16 FIRST PRINCIPLES OF AGRICULTURE. 



upon three conditions, first, the presence of moisture ; 

 second, of warmth, or a proper degree of temperature ; 

 and third, the access of air. The seed of the wheat plant, 

 for instance, will retain its form and remain a seed for a 

 long time if kept dry. As soon as it is enabled to absorb 

 moisture it increases in size, oxygen is absorbed from the 

 air, and heat develops ; it separates the germ, and is no 

 longer a seed, but a young plant. Seeds do not germi- 

 nate below a certain temperature, usually about 37 F. ; 

 the warmth necessary for germination is, however, differ- 

 ent for different seeds. Wheat will not germinate below 

 41 F., and corn below 49 F. There is also a certain 

 temperature above which seed will not germinate; the 

 higher limit is 108 F. for wheat, and 115 F. for corn. 

 The temperature of most rapid germination lies between 

 79 F. and 94 F. 



The presence of air is universal, and care in this 

 respect is only important when plants are grown in 

 comparative confinement. 



Duration of Life. Plants which germinate, grow, 

 and produce flower, fruit, and seed in one year, and then 

 die, are called "annuals." Wheat, rye, oats, buckwheat, 

 peas, and beans are good examples of annual plants. 

 Certain other plants require two seasons for this work; 

 these are called " biennials." During the first season, the 

 organs of growth are developed, viz., the root and leaf; 

 the second season, the flower, fruit, and seed are formed, 

 after which the plants die. Examples of biennial plants 

 are cabbages, turnips, parsnips, celery, lettuce, beets, etc. 



In order to secure seed it is not necessary that the 

 plants remain in the ground throughout the winter. Cex- 



