SEEDS AND SEEDLINGS 161 
of seeds apparently serve, in some cases, as aids in their dis- 
persal and in others as means of preventing the seed from 
being eaten by animals. 
152. Conditions for germination. .\ sound, live seed will 
germinate, or sprout, when suitable conditions are present. 
The requisites for germination are 
1. Proper temperature. 
2. Enough moisture. 
3. Aur or oxygen.1 
4. Seed coats permeable to air and moisture. 
The temperature most favorable to germination varies with 
the kind of seed: for any given kind there seems to be a 
lowest limit, a most favorable (eptinwn) temperature, and a 
highest limit. 
Wheat and barley, for example, will sprout at temperatures 
but little above the freezing point, though they succeed best at 
about 84° F. Muskmelons and cucumbers sprout at 60° F. but 
begin much more promptly at temperatures of 93° and upward. 
Most farmers have learned by experience that the temper- 
ature requirements are not the same for all kinds of seeds. All 
know, for example, that if corn is planted before the ground is 
warm enough, it will decay and have to be replanted, but that 
peas can be sown very soon after the frost is out of the ground. 
There is moisture enough in a few kinds of seeds, like those 
of the willow and the poplar, to allow them to begin to ger- 
minate as soon as they are ripe; but most seeds need to be 
supplied with moisture from without. Too little moisture 
causes them to germinate. very slowly, as is often noticed 
during spring droughts, while immersing them in water causes 
many kinds to rot because the air supply is not sufficient. 
The germination of seeds planted too deep in clay soils is 
very likely to be hindered by lack of air. In warm, open 
soils there is usually air enough; the danger here is that the 
seeds may dry up because of too shallow planting. 
1Some seeds begin to germinate without air, but soon die unless it is 
supplied to them. 
