NOTES AND ABSTRACTS. 



263 



The extent to which the phenomenon of hght convergence is simply 

 a result of cell-turgor and not an adaptation to light perception cannot 

 be definitely determined, but it is suggested that the curvature of the 

 lens cells of the epidermis may be found to bear some relation to the 

 thickness of the cell-wall and cuticle. It is possible that this turgidity 

 may be the starting-point for an adaptation to (1) either light perception, 

 or, as Haberlandt suggests, to (2) the more efficient illumination of the 

 chlorophyll grains, or (3) both; but the evidence is not very conclusive. 



The papillate epidermal cells of petals exhibit a very pronounced 

 convergence of light, with a clear differentiation on the basal wall of a 

 central bright area. It is only in very few leaves, where the cells are 

 highly papillate or where there is a well-marked local thickening of the 

 cuticle, that we get the differential illumination of the basal wall required 

 by Haberlandt 's hypothesis. In some it is not visible at all under any 

 conditions, in others only when a small stop is used, and in a large 

 number of leaves (probably the majority) there is no differential illumina- 

 tion as defined by Haberlandt. 



The experiments which have been made upon the elimination of the 

 lens function by submerging the leaves in water, or by covering them 

 with a layer of paraffin oil, have given results which are so contradictory 

 and unsatisfactory that a much more complete investigation is necessary. 

 In a few special cases, the lens cells appear to bring about a concentra- 

 tion of the light on the chlorophyll grains. In some leaves the general 

 arrangement of the lens cells with respect to the chlorophyll grains seems 

 to indicate that they are effective in promoting a more efficient illumina- 

 tion of the chlorophyll grains. Haberlandt suggests that the stimulus 

 may be brought about by the difference in pressure exerted by the light 

 upon the cytoplasm ; but this is so very slight that it is hardly probable 

 it can be effective. 



There seems to be no good reason why the epidermal cells should be 

 the percipient cells more than the chlorophyll-containing cells, except 

 that the presence of chlorophyll would interfere with the incidence of 

 the light upon the percipient protoplasm. There is, however, some 

 evidence that the perception of light is bound up with its absorption 

 by the chlorophyll grains, in which case the palisade cells would be the 

 percipient cells, and the chlorophyll grains with the cytoplasm in con- 

 nexion with them the actual percipient organs. The evidence for this 

 is as follows : The heliotropic response depends mainly upon the 

 quality of the light and not upon its intensity ; the rays which are active 

 are those which are absorbed by the chlorophyll — of these the more 

 refrangible rays are the most important ; if it were merely the intensity 

 and not the quality of the light there seems to be no reason why the red 

 and yellow rays should not be just as active as the blue and violet rays ; 

 in the more refractive half of the spectrum the amount of light absorption 

 is greatly in excess of that required for assimilation ; when a chlorophyll 

 screen is interposed between the leaves and the light the heliotropic 

 response either ceases altogether or is much reduced; in motile 

 organisms, such as Euglena, the heliotactic response is bound up with 



