388 Ewart . — The Action of Cold and of 
contractions and expansions hour afterwards, stronger after 1 hour, 
but permanently ceasing after 5 hours. 
2. 8-10 S, after 5 min. animals killed, body greenish yellow, 
tentacles bleached. With shorter exposure temporary light-rigor may 
be produced in some of the tentacles, and by localized exposure the 
middle of the animal’s body may be killed and contracted, while the 
base and apex remain living for several hours, and capable of respond- 
ing to light, &c. Hydrae, in which the body is still greenish and 
capable of expanding and contracting, may show no perceptible 
evolution of oxygen when exposed to light. 
It appears therefore as if the plasma of a chlorophyllous 
animal is more sensitive to light than that of a plant such as 
Char a , for the animal is killed before the chlorophyll is 
bleached, whereas almost the whole of the chloroplastids may 
be bleached without producing any permanent injury upon 
the rest of the cell 1 . This is well illustrated in the following 
comparative experiments, in which the light concentrated, 
after passing through an alum-solution, had a photo-chemical 
intensity ten times greater than that of the strongest direct 
sunlight. 
Elodea canadensis . After 5 min. all exposed cells bleached and 
killed. If the end of a cell is exposed, only the chloroplastids 
stationary here bleach, and such cells may show rotation extending up 
to the bleached part but not into it, while cells at the margin of the 
exposed area with green chloroplastids may show variations and 
changes in the direction of the rotation, or the chlorophyll-grains may 
be aggregated into a rotating ball at one end of the cell, or rotation 
may even be shown in cells with partially retracted contents. 
Char a foetida. Complete bleaching requires nearly 8 min. exposure. 
After \ min. rotation may be twice as rapid, but in 4-5 min. slows 
rapidly, is very slow by the time the chloroplastids are bleached, and 
may gradually stop after the exposure has ceased, but in other cases 
may recover again. When the entire cell is exposed, complete 
bleaching almost always involves the rapid or immediate death of 
the cell. 
1 The bleaching of the chloroplastids is an oxidatory process, and less oxygen 
may reach the chlorophyll in an animal’s body than in a plant-cell. 
