REPRODUCTIVENESS AND SEED GERMINATION 391 



{dormancy) is generally due, as Crocker and his students have 

 shown, to one of six causes : 



1. The seed coats are too hard for the growing plant to break 

 open, and they permit the entrance of water, air, etc., in only 

 small amounts. In the soil, freezing and thawing, along with the 

 action of soil organisms and the chemical action of the soil solu- 

 tions, soften the coat, and the seed germinates. Among seeds of 

 this class are the pigweed (Amaranthus) , shepherd 's-purse (Cap- 

 sella), peppergrass (Lepidium), water plantain (Alisma), and 

 blackberry (Rubus) . 



2. The seeds are impermeable to water. Such "hard' seeds, 

 when placed in water, do not swell quickly but only after a long 

 time, owing to the presence on the outside of a hard, varnishlike 

 film, which is impermeable to water. Among the common seeds 

 in this class are many legumes such as sweet clover, red clover, 

 alfalfa, and lupine. Red clover seeds may remain alive and hard 

 for several years when placed in water, but any treatment such 

 as scarifying (scratching), breaking, or removing the coat will 

 result in immediate germination; and if shaken with sharp sand, 

 90% will germinate at once. Treatment with acids and alkalies 

 may have the same result in softening the coat. Sweet clover 

 and alfalfa seeds (air dry) were made permeable without in- 

 jury by freezing in liquid air (at - 190° C:) for several days 

 (Busse, 1930); and Davis (1928) increased by 50% the ger- 

 mination of these same seeds by exposing them to an hydraulic 

 pressure of 2,000 atmospheres at 20° C. and then drying before 

 exposing to germinating conditions. In the soil, freezing and 

 thawing, combined with the chemical action of the soil solu- 

 tions, ultimately soften or break these seed coats and permit 

 germination. 



3. The seeds are impermeable to* oxygen. One of the best ex- 

 amples of seeds of this type is the cocklebur (Xanthium). The 

 fruit or bur contains two seeds with different permeabilities. 

 Both seeds require oxygen but the upper one requires about 35% 

 more than the lower. The result is that in nature the lower seed 

 will germinate the following spring after the fruit is ripe, but the 

 upper seed does not receive enough oxygen till after a year has 

 passed. Both seeds will germinate at once if the seed coats are 

 ruptured so that gases may enter freely. Pisum sativum and 

 Johnson grass (Sorghum halapense) have seeds which behave 



