SEASONAL PHENOMENA IN THE LIFE OF PLANTS 461 



— 5 or even — 10°C. to induce germination. Light is a stimulant 

 to many seeds that fail to germinate or that germinate but 

 poorly in darkness. To this group belong seeds of Poa, tobacco, 

 carrot, Oenothera, and many other plants. Contrariwise, light 

 retards germination of the seeds of some plants, like those of the 

 thorn apple, Crataegus sp., of Amaranthus, etc. Hence, they 

 must always be germinated in darkness. What the stimulating 

 effect of light on the germination of seeds consists in is as yet 

 quite obscure. The action of light may be replaced by fluctu- 

 ating temperature, within the range of a few degrees, or by 

 various chemicals, among which the most important are the 

 d.cids and alkalies. Seeds of bog plants, like Alisma, Sagittaria, 

 and others, will germinate only in an acid medium. Those of 

 certain parasites, like the broomrape Orohanche, also require 

 an acid medium. Others are stimulated by solutions of 

 neutral salts or substances like bromine, iodine, and certain 

 antiseptics. 



The increasingly popular treatment of seeds with various 

 solutions, such as Formalin, copper sulphate, and various organo- 

 mercury preparations, in order to control smut and other fungus 

 diseases, sometimes results in stimulated germination, but 

 frequently it leads to retardation. 



A prolonged delay of germination does not always depend 

 on the state of dormancy in which the embryo of the seed happens 

 to be. Very often it is due to the properties of the seed coat. 

 Many plants produce so-called ''hard seeds," which fail to 

 germinate because their coats are impermeable to water. Such 

 seeds very often occur in clover, alfalfa, lupine, and other 

 representatives of the family Leguminosae. If a certain number 

 of seeds of such plants are placed in water, part of them will 

 rapidly swell and germinate, but others will remain for a long 

 time in water without swelling. Only gradually, after some 

 weeks or months, will a few seeds absorb water and germinate, 

 apparently, as a result of a lesion in the outer layers of the seed 

 coat. Molisch observed in one of his prolonged experiments 

 mth the seeds of the Japanese Gleditsia that 4 seeds out of 57 

 placed in water had swollen on the next day; 11 more, within the 

 first 2 months; 21 additional, during the first year; 6, in the 

 second year; 6 more, during the third year; 3 in the fourth, and 

 3 in the fifth year; while 3 seeds did not swell after having been 

 in water for more than 5 years. 



