8o6 PLANT GROWTH 



lO 



which are outgrowths from the epidermal cells, are known to develop much more 

 strongly in the air than in nutrient solution. Presumably their growth would 

 require much new wall material and thus require both calcium and oxidative 

 metabolism. 



Thus the physiology of elongation in roots has at least three features opposite 

 to those in shoots. On the other hand the general metabolism underlying the 

 growth process has many points of similarity. For example, dinitrophenol and 

 arsenate, which act on phosphorylation, and beryllium, believed to inhibit adeno- 

 sinetriphosphatase, all inhibited elongation strongly (Hopkins, 1952). Arsenite 

 and iodoacetate, both sulfhydryl reagents, acted less powerfully, while fluoride 

 had very little effect (Kandler, 1953). Unsaturated lactones, especially coumarin 

 and its derivatives, inhibit the elongation of oat (Goodwin and Taves, 1950) and 

 of wheat roots (Burstrom, 1954), but as yet it is not clear that this is due to their 

 acting as sulfhydryl reagents, especially since Pollock et al. (1954) could not 

 relieve the inhibition by adding BAL [cf. p. 785). It appears also that coumarin 

 and scopoletin operate in different ways since coumarin causes lateral swelling, 

 which scopoletin does not, while scopoletin inhibits the formation and elongation 

 of root hairs, which coumarin does not (Avers and Goodwin, 1956). 



Isolated roots grown in culture require a complex nutrient medium, containing 

 both macro- and micro-elements, sucrose, four or five B-vitamins and sometimes 

 one or more other nitrogenous compounds. Correspondingly, antimetabolites for 

 pyridoxin, ^-aminobenzoic acid, folic acid, proline, arginine and guanine all 

 inhibit the elongation of seedling roots and in many cases, but not all, these 

 inhibitions can be overcome by adding the parent compound (Fries 1954; Torrey, 

 1956b). This demonstrates that roots on the plant are dependent on the supply 

 (from the shoots or cotyledons) of the B-vitamins, some aminoacids and purines. 



It only remains to consider the development of lateral roots on roots. These 

 organs are not to be compared with lateral buds, for the lateral roots arise far from 

 the apical meristem of the root, by division of cells which have ceased to elongate. 

 The distance between the root tip and the first lateral roots is quite constant during 

 normal growth (Geissbiihler, 1953) and has been ascribed to inhibition of laterals 

 by the main apex. An ether extract of the tip does inhibit lateral formation in the 

 segments below by about 50% (Torrey, 1956a), which supports this view. 



Auxin strongly promotes the outgrowth of laterals in pea roots, but after a root 

 has once responded in this way it cannot for some time produce a second crop of 

 laterals on receiving a second auxin treatment (Torrey, 1950). Evidently another 

 factor is now limiting, a factor which was shown to be coming from the cotyledons. 

 By using isolated segments, it was found that lateral root formation is promoted, 

 in conjunction with auxin, by thiamine, nicotinic acid, adenine and a mixture of 

 micro-elements (Torrey, 1 956a) . Antimetabolites for the three nitrogen compounds, 

 viz. pyrithiamine, 3-acetylpyridine and 2,6-diaminopurine, respectively inhibited 

 the process and in each case the inhibition could be reversed by the parent com- 

 pound. As was noted above, the same three nitrogen compounds are needed for the 

 growth of normal roots, or act as stimulators for the growth of inhibited roots. 

 A pH of 6 is optimal for lateral root formation in pea roots, and 2 to 3 days of 

 treatment with lAA \o'^ M (1.75 mg/l) was the optimum auxin treatment. This 



