President's Address. 321 



lit'}!' in being decolorised than the freely-exposed and unprotected 

 parts. A simple unscreened layer of chlorophyll-corpuscles, or a 

 chlorophyll-plate, such as that of Mi'socarpus, when it lies in a plane 

 at right angles to the light, is decolorised in from one and a half 

 to two minutes. The layer on the upper wall of a Nitdla cell 

 requires in light of equal intensity five to eight minutes of insola- 

 tion. 



But the length of the cell modifies the destruction of the cell- 

 content when exposed to intense light. As the protoplasm in the 

 cells of Nitella is in constant slow streaming movement during 

 examination, any one jiortion of the protoplasm is only subjected 

 for a relatively short period to direct insolation, dependent, indeed, 

 upon the size of the insolated area as compared with the size of the 

 whole cell, and thus the protoplasm is acted upon by the light inter- 

 ruptedly. Each portion of the protoplasm, as it rotates round the 

 sides of the non-illuminated part, is protected from light, and only 

 again is acted on when it reaches the insolated area. 



In Nitella the destruction of the chlorophyll colouring matter 

 depends therefore mainly upon the depth of the cells, whilst the 

 destruction of the protoplasm is also influenced by the length of the 

 cell ; so that the immediate appearances in the local destruction of a 

 Nitella cell may be very difl'erent. 



If the cell be a long and strongly-grown one, the complete 

 decolorisation of the insolated part may occur without any trace 

 of further destruction of content. The chlorophyll-corpuscles in 

 the non-illuminated part remain normal in form, colour, and dis- 

 position. The neutral zone persists, and there is no retraction of 

 the protoplasmic utricle from the cell-wall. Eotation of the proto- 

 plasm and contained bodies shows, as a rule, no visible disturbance; 

 there may, however, occasionally be a momentary cessation of the 

 movement. 



In other cells, and specially if the insolation of a limited area be 

 rapid, death throughout the whole cell may occur long before com- 

 plete decolorisation of the insolated part, or even before the chloro- 

 phyll-corpuscles in this position show much trace of it. The first 

 striking change in such a case is stagnation of rotation, with which 

 is associated an irregular aggregation of the protoplasm at the inso- 

 lated area which hinders the movement. Upon this follows an 

 extending destruction of the cell-content. The green chlorophyll- 

 corpuscles throughout the non-illuminated portion fall into disorder, 

 lose their arrangement in rows, and swell in a manner commonly 

 seen in them when they escape from the cell ; they change their 

 polyhedral or oval form and become, by the absorption of water, 

 transformed into vesicles such as Gtippert and Cohn describe, and 



