GROWTH 295 



refuse to germinate without exposure to special stimulants even 

 after prolonged storage. Thus, for instance, the seeds of the 

 gooseberry, of Lunaria, of the Siberian Impatiens and other plants 

 require more than a simple stratification. They must be frozen at 

 — 5° C. or even —10° C, to induce germination. Light is a stimu- 

 lant to many seeds which fail to germinate, or which germinate but 

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

 carrot, Oenothera, and many other plants. On the contrary, light 

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

 apple, Amaranthus, etc. Hence they must always be germinated 

 in darkness. What the stimulating effect of light on the germina- 

 tion of seeds consists in, is as yet quite obscure. The action of light 

 may be substituted by fluctuating temperature, within the range of 

 a few degrees, or by various chemicals, among which the most 

 important are the acids and alkalies. Seeds of bog plants, like 

 Alisma, Sagittaria, and others, will germinate only in an acid 

 medium. Those of certain parasites, as the broom rape (Orobanche), 

 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 solu- 

 tions, such as formalin, copper sulphate, and various patented 

 preparations, in order to control smut and other fungus diseases, 

 sometimes results in stimulated germination, but more often 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, be- 

 cause their coats are impermeable to water. Such seeds very often 

 occur in clover, alfalfa, lupine, and other representatives of 

 Leguminosae. If a certain amount of seeds of .such plants is placed 

 in water, part of them will rapidly swell and germinate, others will 

 remain for a long time in water without swelling. Only gradually, 

 after some weeks or months, a few seeds will absorb water and ger- 

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

 the seed coat. Thus, Molisch observed in one of his prolonged 

 experiments with the seeds of the Japanese Gleditsia, that 4 seeds 

 out of 50 placed in water had swollen on the next day; 11 more 

 within the first two months, 21 additional during the first year, 6 in 



