396 Morpho genetic Factors 



Went ( 1938 ) here postulated a phyllocaline, analogous to rhizocaline. 

 Later ( 1951/?) he extended his concept of the calines more fully into the 

 details of leaf form and structure. He calls attention to the importance of 

 adenine for mesophyll growth and shows that vein tissue, on the other 

 hand, can be increased by auxin without affecting mesophyll develop- 

 ment. There are thus two morphogenetic tendencies in leaf development: 

 one toward the formation of veins and induced by auxin and the other 

 of mesophyll, induced by adenine. Whatever factor induces the former 

 ( as well as the stem and petiole ) may be called a caulocaline and the lat- 

 ter a phyllocaline, whatever their chemical nature may turn out to be. 

 Leaf shape is affected by the balance between the two. Leaves with a 

 dominance of phyllocaline will tend to be palmate for they will have an 

 excess of mesophyll, whereas those with more caulocaline will tend to be 

 pinnate or parallel-veined, since they will have relatively more vein 

 tissue. One may question, however, whether the problem of organic form 

 can be solved quite as simply as this. 



Harder ( 1948 ) observed that in certain succulents, such as Kalanchoe 

 Blossfeldiarui, variations in leaf shape and structure depend on the day- 

 length. Plants grown under short days have short, apetiolate, and markedly 

 succulent leaves (Fig. 18-18). A single leaf subjected to short days will 

 transmit this "short-day shape" to the immature leaves above it which are 

 developing under a long day-length. This effect Harder and von Witsch 

 (1940b) attribute to a growth substance they call metapkisin, which is 

 not identical with either auxin or florigen. 



Stem Formation. It has proved difficult to demonstrate any substances 

 specifically related to the growth of the stem. Went (1938) decapitated 

 pea seedlings and measured the length of the secondary lateral branches. 

 He gives evidence that stem growth here depended on the roots, not on 

 the cotyledons, and suggests that it was due to caulocaline in conjunction 

 with auxin. In later experiments neither auxin from the apex nor water 

 supply from the root appeared to control stem growth, and Went again 

 attributed this to caulocaline coming from the stem base and the root 

 system. 



Flower Formation. The existence of flower-forming substances has a 

 firmer foundation. They were postulated by Sachs, who held them re- 

 sponsible for changing a plant from a vegetative to a reproductive state. 

 The demonstration that such a change could be induced by altering the 

 carbohydrate-nitrogen ratio (p. 366) and by photoperiodism (p. 315) cast 

 doubt upon this idea. More recent work, however, has come to its support. 

 Kuijper and Wiersum (1936), for example, found that if a soybean plant 

 is brought to a flowering state by exposure to short day-lengths and 

 another kept flowerless by long days a shoot of the former grafted into the 

 latter causes the flowerless plant to form flowers abundantly. Hamner 



