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. A sound, live seed will 

 germinate, or sprout, when suitable conditions are present. 

 The requisites for germination are 



1. Proper temperature. 



2. Enough moisture. 



3. Air 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 (optimum*) 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. 



1 Some seeds begin to germinate without air, but soon die unless it is 

 supplied to them. 



