772 PLANT GROWTH lO 



tree and type of branch from which they are taken, with the age of the wood and 

 season of the year, but, unfortunately, it is not known whether these influences 

 control the initial cell-divisions or the subsequent elongation of the root initials. 

 In addition to auxin, sucrose promotes outgrowth of the roots and a number of 

 nitrogenous compounds increase the number of roots when the auxin treatment 

 is optimal. There is reason to think that these act, at least in part, on the initiating 

 cell-division. For etiolated pea-stems, the promoting factors include biotin, 

 thiamine, adenine, nicotinamide and tryptophan ; for green leafy cuttings, aspara- 

 gine, adenine, guanine, arginine, vitamin K and even nitrate are effective, while 

 woody cuttings in a few instances have responded to purines, pyridoxine and 

 thiamine (see Burstrom, 1953a and Torrey, 1956b, for general reviews, and 

 Thimann and Behnke-Rogers, 1950, for the extensive literature on woody cuttings). 

 The same group of nitrogenous substances seem to be essential for the initiation of 

 lateral roots on the main root (section Vllb, p. 801). 



Why wounding should stimulate cell division has not been satisfactorily ex- 

 plained, since, as a rule, auxin is destroyed by the wounded tissue. The wound 

 response is very variable according to the tissue and its treatment. Potato slices, for 

 example, which give 4 to 5 new cell divisions, parallel to the cut surface, within 

 7 days, fail completely to respond if they have been pre-stored at low temperatures 

 and only recover after returning to room temperature for many weeks (Steward 

 et al., 1943). Long ago, Haberlandt postulated a "wound hormone" liberated in 

 crushed cells and causing cell division. Several of his students and colleagues 

 worked out responsive systems that could be used for tests for this hormone. The 

 formation of outgrowths or intumescences in the parenchyma tissue of the bean 

 pod was one of the most sensitive (Wehnelt's test) and this has subsequently been 

 used as assay for the wound hormone. In this way "traumatic acid" or A^-decene-i, 

 lo-dicarboxylic acid, XXVI, has been isolated from crushed bean extract (Eng- 

 lish et al., 1939). A number of related compounds are also active. Traumatic acid 

 requires co-factors, inchiding glutamic acid, sucrose and phosphate, in order to 

 cause the outgrowths {see Thiina.nn et al . , 1952, for review). The observed outgrowth 

 results from cell enlargement as well as division. In other wound-reactions, how- 

 ever, traumatic acid has so far not been found to show any particular effect and 

 it induces no cell division in the tobacco pith cultures which respond so power- 

 fully to kinetin (below) ^. 



Tissue cultures are stimulated to divide by so many factors that it is evident 

 that no single substance is in general the limiting factor. Endosperm, particularly 

 "when, as in some young fruits, it is in the liquid form, is particularly effective; 

 coconut milk, corn in the milk stage, young walnuts and chestnuts and the tropical 

 fruit Allenblackia have all been used in this way. Coconut milk is particularly 

 useful and since its introduction by Van Overbeek et al. (1941) has become 

 a sine qua non for tissue culturists. The active compounds in coconut milk 

 have recently been shown to include 1,3-diphenylurea, XXVII, (Shantz and 

 Steward, 1955) while those from immature chestnuts include a leuco-anthocyanin, 

 probably leucocyanidin monoglucoside, XXVIII (Steward and Shantz, 1956). 



^ For wound hormones see Chapter 8. 



