EFFECT OF AUXINS ON ROOT FORMATION 



581 



wall. Some auxin is actually used up in the growth process, but the amounts 

 consumed are very small. Thimann and Bonner (1933) have calculated that 

 one molecule of auxin brings about the deposition of 300,000 hexose residues 

 in the form of cellulose during the growth of oat coleoptiles. 



Effect of Auxins on Root Formation. — It has been known for many 

 years that the presence of buds on a cutting favors development of roots when 

 the basal portion is introduced into a suitable rooting medium. Developing 

 buds are more effective in promoting root formation than quiescent buds. 

 Leaves, especially if young, also often favor the production of roots on cut- 

 tings. These observations suggest that root 

 initiation on cuttings is favored by growth 

 substances which are produced in the buds 

 and 3'oung leaves and are subsequently trans- 

 located to the basal part of the cutting. 



The activity of various substances in pro- 

 moting root formation can be tested by a num- 

 ber of different techniques. Went (1934) 

 has described a quantitative method which 

 consists essentially in immersing the apical 

 portions of cuttings from etiolated pea seed- 

 lings in the solution to be tested for 15 hours. 

 The terminal bud is removed and the apex 

 of the stem is split back for i or 2 cm. before 

 immersion. Basipetal translocation carries 

 substances favoring root formation to the 

 lower end of the cutting. The basal ends of 

 the cutting are then immersed for seven days 

 in 2 per cent sucrose solution, followed by 

 seven days in water. The number of roots 

 formed is considered to be a measure of the 

 "root-forming capacity" of the substance in the 

 solution. 



By the use of this technique and others it has been shown that auxin b 

 and heteroauxin are both active in causing root formation. It is now generally 

 considered that at least one of the naturally occurring hormones which favors 

 root formation is identical with one of the auxins, and that one or more 

 hormones is required for the formation of roots, whether they develop on other 

 roots, on stems, or on leaves. 



Subsequently a number of other compounds have been found which pro- 

 mote root formation on stems or cuttings. Zimmerman and Wilcoxon 



Fig. 130. Root formation on 

 stem of tomato plant resulting 

 from treatment with lanolin con- 

 taining 2 per cent alpha naph- 

 thalene acetic acid. Photograph 

 from Zimmerman and Wilcoxon 

 (1935)- 



