Growth Substances 



403 



species-specific, for those from one species will produce this effect in quite 

 unrelated ones. 



Much work has been done in isolating wound hormones and deter- 

 mining their chemical nature. Standard material for estimating relative 

 effects of hormone activity was first sought. Wehnelt (1927) used the 

 layer of parenchyma cells which lines the immature pod of the common 

 snap bean. Such tissue responds sensitively to various types of stimula- 

 tion by abundant cell division and the formation of intumescences, the 

 size of which provides a rough measure of the intensity of the stimulus. 

 This "bean test" has been used by many students of wound hormones 

 (Jost, 1935; Umrath and Soltys, 1936). On such pod surfaces Bonner 

 and English (1938) placed droplets of extract from crushed tissue 



Fig. 18-22. Effect of wound hormones. 

 Section of internode of Kalanchoe below a 

 wound, showing how cortex cells have been 

 induced to divide frequently, and parallel 

 to the wound surface. ( From Sinnott and 

 Block. ) 



(chiefly bean pods) and found that the height of the intumescence 

 which developed after 48 hours was proportional to the concentration 

 of the wound hormone present in the extract. These proliferations are 

 usually higher than the ones induced by other chemical or physical means. 



Considerable progress has been made toward a knowledge of the 

 chemical nature of these wound hormones. Bonner and English isolated 

 from bean-pod juice a substance which in low concentration was very 

 active in the bean test and named it traumatin. English, Bonner, and 

 Haagen-Smit later (1939) purified from the same source a crystalline 

 dibasic acid similar in its effects. Traumatin appears to be active on only 

 a few types of cells, such as those of the potato tuber and the bean pod. 



To understand wound reactions in any plant, organ, or tissue, account 

 must be taken of many internal and external factors as yet imperfectly 

 known (Bloch, 1941). Workers have often been puzzled by differences 



