Public Parks. 35 



Although phmts may derive their food from the air, they receive 

 it mainly through the roots. The aqueous vapor., condensed into 

 rain or dew, and bringing with it to the ground a portion of carbonic 

 acid and of nitrogen or ammonia, &c., supplies the proper food of 

 the plant to the rootlets, and imbibed by these it is conveyed thi^ough 

 the stem and into the leaves where the superfluous water is restored to 

 the atmosphere by exhalation, while the residue is converted into 

 the proper nourishment and substance of the vegetable. 



The water exhaled may be again absorbed by the roots laden 

 with a new supply of the other elements from the air, again exhaled 

 and so on, as illustrated by cultivated plants in Ward's case, where 

 plants are seen to flourish for a long time with a limited supply of 

 water, every particle of which (excepting the small portion actually 

 consumed by the plants,) must repeatedly pass through this circula- 

 tion ; and here is exhibited the actual relations of water, &c., to 

 vegetation on a large scale in nature, where the water is alternately 

 and repeatedly raised by evaporation and recondensed t o such an 

 extent that what actually falls in rain is estimated to be evaporated and 

 rained down on an average ten or fifteen times a year. In this way 

 the atmosphere is repeatedly purified by the rain, and those vapors 

 washed out, which else by their accumulations, would prove injurious 

 to man and animals, and are conveyed to the roots of plants which 

 they are especially adapted to nourish. 



During intensely hot weather the effect of rain is apparent, and 

 the common saying "We have had a refreshing shower," is appre- 

 ciated by all. A shower of rain has been known frequently to stop 

 epidemics, particularly of cholera. 



The lower order of plants, such as the Lichens, Mosses, Ferns, 

 &c., which grow entirely at the expense of the air and are generally 

 found in damp and shady places, gradually form a soil or vegetable 

 mould during their life, which is increased by their decay, while the 

 successive generations live more vigorously upon this inheritance, 

 being supported partly by what they draw from the air and partly 

 from the ancestral accumulations of vegetable mould. It is in this 

 way that, what are called the useless plants create a soil which will in 

 time support the higher plants of immediate importance to man and 

 other mammalia, but which could never grow and perfect their 

 fruits if left like their humble predecessors to derive an unaided 

 subsistance directly from the inorganic mould. The harmony of 

 nature is such that it cannot be disturbed. The greater part of 



