PHOTOLYSIS IN LEAINA TRISULCA. 857 



average for cells set in darkness six days comes out fractionally 

 less. 



Before closing, it may be well briefly to refer to one more point 

 in connection with the present subject. In my first memoir on 

 Light and Protoplasm, it was shown-that in some cases chlorophyll 

 is apostrophised in low as well as in strong light ; so that by 

 graphically representing all grades of illumination upon the " pho- 

 trum," it is possible to mark off a space therein containing all 

 degrees of light capable of bringing positively or negatively apos- 

 trophised grains into epistrophe. This space was called the " epi- 

 strophic interval " ; but with a view of shortening this terminology, 

 it is now proposed to apply to it the term " epistrophion." The epi- 

 strophion of L. tristdca, as determined upon a brilliant day of last 

 September, is shown at Fig. 7, as the space barred with horizontal 

 lines over which the mark I. is placed. To the right of this another 

 space will be observed, shaded with diagonal lines and with the 

 mark II. over it. This contains all grades of illumination sufficient 

 to apostrophise the chlorophyll, but not of intensity equal to the 

 task of systrophising the grains ; for it the term " apostrophic 

 interval " or " apostrophion " may be reserved. At the extreme 

 right end of the photrum there is a small space over which the 

 mark III. is set. This represents all the intensities of light capable 

 of systrophising the grains during the four hours devoted to the 

 experiment, it thus appearing that even systrophe can be induced 

 in diffused light of the highest quality. Except, however, quite 

 at verge of the photrum, systrophe is not very strongly pronounced 

 after the four hours, and may concern only a small percentage of the 

 cells. This space may be designated as the " systrophic interval " 



or " systrophion." ,. . 



Lastly, a few words upon an observation of Borodin's,* that the 

 apostrophised grains of cells bounding those which contain t ho 

 bundles of raphides are systrophised in corners remote from the 

 raphides. There can be no doubt that such is indeed the rule, 

 although I have observed occasional exceptions. Now it the wall 

 abutting on a raphidal cell be longer than the others, and make a 

 wide angle with its meeting walls, it might happen that in the 

 course of their ante-systrophic movement round the cell (slow 

 rotation), the grains might acquire sufficient momentum to carry 

 them round the two corners ; but this explanation is obviously in- 

 sufficient, since it sometimes happens that the wall in contact with 

 the raphidal cell is smaller than the rest— often marked y «>-*£« 

 yet its angles may be denuded of systrophised chlorophyll, a or 

 Want of a better explanation, I am inclined to regard as the cause 

 of this curious fact the reflection of heat from the faces of the 

 crystals : this heat would clearly ma!ce its existence evident upon 

 the walls bounding the cells containing the crystals, and we know 

 that photolysis is in some way related to heat, »£*»•" 

 probability minimum, optimum and maximum points, though this 

 requires further investigation. 



* Op. fit. 



