18 EVOLUTION OF PLANTS 
pregnated with sulphur and other mineral substances 
usually inimical to plant life. 
When in a dormant condition, the protoplasm is able 
to resist much greater extremes both of heat and cold 
than is possible while it is in an active state. Thus 
seeds, spores, and the twigs of woody plants can en- 
dure without injury a degree of cold which would at 
once kill the protoplasm were the cells in a growing 
condition. On the other hand, the same dormant parts, 
especially spores of various kinds, can endure a com- 
paratively high temperature without injury, this being 
especially marked in the case of the spores of certain 
bacteria, which can endure exposure for several hours 
to a temperature above the boiling point of water 
without being killed. 
The amount of moisture necessary for plant growth 
also varies extremely. Water plants are quickly killed 
by exposure to air of ordinary dryness, while many 
desert plants, such as cacti, may remain uprooted 
and exposed to the hot sun for weeks without being 
killed. ‘These desert plants are provided with very 
perfect means of resisting loss of water, both by a great 
reduction of the evaporating surface through the partial 
or complete suppression of leaves, and also by the de- 
velopment of a thick, and almost impervious covering 
to the exposed surfaces. 
In all green plants the arrangement of the chloro- 
phyllons tissue is always regulated by the amount of 
light. If this is weak, the green cells are spread out 
so as to expose a large area to its action; but if the 
light is too intense the area is reduced, and the cells 
are screened by the development of more or less opaque 
