President's Address. 319 



with isolated clilorophyll-corpuscles, some or a single corpuscle may- 

 be acted upon. 



But, besides destroying the colouring matter, the light affects the 

 cell-contents also, and especially those elements concerned in respira- 

 tion and nutrition. In Spirogyra, which, in consequence of its 

 organisation (with the nucleus embedded in a central plasma from 

 which threads radiate to the periphery, and with a granular motion 

 in the protoplasm between the chlorophyll bands), is very suitable 

 for study, either before or after complete decolorisation of the 

 bands, an extending destruction of the cell-contents is observed. 

 The motion of granules in the protoplasm of the cell-wall ceases. 

 The threads of protoplasm contract and pull thereby the middle 

 part of the coils of the chlorophyll-bands deej) into the lumen of 

 the cell, and at the same time a portion of their substance withdraws 

 to the central plasma-mass, which swells into a large vesicle with a 

 distinctly double-contoured membrane of considerable thickness. 

 Then the threads rupture, and the proximal ends remain as frag- 

 ments attached to the central plasma, whilst the distal portions 

 adhere to the amylum bodies. Whilst these changes are progressing 

 there appear upon the threads small, defined, vesicle-like formations, 

 which are undoubtedly distinct morphological structures, and in the 

 normal uninsolated cell are occasionally seen, as before mentioned, 

 at the forks of the threads. These plasma-knots, as they may be 

 called, which are also to be seen in Spirogyra cells killed by other 

 means, are withdrawn with the substance of the tlu-ead to the 

 central plasma, and coat its surface, often in great numbers. 

 Through the contraction and rapture of the threads the nucleus is 

 pushed from its central position and displaced to a greater or less 

 extent, always surrounded by the central plasma with its coating of 

 plasma-knots. It, along with the nucleoli, retains its form, but 

 sometimes^ at least in one-spired species of Spirogyra, appears to 

 have lost portion of its substance, whilst in other cases it becomes 

 granular in aspect and acquires a red tinge. The turgescence of the 

 cell is markedly affected, and alterations in the substance of the 

 layer of protoplasm lining the cell- wall, which are chiefly recognisable 

 by colour changes, occur. 



Many of these effects in the protoplasm are familiar as appearances 

 connected with death of the cells frbm other causes, e.g., from heat 

 or mechanical injury ; but in such cases the changes are not identical 

 in their whole course with those just described. Here the effects 

 are the result of a definite photochemical action, and many of them 

 are specific of death from light, although they are combined with 

 the general changes in the protoi)lasm, which appear ai'Ler death 

 from any cause. 



