DORMANCY OF SEEDS 631 



the seed coats are ruptured, however, aerobic respiration replaces anaerobic 

 oxidative processes even in seeds of this type. 



3. Suitable Temperature. — In the absence of other limiting factors the 

 seeds of any species will germinate within a certain range of temperatures, but 

 at temperatures above or below this range no germination will occur. As a 

 rule, the seeds of species indigenous to temperate regions germinate in a lower 

 range of temperatures than seeds of species whose native habitat is in tropical or 

 subtropical regions. Wheat seeds, for example, germinate at temperatures only 

 slightly above 0° C. and at temperatures as high as 35° C, while the range 

 of temperatures for germination of seeds of maize (a species of subtropical 

 origin) lies between a lower value of 5-10° C. and an upper limit of about 

 45° C. The optimum temperature is usually about midway between the two 

 extremes of temperature at which germination will occur. It is not possible 

 to designate any exact temperature as the optimum for germination as this 

 varies with the other prevailing environmental conditions and also with the 

 exact criterion selected as an index of germination. The most favorable tem- 

 perature for the elongation of the primary root, for example, does not always 

 correspond to the most suitable temperature for the development of the 

 plumule. 



4. Light. — A few species of plants including the strangling fig {Ficus 

 aurea), mistletoe {Visciim album) and some other epiphytes, have seeds which 

 fail to germinate unless exposed to the light. Many kinds of seeds germinate 

 better when exposed to the light than when kept in total darkness. Ex- 

 amples of these light-favored seeds are those of many grasses, especially species 

 of Poa, the evening primrose {Oencthera biennis) and mullein {Verbascufn 

 thapsus). On the other hand the germination of the seeds of some species 

 appears to be retarded or even prevented by exposure to light. Seeds of the 

 onion and many other members of the lily family belong in this group 

 (Crocker, 1936). The effect of light upon the germination of seeds is pro- 

 foundly influenced by other environmental factors. For example, the germina- 

 tion of the seeds of some species of grass {Poa sp.) is ordinarily influenced by 

 light, but after a period of dry storage this effect disappears. Similarly the 

 ordinarily light sensitive seeds of the pampas grass {Chloris ciliata) will 

 germinate readily in complete darkness in an atmosphere of pure oxygen. 



Dormancy of Seeds. — Many kinds of seeds, apparently ripe, fail to ger- 

 minate even if placed under such conditions that all environmental factors are 

 favorable. In such seeds resumption of growth by the embryo is arrested by 

 conditions within the seeds themselves. The state of inhibited growth of 

 seeds or other plant organs due to internal causes is usually called dormancy, 

 but is sometimes referred to as the "rest period." 



