288 PLANT GROWTH SUBSTANCES 



physiological concepts are embraced. When the differential effects of 

 plant nutrition became known, especially through the work of Klebs, 

 differences in form were generally attributed to differential nutrition 

 (see 8). Thus the earlier explanations based on specific growth substances 

 (22,3) were more or less abandoned. With the increase in our knowledge 

 of plant hormones, they again assume a central part in morphogenetic 

 considerations. 



With the knowledge gained in the last ten years about leaf growth 

 factors it is interesting to develop a unified view of leaf shape, which can 

 account for many of the variations we observe within a single plant, 

 differences between closely related genetic races or effects of external 

 conditions, effects of viruses and other diseases, or teratological phe- 

 nomena. 



As a basis for the following analysis we assume that petiole, vein, 

 mesophyll, and stipule growth are independent of each other. This has 

 been shown experimentally in transplantation experiments with peas, 

 in which these parts on the scion were independently influenced by the 

 stock (36,38). The work of D. Bonner has shown that mesophyll growth 

 is controlled by substances such as adenine which are not primarily 

 concerned with stem and vein growth (4,5). There are certainly many 

 other growth factors for mesophyll (21), but for the following considera- 

 tions it is sufficient that adenine specifically increases mesophyll de- 

 velopment in young radish and pea leaves without affecting vein growth. 

 On the other hand, it is possible to increase vein growth without 

 mesophyll development by applying auxin to leaves (40). In general it 

 seems that petiole and vein growth are influenced by the same factors 

 which increase stem elongation. Thus there is a marked separation in 

 physiological response of stem, petiole, and vein growth on the one hand, 

 and mesophyll growth on the other, a difference based upon a different 

 set of growth factors for each type of tissue. For the sake of convenience 

 in this paper the vein growth factors will be named caulocaline, and 

 the mesophyll growth factors will be summarized under the name of 

 phyllocahne without further reference to their chemical nature. Caulo- 

 caline and phyllocaline thus arc purely physiological names, that is 

 functional, not chemical. 



For the above reasons we should consider leaf form as a synthesis of 

 two separate tendencies which are independently controlled by separate 

 sets of growth factors: a tendency to hnear development of veins, and to 



