TRANSMISSION OF ACQUIRED CHARACTERS. 393 



There is nevertheless a difference between these two cases in that 

 the T/iiy'a-shooi possesses the possibility of development in two 

 different ways instead of only one. The upper side of the shoot 

 can assume the structure of the under side and vice versa, and this 

 structural reversal depends upon the way in which the light is 

 thrown upon the shoot. But even if the light causes the structural 

 reversal, does this justify us in assuming- that the structure itself 

 is also the direct consequence of the influence of light ? I see no 

 reason for rejecting- the supposition that the physical nature of part 

 of a plant may be of such a kind that this or that structure may be 

 produced according as this or that condition of development pre- 

 vails. Thus with stronger light the structure of the upper side of 

 the shoot developes ; with weaker light, the structure of the under 

 side. But this physical nature of the T/mja-lmd depends, like that 

 of a bird's egg, upon its phyletic history, as we must assume to be 

 the case with, the germs producing all individual developments. 

 It is therefore quite impossible to interpret the reversal of the 

 structure in the Tkvja -shoot as the result of modification produced 

 by the direct influence of external conditions. It is an instance 

 of double adaptation one of those cases in which the specific 

 nature of a germ, an organism, or a part of an organism, possesses 

 such a constitution that it reacts differently under the incidence of 

 different stimuli. 



An entirely analogous example of reversal occurs in the climbing 

 shoots of the Ivy, and is described in Sachs' lectures on the 

 physiology of plants. Such shoots produce leaves only on the side 

 directed towards the light, and roots (which are made use of in 

 climbing) only upon the opposite side. If however the position of 

 the plant be altered so that the root-bearing side is turned towards 

 the light, while the leafy side is shaded, a reversal occurs, so 

 that from that time the former only produces leaves, and the latter 

 nothing but roots. In other words, the Ivy-shoot reacts under 

 strong light with the production of leaves and under weak light 

 with the production of roots, just as litmus-paper becomes red with 

 an acid and blue with an alkali. The physical nature of the Ivy- 

 shoot was present before the production of either structure, and was 

 no more due to the action of light itself, than the physical nature 

 of litmus-paper is due to an acid or an alkali. But this is quite 

 consistent with the possession of a physical nature which reacts 



