IRRITABILITY. 445 



Cucurbita) becomes expanded by means of epinasty when the 

 light falls on its lateral margin. A similar result was obtained 

 by de Vries, but he failed, apparently, to see the signifi- 

 cance of it. 



But light does, as a matter of fact, exert a directive 

 influence on dorsiventral organs. It is a matter of common 

 observation that these organs take up a definite position with 

 regard to the direction of the incident rays, such that the 

 dorsal (morphologically upper) surface of the organ is placed 

 perpendicularly to it. This has been observed by Frank and 

 others in the case of branches, and by Frank and Sachs in the 

 case of the Marchantia-thallus, but, inasmuch as the actual 

 influence of light has not been estimated by eliminating the 

 effect of other directive forces that of gravity, especially, by 

 rotation on the clinostat the further consideration of them 

 would not lead to any conclusion. The phenomena are 

 much more strikingly exhibited by dorsiventral leaves, and 

 moreover they have, in this case, been to a certain extent 

 investigated by the experimental method just mentioned. 

 Leaves of this kind take up any definite fixed light-position. 

 In the vast majority of cases, this position, when the plant is 

 fully exposed on all sides to light, is such that the dorsal 

 surface of the leaf is directed towards the sky, and the ventral 

 surface towards the earth. But to this general rule there are ex- 

 ceptions. Frank mentions the remarkable fact that in Allium 

 ursinum the fixed light-position of the leaf is such that the 

 ventral (morphologically lower) surface is the one which is 

 presented to the incident rays. In a great many cases the 

 fixed light-position is such that the surface of the leaf is placed 

 vertically so that one lateral margin is directed towards the 

 sky and the other towards the earth. 



Stahl mentions the following examples of the last-mentioned case : 



Santalacece ; species of Thesium. 



Composite ; Picris hieracioides, Cirsium arvense, lanceolatum, 



eriophorum, Silphium laciniatum, Lactuca scariola. 

 Labiatae ; Marrubium vulgare. 

 Cruciferae ; 

 Umbelliferae ; Pcucedanutn cervaria. 



