PBACTICAL IKFEEE^CES. 109 



substance on some other plant or plants, growing in as- 

 sociation with them, is greater than the direct mischief. 

 The manures act very differently on different plants, and 

 vary in their action, even in the same species, according 

 to the time and stage of growth at which they are em- 

 ployed. Some encourage the growth and development 

 of their cellular tissues, at the expense of the woody and 

 fibrous constituents, others favor the consolidation of 

 the tissues, hasten the flowering period, and bring about 

 an increased production of seed. But any change that 

 may be induced is of a physiological kind, affecting the 

 development of the individual, not the character of the 

 species. By no combination of manurial elements is it 

 possible to bring about that kind of change which a 

 naturalist would consider specific. 



CHAPTER VIII. 

 PRACTICAL INFERENCES. 



Objects for which plants are cultivated, and the means of promoting 

 them. Plants cultivated for their roots for their foliage for their 

 fibre for their seeds. Farming operations as aids to propitious cli- 

 matal influences and as counteracting the evil effects of injurious 

 ones. Drainage. Tillage. Manures. Change and variety of crop- 

 ping. Rotation. Improvement of cultivated plants. Selection, 

 Change of seed. Cross breeding. 



Having in the preceding chapters given an outline of 

 the life-history of the plant, the machinery by which it 

 is carried on, the manner in which that machinery fulfils 

 its purpose, and the contest and competition in which 

 living plants are always engaged, it may be well to indi- 

 cate some of the points in which the history so outlined 

 affects the practice of agriculture. Of course, were 



