VII GROWTH OF SPECIFIC ORGANS 805 



lower side, as required. However, geotropic curvature is not easy to imitate by 

 local application of auxin, and besides, one must make a reservation for the case 

 of those roots whose growth is promoted by low auxin concentrations; these ought 

 to curve upwards. It may be that two components, positive and negative, interact 

 in normal geotropism (Rufelt, 1954). Since some roots begin to show geotropic 

 curvatures after 8 min., while in general the promotions of growth due to auxin 

 only occvir after much longer times [cf. above), the above reservation may not 

 need to be made (Larsen, 1956), though it remains true that auxin applied for 

 short times and then removed may cause marked growth accelerations, measurable 

 within min., (Gast, 1942), an observation which seems to have been overlooked 

 recently. It is interesting that some auxins do cause upward curvatures (Rufelt, 

 1954). If the classical auxin theory of geotropism does prove to hold in roots then 

 the contrast with shoots is complete. 



Phototropism cannot be similarly compared, for lack of critical data on photo- 

 tropism in roots. 



A third sharp contrast with shoots is in the response to calcium, which has been 

 known to be necessary for root growth since the work of Hansteen-Cranner in 19 10 

 and of Mevius in the 20's. Calcium is not stored in roots in appreciable amounts but 

 must be added to the nutrient solution. Not only is it necessary, it clearly promotes 

 root growth; at the optimum concentration, which is io~'^M for wheat roots, it 

 increases the total root length by about 200%, more or less, depending on the pH 

 (Burstrom, 1952, 1954). The effect on cell length is greater than that on cell 

 division, but even the latter is increased nearly 50%. Burstrom found that on plas- 

 molysis the root epidermal cells shrink more if they have been exposed to high 

 calcium than do those of controls ; this means that calcium has increased their 

 elastic extensibility. A sample of the data is shown in Table 5. The results are 

 explained in terms of the postulated two phases of growth [cf. p. 794) :-the first, wall 

 softening, promoted by auxin in roots as in shoots and independent of calcium; 

 the second, deposition of new wall material, inhibited by auxin in roots, but pro- 

 moted by calcium. 



The action of calcium in promoting elongation of the root is in line with another 

 well-known effect, namely that of promoting the outgrowth of root-hairs. The 

 basis for this is not well understood (see Cormack, 1949, for review). Root hairs, 



TABLE 5 



ELASTIC TENSION OF WHEAT ROOT EPIDERMAL CELLS AS A FUNCTION OF 

 CALCIUM CONTENT OF THE MEDIUM. FROM BURSTROM (1954) 



Calcium concentration 

 M 



Literature p. 816 



