320 President's Address. 



Effects similar to those seen in Spiror/>/ra occur, with modification 

 dependent upon organisation, in other Algce, with so-called formless 

 chlorophyll-masses ; for example, in Mesocarpus, (Edogonium, Dra 

 parnaldia, Vaucheria, &c., always varying in extent, up to complete 

 death of the cell, according to the duration of exposure to light. 

 Also in cells with definite chlorophyll-corpuscles, such as occur in 

 Characece, Musci, ferns and phanerogams, like results obtain. 

 Amongst these, Characece are most favourable for investigation on 

 account of the power of recovery of their cells even after consider- 

 able injury, the structural relationships of these, and especially their 

 great length, which permits of a small portion of one cell being 

 insolated. 



If a portion, '3 mm. in diameter, of a NitelJa cell, which is about 

 1 mm. long, and not too thick, be insolated, it is more or less com- 

 pletely decolorised in a few, usually three to eight, minutes. 

 Quite independently of the decolorisation, except in so far as the 

 chlorophyll colouring matter acts as a screen, the destruction of the 

 cell-contents begins, and may extend until the wdiole content is 

 killed. It is possible, however, to regulate the action so that death 

 of the cell shall precede complete decolorisation of the insolated 

 area, or shall succeed it, or that the only effect shall be the destruc- 

 tion of the colouring matter without injury to the life of the cell. 

 This depends, in the first instance, upon the specific energy of the 

 plant, but also ujion the depth and the size of the cell. 



In deep plant cells, such as those of adult leaves, or the inter- 

 nodal cells of Nitella, which are lined with a dense persistent layer 

 of chlorophyll-corpuscles, the depth of the cell and the j^rotection of 

 the layer of chloroj)hyll-corpuscles have great influence in modifying 

 the action. The cell in such a case lies with its lower, or an ideal 

 median, or its upper surface in the plane of most intense illumina- 

 tion, and the parts above or below such plane must be affected in a 

 different degree. The layer of chlorophyll-corpuscles on the lower 

 surface of the cell will protect in great measure the portions above 

 so long as the chlorophyll colouring matter is not destroyed, and in 

 like manner the centre of the layer of chlorophyll-corpuscles on the 

 upper wall of the cell will be sooner decolorised than the sides of 

 the cell, where many rows of chlorophyll-corpuscles lie over one 

 another. In Sjiirogi/ra like circumstances have to be considered, 

 but only in small degree even in the species with deep cells, because 

 of their small diameter, and also because the lower flexures of the 

 bands afford but a small protection to the upper. Yet one finds 

 that the points on the upper flexures where the lower flexures cross 

 them, especially at the edges of the cell-wall, and the spots where 

 the bands commence to bend on the lower surface of the cell, are 



