198 PHYTOHORMONES 



centration which just exerts a detectable effect on yeast 

 growth. 



Other factors. There is good evidence that pure theehn 

 (female sex hormone or oestrin) also increases the number 

 of roots produced by pure auxin, when it is appUed at the 

 base (u). The effect is very much smaller than that of biotin, 

 but like that of biotin it does not appear in the absence of 

 auxin. Whether theelin plays any part in root formation in 

 nature is doubtful, although it is frequently present in 

 plant material. 



Bouillenne (1936) has stated that root formation in Im- 

 patiens seedlings is greatly hastened by carotene. The total 

 number of roots produced was, however, not affected. 



The role of sugars, substances Uke biotin, and the auxins, 

 provides an excellent example of an interlocking system 

 of limiting factors (see Figure 62). By varying the condi- 

 tions any one can become limiting; the activity of each can 

 only be shown in the presence of sufficient of the other 

 factors. Such a relationship is further borne out by the 

 behavior of different races of peas; some, like Alaska and 

 Gradus, give few roots unless supplied with auxin, while 

 others, such as Dehcatesse, Dark Laxtonian and Perfection, 

 give large numbers of roots on the controls, and auxin does 

 not increase the number further (w). In the latter types 

 auxin is evidently not a limiting factor. Others again give 

 few roots even if supplied with auxin; in these evidently 

 one of the other factors is limiting. 



Thus far the factors considered have been those which 

 influence the formation of visible roots. In which stage of 

 development each factor exerts its influence is unknown. To 

 produce a visible root at least three processes must take 

 place in succession: redifferentiation of pericycle cells into 

 root initials, formation of a root primordium by these initial 

 cells, and the outgrowth of the root primordium. From a 

 standpoint of morphogenetics the first two processes are 

 the most important; physiologically speaking they can be 

 regarded as one process. There are several reasons for 



