^ BIOLOGY OF PLANTS. 



food containing that substance, in a form in which they can 

 assimilate it, as in ammonia, nitric acid, crenic and apocre- 

 nic acids. Some plants, as wheat, require potash and phos- 

 phates. All kinds of grasses require silicate of potash. Sea- 

 plants require soda. And, generally, plants require differ- 

 ent substances to enable them to develope their organs. So 

 also wheat requires more alkalies in quantity than barley or 

 oats. Saussure found that wheat requires different quantities 

 at different periods of its growth. The same fact has been 

 observed of other plants.* 



The absolute necessity of supplying plants with appropriate 

 nourishment, of nutriment derived from animal and vegetable 

 manures, has been proved, by the most carefully conducted 

 experiments, whatever be the particular form in which the 

 food is presented, whether as carbonic acid, water, ammonia 

 and saline compounds, or, in addition, as geates or humates, 

 crenates and apocrenates. A continued course of cropping 

 will exhaust the soil, both of vegetable matters and salts, and, 

 unless they are restored, it will become in time, wholly bar- 

 ren ; and in proportion as these matters are wanting, or in a 

 state unfitted to enter the organs of plants, will the soil become 

 sterile, its productions scanty, and of an inferior quality. 



3. Tillage. The third general condition necessary to the 

 growth of plants, is proper tillage. The object of tillage, is 

 to break up the entire soil, and give it such a degree of fine- 

 ness, as to render it permeable to atmospheric agents and wa- 

 ter, and to incorporate the manures with the soil ; thus to pro- 

 mote an equal and economical distribution of food to the roots 

 of plants ; to bury the seed at the proper depth ; and finally, to 

 destroy weeds, which rob the crop of food, and check its 

 growth. 



( 1 ) The soil should be thoroughly 'ploughed ; every part of it 

 turned over and stirred at a sufficient depth to allow the roots 

 of plants to extend themselves freely in every direction. If 



* For a further notice of this subject, see third chapter. 



