296 PLANT GROWTH SUBSTANCES 



leaf form can be accounted for by the two-factor scheme of phyllocaHne- 

 caulocaHne in their relative proportions. 



Stem Growth. — Very little new evidence has been collected during the 

 last few years concerning the role of factors other than auxin in the 

 elongation of stems. That such other factors exist was assumed as soon 

 as the work with auxin started (33) and was further supported by subse- 

 quent experiments (34,35)37)- Schneider (24) also found that in addition 

 to auxin and sugars some other factor controlled growth of Avena 

 coleoptile sections. In a few experiments Went and D. Bonner (39) 

 were able to replace this factor with pea diffusate or yeast extract, but 

 these experiments could not be repeated consistently. Therefore all 

 our knowledge about caulocaline, as this factor has been named, is 

 circumstantial. In this respect it rather resembles florigen, with which 

 it also has in common the fact that it moves only through continuous 

 vascular connections. When a stem is cut and grafted on another base, 

 growth is resumed only after the graft has "taken" or in other words has 

 new vascular tissue between the graft partners (11). This was also found 

 in the case of the transmission of the flowering stimulus (42). 



Synthesis of caulocaline takes place in roots, and in exceptional cases 

 in stems (for example, Asparagus 18). In branches of deciduous plants and 

 in seeds it can be stored in limited amounts, but otherwise stem elonga- 

 tion occurs only when such stems are connected through vascular ele- 

 ments with roots. Therefore stem cuttings of nondeciduous plants do 

 not start to grow until new roots have appeared. Similarly detached 

 buds grown under sterile conditions start to elongate only after roots 

 have been formed on them (21,28). This is even partly true for Asparagus 

 stem tips, whose growth is much speeded up when roots regenerate on 

 them (18). 



Summary. — Growth of leaves can be differentiated into vein growth, 

 under the influence of the same factors as stem growth (caulocaline), 

 and mesophyll growth, controlled by different growth factors (phyllo- 

 caUne) among which adenine has been identified. Difl^erences in leaf 

 form can often be interpreted as being due to differential vein and 

 mesophyll growth. Thus it seems that the form of submerged leaves, of 

 shade leaves, of many virus-infected leaves, of laciniate and other 

 genetically controlled leaf forms is due to lack of mesophyll growth 

 factors. Also in other cases, where for example, virus has caused decreased 



