IRRITABILITY. 383 



though it must be taken into account in endeavouring to 

 explain the fact. 



Finally, Prantl ascribes the smallness of the leaves to an 

 unhealthy condition induced by the continuous darkness ; he 

 regards it, in fact, as a pathological phenomenon. But this is 

 no explanation. The smallness of the dorsiventral leaves is 

 not more of a pathological phenomenon than the excessive 

 elongation of the internodes ; if the one is a pathological 

 phenomenon, then so is the other. And why should radial 

 or bilateral leaves become excessively elongated in darkness ? 

 The question still remains why unhealthiness should express 

 itself, in the one case, in arrested growth, and in the other, in 

 excessively active growth. 



We shall best obtain, not perhaps a full explanation, but a 

 suggestive insight into the nature of this phenomenon by a 

 consideration of the resumption of growth under the influence j, ..L 

 of light Exposure to light, as mentioned above, restores to- ' 

 leaves which have been long kept in darkness the condition of 

 phototonus ; it enables them to grow and to respond to the 

 stimulating action of light ; this is probably also true with 

 regard to their power of responding to other stimulating agents. 

 We may attribute, then, the effect of prolonged absence of light, 

 or of exposure to too intense light, like the effect of exposure 

 to extreme temperatures, to a destruction of either the irrita- 

 bility or the motility of the protoplasm of the growing cells ; 

 probably the latter is the more important factor in producing 

 the result. We cannot, however, in any way account for 

 the difference of behaviour in this respect between stems and 

 leaves. 



Before leaving the subject we may briefly notice the facts 

 which have been observed with regard to the behaviour of 

 young etiolated leaves on their exposure to Jightv* Detmer 

 found that on exposing young etiolated seedlings of Phaseolus 

 and Cucurbita for short periods to light of a certain intensity, 

 they resumed their growth and exhibited distinct epinasty. 

 He explains this by assuming that light induced a more rapid 

 growth of the upper surface, and he speaks of this induced 

 epinasty as photo-epinasty. It must be remembered, how- 



