250 TRANSLOCATION IN PLANTS 



of some stimulus or a specific hormone. There seems to be 

 a quantitative relation, however, between leaf area and 

 growth, so it would seem to be more controlled by food 

 supply unless there is also a quantitative response to a 

 hormone for which there is evidence. 



It has also been demonstrated (Munch, 1930; Curtis, 

 1920a; Haller, 1931) that fruits on a completely defoliated 

 shoot may grow at approximately normal rates, which 

 proves that food is carried through these stems, and yet 

 cambial growth may come to a standstill. Possibly the 

 movement of the substance necessary for cambial growth 

 is strictly polar. 



As pointed out by MacDaniels and Curtis (1930), both 

 cambial development itself and also orientation of the 

 cells seem to be determined or at least influenced either 

 by lines of movement of necessary solutes, carbohydrates 

 from above and nitrogen from below, or by some sort of 

 electrical gradient through the phloem or cambium. Lund 

 (1931) has given evidence that there is such a gradient 

 in stems and that ringing between two contacts alters the 

 electromotive force between the contacts. It is conceiv- 

 able that polarity and dominance may be partly under 

 the control of such gradients. The ringing of a stem by 

 interrupting such a system may interrupt both the trans- 

 port of materials through the conducting tract as well as 

 the continuity of a system showing electrical polarity. 

 That a cambium to become active merely needs to lie 

 within such a gradient and parallel to it is not adequate, 

 for if leaves are not allowed to develop on a defoliated 

 shoot, the gradient may be still present and yet cambial 

 activity ceases. It would seem, therefore, that mature 

 leaves favor cambial growth partly by supplying carbo- 

 hydrates and possibly partly by producing some other 

 specific substance which shows a polar movement. 



There seems to be a common tendency to explain these 

 at present obscure phenomena as controlled by hormones, 

 and often a special hormone is assumed for each obscure 

 point. The evidence seems conclusive that a hormone is 



