462 PLANT PHYSIOLOGY 



The impermeability of the coats of hard seeds to water depends 

 on very close-packed dry thick-walled cells forming the outer 

 layers of the seed coat. When these outer layers are injured 

 by scratching with a knife, by filing, or simply by scratching 

 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 a nonpermeability of the seed coat to gases is the real 

 cause, because 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 germination, though they swell readily. 

 For seeds of many of the common weeds, like plantain, black 

 mustard, shepherd's-purse, wild oats, and chickweed, it is not 

 sufficient merely to scratch their seed coats. They must be 

 split to permit the escape of carbon dioxide and admit oxygen. 

 Seeds that happen to be covered by a thick layer of earth may 

 be in a similar condition. They may remain alive in the ground 

 for many years without germination but will germinate rapidly 

 as soon as they have reached the top of the soil. For example, 

 in samples of soil taken from a depth of about 20 cm. in a forest 

 growing on ground that 30 to 40 years before had been occupied 

 by a field or meadow, seedlings of weeds or meadow plants could 

 be obtained. Evidently the seeds had retained their power of 

 germination throughout this long period. Seeds of Nelumho 

 have been found viable after more than 100 years under certain 

 conditions. 



Such long preservation in moist ground without decaying is 

 possible only because the seed coat excretes on its surface some 

 antiseptic substances that inhibit the development of bacteria 

 and molds. It is interesting to note that in dead seeds this 

 resistance to rots soon disappears, and they rapidly become the 

 prey of microorganisms that 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 in those that have been stored for a long time. It 



