ELEMENTS OF AGRICULTURE. 
23 
equally useful to plants; for without it they could not 
grow. Air, however, is not always favorable to vegeta¬ 
tion : if stagnant, it is fatal; if too brisk (as is sometimes 
the case on high hills), it is injurious, as it causes the 
plant to transpire too freely, without yielding it anything 
in return. Air, by its weight, acts as well upon vegetable 
as upon animal economy. By its pressure upon the pores, 
it prevents the escape of the sap and blood from the ves¬ 
sels in which they are enclosed. Plants perspire mosl 
during the continued prevalence of dry winds. On the 
approach of a storm or gust, the air is particularly favor¬ 
able to the development of plants; for then it is warm, 
and holds suspended a vast quantity of vapors. The vi¬ 
cinity of cities, and generally of those places where much 
gas is generated by the decomposition of organic bodies, is 
stimulating to vegetation. 
52. Without light, germination can take place, but 
vegetation can not; or at most it will be very feeble, and 
the plant will soon perish. The flavor and color of fruits 
are due to the influence of light upon the plants producing 
them. A plant deprived of light will be pale and of little 
consistence. So, when we heap earth upon celery-plants, 
the object is to deprive them of light, that they may bleach. 
The outside leaves of a drumhead cabbage are green ; but 
the inner leaves, that are deprived of light, are white. 
QUESTIONS. 
1. How does germination take place ? 
2. What is germination ? 
3. Is the soil alone all that is necessary to the germination of plants ? 
4. What is humus ? 
5. Besides the soil, what other agents are necessary to the development 
>i plants ? 
G. What is necessary to make the germination and vegetation perfect ? 
7. Why is air indispensable to plants ? 
8. Is the air always favorable to vegetation ? 
9. When and where does air stimulate vegetation 1 
10. Is light necessary to plants ? 
