304 [Assembly 



This principle, sir, needs no farther explanation. Long expe- 

 rience has fully established the facts. The severe test of the pre- 

 sent season has resulted in another complete triumph of the sys- 

 tem, and all that is necessary is that it should become well known 

 to all. 



When we have a Department of Agriculture at Washington, 

 sir, (which I believe we shall finally succeed in obtaining,) we 

 may hope to be secure against the ravages of drouth, for then the 

 necessary information may be disseminated in a manner to be 

 accepted by our farmers. 



Mr. Maxwell — The law authorizes you in a reasonable use of 

 streams of water running through your land. 



Prof. Mapes — It is often asked what is the action of drought on 

 plants. As drought is the absence of water, as cold is the ab- 

 sence of heat, we should first examine of what advantage is water 

 to plants, for on the absence of these advantages depend of course 

 the influence of drought. Water then may be called the lubrica- 

 tor of the plant, swelling its fibres, causing them to be supple. 

 A large majority of the weight of most plants is water. It is a 

 communicating medium for much of the pabulum of the plant. 

 Gases when entering the lower part of the capillary tubes of 

 plants may rise through water, and be appropriated during their 

 passage ; but in a plant too dry for the full exercise of its func- 

 tions, no such appropriation can take place. 



Even the outer surface of plants cannot be in a healthy condition 

 without the presence of so much water as wall prevent their ter- 

 mini from losing their organism, and ceasing to assist in the fur- 

 ther development of the plant. A certain amount of water is re- 

 quired to be evaporated from the surface of plants for the abstrac- 

 tion of the excess of heat; for as all substances render present 

 heat latent by the increase of their bulk, so the water evaporated 

 from the surface of a plant by increasing itsbulk 1,700 times, is 

 capable of rendering sensible heat latent, and does so by abstract- 

 ing the sensible heat from the plant. One's head may be cooled 

 by first wetting and then fanning, and the same facts are con- 



