44$ LECTURE XVII. 



The cases which we have so far considered as to the direc- 

 tive influence of light upon leaves are such as present them- 

 selves in nature ; we will now consider the facts which have been 

 elicited by experiment. Bonnet long ago noted that most leaves 

 are so placed that their morphologically superior surfaces are 

 directed towards the sky, and their morphologically inferior 

 surfaces towards the earth, and found that whenever he altered 

 this, the normal position, the leaves resumed it by curvatures 

 or torsions. More striking demonstration of the tendency of 

 leaves to place themselves so that their dorsal surfaces are 

 directed perpendicularly to the direction of incidence of the 

 stimulating light is afforded by the observations of F. Darwin. 

 In speaking of the positions taken up by plagiotropic branches 

 under the directive influence of light, it was mentioned that it 

 is at present impossible to estimate the extent and nature of 

 their response in the absence of observations made in such a 

 way as to eliminate as far as possible any effects which might 

 be induced by other directive influences. This has, however, 

 been done to a certain extent in the case of leaves by F. Darwin 

 by observing the positions taken up by leaves under the 

 influence of light whilst rotating on the clinostat. His obser- 

 vations are, unfortunately, not quite complete, inasmuch as 

 they refer only to the curvatures, and not to the torsions, 

 performed under these circumstances. Without entering into 

 a detailed description of his experiments, it may be stated that 

 the leaves were exposed in three positions to the incident rays 

 of light: (i) so that the rays fell on the dorsal (morphologically 

 upper) surface (zenith-position) ; (2) so that the rays fell upon 

 the ventral (morphologically lower) surface (nadir-position) ; 

 (3) so that the rays fell on the margin, the apices or bases 

 being directed towards the light (lateral position). The plants 

 to which reference will now be made were Ranunculus Ficaria, 

 Cucurbita orifera (Vegetable Marrow), Plantago media. In so 

 far as the various forms of experimentation were applied to 

 these plants, it was found that under all circumstances their 

 leaves so curved as to place their dorsal surfaces more or less 

 nearly at right angles to the direction of the incident rays. 

 With the plant in the zenith-position the leaves curved either 



