MADDEN ON DEAINAGE, 169 



this point Ave want much more satisfactory evidence than we 

 yet possess. 



Dr. Madden' s views of the effects of drainage may be quoted in con- 

 nection with, this subject (see his prize essay). 1. One great evil pro- 

 duced by an excess -oi water in soil, is the consequent diminution in the 

 quantity of air within it; which air we have proved to be of the 

 greatest consequence, not only in promoting the chemical changes 

 requisite for the preparation of the food of plants, but Kkewise to the 

 roots of the plants themselves ; for Saussure and Sir H. Davy have 

 proved that oxygen and carlaonic acid are absorbed by the roots ; which 

 gases, however, especially the former, can be conveyed to them only by 

 the air. 2. An excess of water injures soil by diminishing its tempe- 

 lature in summer and increasing it in winter — a transposition of nature 

 most hurtful to perennials, because the vigour of a plant in spring 

 depends greatly upon the lowness of temperature to which it has been 

 subjected during winter (within certain limits of course), as the difference 

 of temperature between winter and spring is the exciting cause of the 

 ascent of the sap. 3. The presence of a large quantity of water in soil 

 alters the result of putrefaction, by which some substances are formed 

 which are, in all probabilityj useless to plants, — such, for example, as 

 carburetted hydrogen, — and diminishes the proportion of more useful 

 ingredients, as ulmio acid. 4. An increase in the proportion of fluid in 

 soil has a most powerful effect upon its saline constituents, by which 

 many changes are produced diametrically opposite to those that take 

 place in soil where the vrater is in much less quantity; and in this 

 manner the good effects of many valuable constituents are greatly 

 diminished, as, for instance, the action of carbonate of ammonia upon 

 humus, and of gypSum upon carbonate of ammonia. 5. The directions 

 of the currents which occur in wet soil are entirely altered by drainage; 

 for whereas in undrained soil the currents are altogether from below 

 upwards, being produced by the force of evaporation at the surface, and 

 consequently the spongioles of the plants are supplied with exhausted 

 subsoil water, when land is drained the currents are from the surface 

 to the drains, and the roots are consequently in this manner supplied 

 •with fresh aerated water. Lastly, an excess of water in soil produces 

 a constant dampness of the atmosphere, which we have shown to be 

 injurious to plants in three distinct ways : — ^1. by diminishing evapora- 

 tion, and thus rendering the process of assimilation slower; 2. By 

 diminishing the absorption of the carbonic acid, and thus lessening the 

 atmospheric supply of food ; 3. By creating a tendency in the plant to 

 produce leaves possessing a different structure from those which the 

 same plant produces in dry situations. Thus we have sis distinct 

 methods in which an excess of water in soil has been proved to be 

 greatly injurious to plants. 



