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- 

 phyllous 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 



