210 The Phenomena of Morphogenesis 



plexity. Hammond (1941; Fig. 8-17) and Stephens (1944) have described 

 similar changes in leaf shape in cotton, and Montfort and Miiller ( 1951 ) 

 in mistletoe. Von Maltzahn (1957; Fig. 8-18) has compared leaf char- 

 acters throughout plant development in large and small races of Cucur- 

 bita and the hybrid between them. 



Similar alterations have also been found in reproductive structures. In 

 Chamaecyparis there is a gradient of sexuality in the branches, the tips 

 being sterile, with female cones below and male ones still farther back 

 (Courtot and Baillaud, 1955). There is a flower bud in the axil of each 

 leaf of Cucurbita pepo but the type of flower produced by it tends to vary 

 with the position of the leaf on the plant, in the following sequence: 

 underdeveloped male, normal male, normal female, inhibited male, and 

 parthenocarpic female (Fig. 8-19). The order of these steps in progressive 

 feminization is constant but the length of each is affected by temperature 



Fig. 8-18. Change of leaf size during 

 plant growth. Lamina length of succes- 

 sive leaves in cucurbit plants of small- 

 fruited and large-fruited types and the Fi 

 between them. ( From von Maltzahn. ) 



I 3 5 7 9 I I 13 15 I 7 19 21 



and day-length, high temperatures and long days extending the male 

 phase and delaying the female one ( Nitsch, Kurtz, Liverman, and Went, 

 1952). 



Leaves are especially good material in which to study such changes, 

 and Ashby ( 1948/?, 1950a, and Ashby and Wangermann, 1950) has made a 

 thorough investigation of the changing character of the leaves in Ipomoea, 

 describing the progressive differences in their size and shape and in the 

 size and number of their cells from lower nodes to upper ones (Fig. 

 8-20). He presents evidence that these changes are not primarily due to 

 environmental factors (although such are operative) but to alteration of 

 inner conditions. In his 1948 papers Ashby reviews this field and discusses 

 at some length Krenke's theory (1940) that such changes are due to the 

 physiological age of the plant, as contrasted to its age in time. Krenke 

 regards aging as progress toward maturity, particularly reproductive ma- 

 turity, which is sometimes followed by further changes, in a cycle. At 

 points along this progression rejuvenescence may occur, as on shoots 

 grown from lateral buds. Successive nodes are units in a developmental 



