208 Irrigation and Drainage 



direct lass of moisture from the wet, unmulehed soil ; but it may 

 have a tendency, as has been pointed out, to induce a superficial 

 development of roots, causing the crop to show signs of need of 

 water sooner than would be the case if a smaller number of more 

 thorough irrigations were resorted to. This is so, not only be- 

 cause the water disappears sooner from the soil, but also because 

 of the larger amount of root-pruning which results from culti- 

 vation where the roots are developed near the surface of the 

 ground. 



It is probable that a large supply of water in the soil during 

 the early stages of growth of many plants tends to develop in 

 them a possibility for using more water. In some, at least, of 

 our experiments with corn, oats, potatoes and clover, where we 

 have started with like amounts of water in the soil, and have 

 watered one set of plants every seven days while the others 

 were allowed to go without water until the soil was so far ex- 

 hausted that the plants were plainly suffering for want of mois- 

 ture, it was found that these plants not only did not use water as 

 rapidly after they were given it as did those which had been 

 watered every week, but they used the water they did have with 

 relatively greater economy. Whether this was because the plants 

 were smaller, and thus presented a smaller surface to the air and 

 sun, or whether the size or number of breathing pores per unit 

 area of foliage was actually less, cannot yet be stated ; but it 

 appeared evident that for some reason the plants which had not 

 been watered at first were later not able to use the larger amount 

 of water which was given to them, as they might have done had 

 they been more freely watered at first. 



THE AMOUNT OF WATER USED IN IRRIGATION 



It is very difficult, indeed, to get data bearing 

 upon this important subject which may be regarded as 

 in every way satisfactory and trustworthy. Nearly all 

 statistics are necessarily so general in their character, 

 t;he exact amount of land to which the water of ^ 



