lobing) in all four categories of organs. Such correla- 

 tions are examples of what is sometimes called homeosis.. 



Many examples of growth correlation are found in internal structures. 

 Thus Buchholz (1938) in Sequoia has shown that in stems of different 

 sizes the vascular cylinder occupies a relatively larger portion, as meas- 

 ured in cross section, in large stems than it does in small ones. In pine 



