120 THE VEGETABLE KINGDOM. 



and act, in the heart of hot countries, as inland seas, giving 

 out clouds of vapour, which ascend and are condensed in the 

 form of rain ; in this way the surface of the earth is cooled, 

 the fluid (far away from its proper source, the ocean) is 

 economised and fertilises a greater space of surface. Ve'ge- 

 tation also acts in preserving the surface of the earth from 

 those ravages which both wind and rain would otherwise 

 effect ; the great deserts have been levelled by these causes, 

 the earth not being of a quality to support vegetation. 

 Cattlin says, that in North America he frequently met with 

 great conical-shaped mounds of earth devoid of vegetation, 

 and which were fast being levelled by the rains, at every fall 

 of which gullies were formed on their sides, down which the 

 mud poured in streams ; now, had these been of a quality 

 to support a covering of grass, their size and form would 

 have remained unchanged for centuries. Railway embank- 

 ments are often sown with grass seeds, to prevent injury to 

 them, especially where this covering is not likely to come 

 soon, as in the vicinity of large towns. 



Thus it is seen that the vegetable kingdom far exceeds 

 the animal in quantity, that it is everywhere distributed, 

 that it affords nourishment and shelter to animals, preserves 

 the air fit for respiration, keeps down the excess of heat, and 

 preserves the form of the surface of the earth. 



MODE OF GBOWTH AGENTS AFFECTING CONSTITUTION 



AND POSITION IN CEEATION. 



The green colour of plants is due to the formation and 

 deposition in their cells of a peculiar compound called 

 " chlorophyll," and it is ascertained that this compound is 

 produced in the plant in consequence of its exposure to 

 light, for no plant grown in the dark is green, but will 

 become so when light is admitted. This is shown in many 

 cultivated vegetables, as celery, lettuces, &c., in which by 

 banking up the celery, or tying together the lettuce, the 

 light is excluded, and the stalks of the one and the heart- 

 leaves of the other become mild and white, whereas the 

 same grown in a state of nature are so green and rank 

 as to be almost poisonous. 



