POOD OF PLANTS. 119 



although one animal feeds upon others, yet these must 

 previously have derived their nourishment from vegetable 

 matter. Vegetables not only supply food properly so called, 

 but likewise that which is essential to respiration, for 

 besides separating all noxious excess of carbonic acid from 

 the air, they are an inexhaustible source of oxygen, the 

 element essential to respiration, and consequently animal 

 life. This supply of oxygen by vegetables compensates for 

 that which is consumed in respiration by animals, and 

 thereby maintains the atmosphere in a state proper to be 

 breathed. That plants do thus absorb carbonic acid and 

 give out oxygen, can be proved in the most certain manner, 

 if the green parts of a plant be placed in water holding 

 carbonic acid in solution, and exposed to sunshine ; the car- 

 bonic acid will shortly be found to have disappeared from 

 the water, while oxygen gas is evolved and can be collected, 

 its quantity being exactly equal to the carbonic acid which 

 the water contained. 



Besides furnishing food and oxygen for the nourishment 

 of animals, vegetables afford a shelter from the burning rays 

 of the sun in hot climates, not only to man, but to those 

 hundreds of wild animals whose proper home is in the forest. 

 Humboldt says he found in South America forests com- 

 posed of such close growth, that it was quite impossible 

 even for the wild animals to penetrate into them, except at 

 a few places, and that the jaguar often lives for weeks in 

 the trees without descending to the ground, preying upon 

 the monkey tribes and other animals, which are found in 

 almost incredible numbers there ; and Dr. Livingstone 

 thus describes the forests of Africa : " The forests became 

 more dense as we went north. "We travelled much more 

 in the deep gloom of the forest than in open sunlight. 

 No passage existed on either side of the narrow path made 

 by the axe. Large climbing plants entwined themselves 

 round the trunks and branches of gigantic trees, like boa- 

 constrictors, and they often do constrict the trees by which 

 they rise, and, killing them, stand erect themselves." 



These woods, as well as the grass and herbage of the 

 plains, afford an enormously extended evaporating surface, 



