121 



harvest increases with increasin<i: water supply up to a definite limit, 

 beyond which the hai-vest diminishes steadily for any further increase 

 in the water sup{)ly, until when the earth is completely saturated with 

 water the harvest in some cases becomes almost nil. The most, 

 advantageous percentage of moisture in the soil varies foi" the differ- 

 ent plants, depending on their own method of using the water, on the 

 evaporation from their leaves, and on the number of j^lants to the 

 unit of area of the field, namely, their closeness to each other. 



In reference to the needs of practical agriculture it would be 

 improper to consider in such experiments as these only the water 

 that has been used, since the number of plants to the miit area is of 

 equal if not greater importance. It would therefore be improper 

 to reason from these experiments up to the needs of another field or 

 tub having a greater or less plant density. Again, as also shown by 

 AVollny, more water is used in proportion as more nutriment is avail- 

 able in the ground, because the development of the organs of tran- 

 spiration or the leaves is thereby increased. Therefore, in general, 

 the quantity of water required to attain the maximum crop will 

 increase with the richness of the soil and the closeness of the plants as 

 well as the dryness and velocity of the wind. For different crops, 

 moreover, the absolute quantity of water wall depend upon the dura- 

 tion of the whole process of vegetation, from germination to harvest, 

 (See Wollny, 1881, IV, p. 109.) 



The character of the plant affects the quantity of necessary water, 

 not only by the duration of the process, but by the relative quantity 

 of auxiliary organs that the plant develops in order to produce the 

 ripened seed, which we call the harvest. The ratio of the grain to the 

 straw and chaff when the maximum crop of grain is produced in 

 each of Wollny's seven cases is shown in the following table : 



Ma.rirniuti liarrrst (IrlciJ in air. 



These percentages show the success with which the plant labors 

 to perpetuate its species with the least possible waste of molecular 

 energy on extraneous matters. 



