PRACTICAL INFERENCES, 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 
