196 BIOLOGICAL LECTURES. 



the production of new leaves and new roots. Sachs made 

 numerous experiments which showed, he thought, that the 

 direction of the flow of these substances is determined by 

 the action of gravity ; the leaf -forming substances flowing up- 

 wards, and the root-forming downwards ; the direction of 

 the flow being thus determined by some factor outside of the 

 plant itself. It had been shown that if twigs of the willow be 

 planted with the distal end in the ground, new roots arise from 

 the end in the ground, and leaves from the free end. This 

 result follows, Sachs thinks, from the direction of flow of the 

 root-forming and leaf-forming substances. 



In considering Sachs's view, it will be well, I think, to keep 

 apart the two ideas, that specific substances produced in the 

 plant bring about the change, and that these substances may be 

 transported from one part to another in definite directions. 

 We might think of the transportation taking place in a given 

 direction as due to some peculiarity of the substance itself, 

 or of the tissues of the organism, or, as Sachs supposed in this 

 case, as the result of some outside influence. 



It is interesting to notice that this idea of a transportation 

 of specific stuffs goes back to Bonnet. The latter imagined 

 head-nourishing and tail-nourishing stuffs to flow, respectively, 

 forwards and backwards, and, acting on the germs in those 

 parts, determine the kind that develop. Sachs does not intro- 

 duce the idea of preformed germs, and correspondingly sim- 

 plifies his hypothesis. 



The idea of specific substances determining the regeneration 

 of a part is, in my opinion, one deserving of very careful con- 

 sideration. We have seen that the idea first suggested itself 

 to Bonnet when searching for an explanation of the development 

 of a new head or tail from the same region of Lumbriculus. 

 A head developed if the exposed part lay at the anterior end 

 of a piece, and a tail if the exposed part lay at the posterior 

 end. Bonnet said that something must awaken the one or the 

 other kind of germ, since both were assumed to be present at 

 every level. Hence the idea of two specific stuffs. It is evi- 

 dent, of course, that the difficulty is only shifted from the 

 germs to the stuffs, for no such stuffs were known, much less 



