28 THE SOILS AND CROPS OF THE FABM. 



The degree of heat necessary varies much with dif- 

 ferent plants. Some seeds will start growth at a tem- 

 perature but little above the freezing point; others 

 may need 60 deg. F. The quantity of moisture nec- 

 essary, also varies considerably. There may be 

 enough to cause the growth to- commence but not 

 enough to contiuue it. If the seed is deeply buried, 

 the food supply contained in it may be exhausted be- 

 fore the plumule reaches the surface. Usually there 

 is no good reason for covering seeds more deeply than 

 is sufficient to secure the needed supply of moisture. 

 Deep covering of small seeds especially, is not desira- 

 ble. 



The order of growth and development of most 

 plants is much the same, with variations depending 

 upon whether they are aMni^aZs; that is, plants like 

 corn or wheat, which complete their growth and die 

 within a year; biennials, which live two years, produc- 

 ing seeds only the second year, or perennials, which 

 live several or many years, sometimes producing seeds 

 each year and sometimes only rarely. 



In each class there is first a considerable growth 

 of both roots and leaves. With annual plants there 

 is next a rapid growth of the seed stalk; then bloom- 

 ing and the production of seed. With biennial plants 

 there is a storing up of plant food in the roots, stems, 

 thick leaves or tubers, near the close of the first year's 

 growth. The second year a seed stalk is produced, 

 beariag sefid largely made up from the food stored up 

 the first year. B9ets and cabbages are good illustra- 

 tions of this class of plants. 



In the young plants the roots furnish a larger 

 percentage of food than after growth is further ad- 



