296 TEXTBOOK OF PLANT PHYSIOLOGY 



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

 and fifth years, while 3 seeds did not swell after having been in 

 water for more than 5 years. 



The impermeability of the coats of hard seeds to water depends 

 on very close union and drying of the cells forming the outer layers 

 of the seed coat. When these outer layers are injured by scratch- 

 ing with a knife or by filing or simply rubbing them with sand, 

 swelling and germination will result. The corroding action of 

 strong sulphuric acid also may be used for this purpose. 



Not always, however, is the delayed germination of viable 

 seeds due to the impermeability of their coats to water. Very 

 often an impermeability of the seed coat to gases is the real cause, 

 as a consequence of which the carbon dioxide produced during 

 respiration is not eliminated, but accumulates within the seed 

 and inhibits the active growth of the embryo. Such seeds may be 

 kept for years without germinating, though they swell readily. 

 The seeds of many of the common weeds, like plantain, black mus- 

 tard, shepherd's purse, wild oats, chickweed, etc., are of this type. 

 In order to make them germinate readily, it is not sufficient merely 

 to scratch their coat. It must be split, to permit the escape of 

 carbon dioxide and admit oxygen. In a similar condition may be 

 seeds which happen to be covered by a thick layer of earth. They 

 may remain alive in the ground for many years without germina- 

 tion, but will germinate rapidly as soon as they have reached the 

 top of the soil. Thus, for instance, in samples of soil taken from a 

 depth of about 20 cm. in a forest growing on ground that 30 to 40 

 years ago had been occupied by a field or a meadow, seedlings of 

 weeds or meadow plants could be obtained. Evidently the seeds 

 had retained their power of germination throughout this long 

 period. 



Such long preservation in moist ground without decaying is 

 possible only because the seed coat eliminates on its surface some 

 antiseptic substances inhibiting the development of bacteria and 

 molds. It is interesting to note, that in dead seeds this elimina- 

 tion soon ceases and they rapidly become the prey of microorgan- 

 isms which produce decay. 



As the seed matures the impermeability of its coat gradually 

 increases and desiccation often makes it still more impermeable. 

 This explains the commonly observed phenomenon that in a fresh 

 supply of seeds the percentage of hard seeds is somewhat lower than 



