18 PHYTOHORMONES 



In experiments with leaves of succulents and kohl-rabi, 

 Haberlandt (1914, 1921) and Lamprecht (1918) also showed 

 that cell divisions could be induced by spreading upon the 

 cut surface some of the crushed tissue of other leaves. Haber- 

 landt concluded (1913) that cell division was determined by 

 two substances, one coming from the wound ("wound- 

 hormone"), and the other from the phloem tissue ('^lepto- 

 hormone"). The latter was assumed to be present to a 

 lesser extent in some other tissues. Subsequent investigators 

 have tried to extract these hormones and to identify them 

 with known substances (Reiche, 1924; Wehnelt, 1927). 

 Their methods of testing were not specific enough to allow 

 the drawing of any definite conclusions, but Jost (1935a) 

 has confirmed and extended some of their results (c/. 

 XIII A), and Umrath and Soltys (1936) have made a partial 

 purification (see below) . 



Further, Bonner (1936), using Wehnelt's test not as a 

 measure of cell division but simply of growth, has demon- 

 strated that tissue extracts contain a growth-promoting 

 factor not identical with auxin. By the addition of alcohol 

 extracts of tissues to a culture medium he has obtained 

 growth of undifferentiated parenchyma cells in vitro, which 

 closely approached true plant tissue cultures. 



The experiments of Jacques Loeb (1917-1924) on cor- 

 relations in Bryophyllum occupy a very isolated place in the 

 literature. He studied regeneration, bud growth, and geo- 

 tropic behavior in isolated leaves and stems. At first his 

 explanations were based upon Sachs' ideas, as for instance: 

 ''There must, therefore, be associated with the material 

 which causes geotropic bending also something which favors 

 the growth of roots, and this may be one of the hypothetical 

 substances of Sachs" (1917a, p. 118). Subsequently he 

 transferred the emphasis to more general factors of nutri- 

 tion, controlled by simple mass action, as for instance: 

 "These facts show that the regeneration of an isolated leaf 

 of Bryophyllum is determined by the mass of material avail- 

 able or formed in the leaf during the experiment" (1923, 



