392 Ewart . — The Action of Cold and of 
cells take only a minute or two longer to bleach than normal 
living ones do. Hence it appears that the difference is due 
to the fact that in the dead cell light exercises a photo- 
chemical action upon a dead inert substance, whereas in 
the living cell it acts upon a working vital mechanism 
composed of substances which are continually undergoing 
change, and causes katabolic and oxidatory changes to pre- 
ponderate over the anabolic ones. It must be remembered 
that there is no necessary connexion between the bleaching 
of the chlorophyll and the death of the chloroplastid, and 
it may be found possible, under certain circumstances, to 
completely bleach chloroplastids without killing theimA As 
a matter of fact, cases have been observed in Elodea in which 
the chloroplastids were almost entirely bleached, but yet 
remained living and capable of recovery. 
Even assuming that the rate at which the chlorophyll is 
decomposed in the presence of oxygen is directly pro- 
portionate to the intensity of the light, it does not necessarily 
follow that in unconcentrated sunlight decomposition is also 
more active in the living chloroplastids than it is in dead 
ones. A living end-cell of Chara is, however, bleached by 
6-8 hours’ exposure to continuous sunlight (6-7 minutes 
when exposed to sunlight 10 times concentrated), whereas 
an end-cell killed by chloroform is bleached under similar 
conditions in about 2 hours, but when exposed to 10 times 
concentrated sunlight 12-15 minutes’ exposure is required. 
It seems therefore justifiable to conclude that during the 
period of exposure to direct sunlight the living chloroplastids 
had formed at least five times as much chlorophyll as they 
originally contained. Without attaching too much impor- 
tance to these calculations, they are nevertheless interesting 
as showing the perpetual change to which the living chloro- 
plastid is subject when exposed to light. Under prolonged 
exposure to the same intensity of sunlight, old chlorophyllous 
cells bleach much more rapidly than young but fully 
grown cells do ; whereas in concentrated sunlight, in which 
the bleaching is produced in a few minutes, no marked 
