PHOTOTROPISM 123 



the stimulus to the blade secures a " fine " adjustment. 

 Among plants showing this mode of response are the 

 garden nasturtium, the hop, the mallows, and the Virginia 

 creeper. 



The recognition of the roles of blade and petiole does not 

 take us far towards an understanding of these very compli- 

 cated movements, but at present our knowledge stops here. 

 Mention must, however, be made of Haberlandt's ingenious 

 theory of the mode of perception of the light stimulus by 

 the leaf blade. He has shown that in many cases the 

 epidermal cells, by virtue of the curvature or structure of 

 their external walls, act as concentrating lenses, so that, if 

 they are brightly lit from above, a spot of light is thrown on 

 the back wall of the cell. So perfect is the lens action 

 that photographic images of external objects may be obtained 

 with these structures. He supposes that the leaf is in 

 equilibrium only when the light spot falls on the centre of 

 the cell. This is not the case when the illumination is 

 oblique, and then the leaf moves until the light falls normally 

 to its surface and brings the spot to the central position of 

 equilibrium. Ingenious as this theory is, and supported by 

 a mass of observation, it has not received much experimental 

 support. It has been shown, for example, that leaves with 

 the epiderm removed still respond, as do those which have 

 been covered with a layer of liquid paraffin, which converts 

 the cell w^alls into dispersing lenses. The most highly 

 developed type of lens was found in Fittonia Verschaffeltiiy 

 where it is a reduced trichome, consisting of a small cell 

 resting on the epidermal cell beneath ; but in two very 

 closely related species, the leaves of which are just as 

 sensitive in their adjustment to light, these cells are entirely 

 wanting, and this tells distinctly against Haberlandt's theory. 



§ 12. Orientation of Strongly Insolated Leaves 



Not all leaves are oriented so that the maximum amount 

 of light falls on the blade. Where insolation is very strong 

 the leaf may take up a position in which a minimum surface 



