394 Morpho genetic Factors 



(p. 367). Torrey (1950) observed that pea root tips transferred directly 

 to a culture medium provided with IAA produce lateral roots at once 

 but that tips transferred after growing a week in culture do not do so for 

 some time. A substance (not auxin) stimulating lateral root formation 

 seems to originate in the lower part of the root and moves upward, 

 producing laterals in acropetal succession. 



It has been observed that the rooting response is often altered (gen- 

 erally increased ) when two different substances, such as indoleacetic acid 

 and naphthaleneacetic acid, are combined. Went (1939) applied dif- 

 ferent substances successively rather than in mixtures and found that 

 cuttings of etiolated pea seedlings, which do not root after treatment 

 with auxin alone, will do so if phenylacetic acid is first used, though 

 this substance by itself is ineffective. He believes root formation results 

 from the interaction of two factors and suggests that phenylacetic acid 

 may act to mobilize or activate a specific root-forming factor, rhizocaline 

 (Bouillenne and Went, 1933; Bouillenne, 1950; Libbert, 1956). The 

 question of the existence of such a specific factor has been studied by a 

 number of workers. Evidence for it is largely indirect, and rhizocaline 

 has not been isolated; but auxin is evidently not the only factor operative 

 in the initiating of root primordia. 



It must not be concluded that growth substances can produce roots 

 anywhere on the plant. They are often formed, to be sure, in unusual 

 places, as along the surface of the stem. Even in such instances, how- 

 ever, the initiation of root primordia does not take place anywhere and 

 indiscriminately but only in certain cells or at certain zones that are 

 potentially capable of forming them. At such points roots may be formed 

 under the stimulus of other factors such as ethylene, carbon monoxide, 

 wounding, or abnormal nutrition. The nature and location of such re- 

 gions are variable and depend on the general growth pattern of the plant 

 treated. There is a difference, for example, between monocotyledons and 

 dicotyledons as to rooting response. Treatment with growth substances 

 is one of the methods by which knowledge may be gained as to the 

 potentialities of various cells and tissues, not only for root formation but 

 for other developmental activities. 



Auxin and Rhizoids. Auxin is present in the coenocytic alga Bryopsis 

 and is most abundant in that part of the plant where rhizoids are com- 

 monest. Jacobs (1951) finds that an application of indoleacetic acid 

 stimulates rhizoid formation in the region where these are least abundant. 

 He regards this as analogous to the effect of auxin on root initiation in 

 higher plants. 



Leaf Formation. Attempts have been made to find substances which 

 might be involved specifically in the development and growth of the 

 leaf blade. In some cases leaf growth is dependent on the presence of a 



