IMPORTANCE OF DRAINAGE. 385 



altogether upon the artificial supply of moisture which 

 it furnishes to the plant. " The mechanical action of 

 the irrigatory current of water, in exercising the plants, 

 strengthening their organisms, keeping their stems and 

 root crowns clear of obstruction, promoting the equa- 

 ble circulation of water and oxygen around them, and 

 causing an equable distribution of the soluble materials 

 of their food, probably plays a considerable part in irri- 

 gatory fertilization. The differences of effect, from the 

 mere circumstance of flowing or stagnation of the water, 

 are prodigious ; for, while flowing water coaxes up the 

 finest indigenous grasses of the climate, and renders 

 them sweet, and wholesome, and nutritious, and luxu- 

 riant, stagnant water starves, deteriorates, or kills, all 

 the good grasses." 



The effect which surface irrigation produces on the 

 nutritive qualities of the grasses may be seen by 

 reference to the tables of analyses found in a pre- 

 ceding chapter. 



But, if one thing more than another may be said to 

 lie at the foundation of all real improvement of grass 

 lands, or lands under a course of rotation, it is a proper 

 system of drainage. Especially is this important for 

 low, wet lands, since it not only frees them from super- 

 fluous water, thus making them more susceptible of 

 tillage in early spring, but actually increases their tem- 

 perature several degrees, in some cases as much as 

 from eight to ten, and rarely less than from two to four, 

 and admits the air to circulate more freely around the 

 roots of the plants. The aquatic grasses require large 

 and constant supplies of moisture, and when the soil is 

 changed by drainage the more valuable species of grass 

 may be introduced and cultivated in it. 



With regard to the management of salt marshes, 

 though they cannot be under-drained, there are few 

 33 



