IRRITABILITY. 441 



Now that we have concluded our consideration of the 

 directive influence of light upon growing radial organs, we 

 turn to the study of the phenomena presented by bilateral 

 organs. It will be convenient to consider separately the 

 phenomena presented by isobilateral organs, and those pre- 

 sented by dorsiventral bilateral organs. 



With regard to the former, of which the flattened leaves of 

 some species of Iris and Xyris may be taken as examples, it 

 will suffice to say that they are positively heliotropic. 



The characteristic position of dorsiventral organs is such 

 that, under normal conditions, their long axes are inclined at 

 a greater or a smaller angle to the vertical. This position is, 

 like that of all plant-organs, a resultant one, the resultant 

 effect of the action of inherent tendencies and of external 

 influences, and the problem now before us is to ascertain 

 the extent to which light takes part in inducing this 

 position. 



It has been shewn in many cases that light is an important 

 factor. Rauwenhoff, in particular, has called attention to the 

 fact that whereas in a large number of plants the lateral branches 

 are directed obliquely under normal conditions, in etiolated 

 specimens they are more or less nearly vertical. Frank, again, 

 has observed that the shoots of Lysimachia Nummularia, 

 Polygonum aviculare, Panicum Crus-Galli and many other 

 Grasses, A triplex latifolia, Chenopodium polyspermum, Matri- 

 caria Ckamomilla, and others, run horizontally along the 

 surface of the soil when the plants are growing in sunny 

 localities, whereas they grow erect when the plants are shaded 

 or are kept in darkness. He has observed the same thing in 

 the branches of thalloid Liverworts, such as Marchantia, which 

 under these circumstances are narrow and channelled on the 

 upper surface. Vochting has made similar observations on 

 the peduncles of Erodium cicutarium and of Taraxacum 

 officinale, and Wiesner on the runners of Fragaria vesca and of 

 Glechoma hederacea. This peculiar relation to the intensity of 

 light is strikingly illustrated in Vaucheria. Stahl has found 

 that in weak light the shoots of this plant are positively helio- 

 tropic, that is, that their apices are directed towards the source 



