RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 569 



Coprinus drooped in darkness, and became turgid on restora- 

 tion to light ; and the further supposition that light dimi- 

 nished growth was held to be negatived by the instance 

 of a dark-rigored plant, in which growth, so far from being 

 retarded, was accelerated or renewed by simple exposure to 

 light. 



In both these instances we notice the abnormal condition 

 induced in the plant. We must bear in mind that that 

 rhythmic activity which is essential to growth depends, like 

 all other rhythmic activities, not only on the turgor of the 

 tissue, but also on the energy which it has absorbed. We 

 have seen that when the sum total of independent stimulating 

 factors present in the plant is adequate to raise its tonic condi- 

 tion above par, then rhythmic activity is initiated or renewed. 

 Among these factors are, as has been shown : (a) a proper 

 condition of turgidity ; (b) favourable temperature ; and (c) 

 that previous absorption of energy of light which determines 

 what we may distinguish as the phototonic condition. 



The fact that, besides turgor, a certain amount of energy 

 is also necessary to initiate growth has been fully demon- 

 strated in a previous chapter. Taking now the case of the 

 dark-rigored plant, we see that the arrest of its growth is due 

 to a deficit of absorbed energy — in this instance, phototonus. 

 Under these conditions, the renewed exposure to light would 

 be sufficient, by supplying the missing factor, to re-initiate 

 rhythmic activity and consequent growth. The drooping of 

 the pileus of Coprinus in darkness is another expression of 

 the sub-tonic condition of the plant. It must be remembered 

 that the suctional activity which determines turgor is itself 

 dependent on the rhythmic activity, and therefore on the 

 tonic condition, of the plant. The critical element of this 

 tonic condition may in certain cases be the absorption of 

 light. I have noticed a similar drooping in flowering plants 

 kept in the dark for a long time. Exposure to light, restoring 

 the tonic condition, is in such cases, as also in Coprinus, 

 sufficient to restore the normal turgidity of the plant. 



In connection with this, I may draw attention to the fact 



