8 FIRST PRINCIPLES OF AGRICULTURE. 



less than four-fifths, while vegetables and young plants 

 are frequently nine-tenths water. 



When plants are removed from the soil, and brought in 

 contact with the air and warmth, a large part of the water 

 contained in them escapes in the form of water vapor ; 

 they become what is termed " air dry." The proportion of 

 water lost in this way, and the time or rapidity of loss, 

 depend upon the moisture contained in them, and the 

 warmth of the atmosphere; the drier and warmer the 

 atmosphere, the greater the loss of water. This loss of 

 water is nicely illustrated in hay-making, the time re- 

 quired to dry or cure depending upon the kind of plant 

 and the character of the weather. 



Air-dry Plants. In temperate climates, air-dry plants 

 still contain from eight to twelve per cent of water. To 

 effect its complete removal they are heated to a temper- 

 ature of 212 F., that of boiling water, until there is no 

 further loss of weight. The portion remaining after the 

 drying is termed the "dry matter." The dry matter of 

 plants contains all the constituents of their growth; that 

 is, all those necessary for perfect growth, no one of which 

 can be removed without destroying it. 



Water, while it is essential in the growth of plants 

 and serves a useful purpose, is not a constituent in the 

 same sense as those which are contained in the dry 

 matter, since, as has already been noted, it can be removed 

 without destroying the structure and form of the plant. 



The Total Dry Matter. The total dry matter of 

 plants contains all those substances or compounds which 

 are useful for the purposes for which they are grown. 

 The constituents contained in it may also, for convenience 



