REGENERATION IN PLANTS 8 1 



upper part of the basal arm. The results are due to two factors, 

 gravity and an inner "force" that is supposed to be the resultant of 

 a growth phenomenon taking place in the bent portion. Vochting 

 supposes that a process of growth takes place as a result of the bend- 

 ing ; " the plasma streams to this region, and a new development 

 takes place here more easily." Vochting adds that this view will not 

 explain the morphological character of the new organs, and that this 

 must be due to quite other causes. The results may, I venture to 

 suggest, find a simpler explanation as the result of the bending, dis- 

 turbing the tensions of the protoplasm, causing the two arms of the 

 piece to act as if they had been separated from each other. This 

 idea is more fully developed in a later chapter. 



Sachs has criticised Vochting's general conclusion in regard to the 

 internal factors that determine the regeneration in a piece of the stem 

 of a plant. He gives very little weight to the innate polarity of the 

 piece, and attempts to explain the results as due to certain substances 

 in the stem of such a sort that, accumulating in any region, they 

 determine the kind of regeneration that takes place. Sachs also as- 

 sumes that gravity acts on these substances in such a way that 

 the root-forming substances flow downward and the shoot-forming 

 substances flow upward. In a piece of a stem, the two formative sub- 

 stances contained in it accumulate at the two ends, and determine 

 the kind of regeneration that takes place. It is evident that Sachs' 

 hypothesis fails to explain the method of regeneration of an inverted 

 piece suspended in a vertical position, since the roots appear at the 

 upper end and the shoots at the lower end. Sachs explains this as 

 the result of the previous action of gravity on the piece, while the piece 

 was a part of the tree and stood in a vertical direction. He supposes 

 the longer time that gravity has acted on the piece has determined its 

 basi-apical directions, so that this influence is shown in the inverted 

 piece, rather than the action of gravity on it in its new position. 

 This conception involves quite a different idea from the original one 

 of formative substances flowing in definite directions. Moreover, 

 Vochting has met this interpretation by using the twigs of the weep- 

 ing willow, that hang downward on the tree. If gravity has acted on 

 these drooping twigs in the way that Sachs supposes it can act, then 

 we should expect to find, if Sachs' view is correct, that roots would 

 develop at the apical end of a piece of the twig, and leaves at the 

 basal end, if the piece is hung vertically with its basal end (i.e. the 

 end originally nearer the trunk of the tree) upward. The regeneration 

 of these pieces shows, however, that they behave in the same way as do 

 pieces of twigs that have always stood vertically on the tree. There 

 can be, therefore, no doubt that the distinction between base and apex 

 is an expression of some innate quality of the plant itself. That an 



