632 GERMINATION AND DORMANCY 



Failure of seeds to sprout does not necessarily mean that they are dormant. 

 Environmental conditions may be unfavorable for germination. The water 

 supply may be inadequate, or the temperature may be unfavorable. Deeply 

 buried seeds are often prevented from germinating by an inadequate oxygen 

 supply, or certain kinds of light-sensitive seeds may fail to germinate because 

 of unfavorable light conditions. The term dormancy as applied to seeds is 

 generally restricted to those which fail to germinate as a result of internal 

 causes. For convenience we shall use the term quiescence as a designation 

 for the situation in which failure of a plant organ to grow is due to environ- 

 mental conditions. In practice it is often difficult to determine whether seeds 

 or other plant organs under natural conditions are actually dormant or merely 

 quiescent without resorting to an experimental test. 



Dormancy of seeds is due to one or a combination of several different 

 factors (Crocker, 191 6) : 



1. Seed Coats Impermeable to Water. — The seed coats of many species 

 are completely impermeable to water (and probably also to oxygen) at the 

 time the seeds are ripe. This condition is very common in the seeds of many 

 legumes (clovers, alfalfa, black locust, honey locust, etc.), of the water lotus, 

 and of the morning glory. Germination fails to occur until water penetrates 

 through the seed coats. In many such seeds the permeability of the coats to 

 water increases slowly in dry storage but it occurs more rapidly when they 

 are exposed to the fluctuations of temperature and moisture that are present 

 in soils under natural conditions. The action of bacteria and fungi also in- 

 creases the permeability of the seed coats to water and so shortens the dormant 

 period of seeds of this kind that are buried in the surface layers of soil. 



2. Mechanically Resistant Seed Coats. — The seeds of some of the com- 

 monest weeds such as mustard {Brassica sp.), pigweed {Amaranthus sp.), 

 water plantain (Jlisma), shepherd's purse {Capsella) and peppergrass 

 (Lepidium), remain in the dormant condition because the seed coats are strong 

 enough to prevent any appreciable expansion of the embryo. In the seeds of 

 the pigweed (Amaranthus retroflexus), for example, water and oxygen pene- 

 trate through the seed coats readily, but the enlargement of the embryo is 

 limited by the mechanical strength of the seed coats. As long as the seed 

 coats remain saturated with water, dormancy persists, and may last for a 

 period of thirty years or even longer. If the seed coats become dry, however, 

 certain changes occur in the colloidal compounds that make up the walls of 

 the cells of the seed coats so that upon being again saturated with water 

 they are no longer able to resist the pressures developed by the imbibitional 

 forces in the embryo. The coats are ruptured and germination occurs. High 

 temperatures (above 40° C.) may also induce some germination of pigweed 



