SEED COATS AND DORMANCY 425 



and other families have already been mentioned. The 

 seeds of the broom, for instance, may lie for months on 

 moist filter paper without change ; a sUght scratch on the 

 seed coat is at once followed by absorption of water, the 

 seed swells, and germination follows in a day or so. This 

 behaviour is of practical importance in such commercial 

 hard seeds as the red clover and the spinach, which are 

 sometimes artificially abraded to secure more rapid and 

 complete germination. De Vries (19 15) found that the 

 germination of CEnothera seeds was improved by forcing 

 water into them under pressure. 



A number of cases are known of seeds and fruits in which 

 the wall interferes with the supply of oxygen. The most 

 fully investigated is that of the fruits of Xanthium, the 

 cocklebur (C. A. ShuU, 191 1, 1914). Germination takes 

 place if the walls are removed, and may also be secured by 

 placing intact fruits in a high concentration of oxygen. The 

 increased supply of oxygen may be necessary for respiration, 

 or it may remove the narcotic eff"ects of carbon dioxide. 

 ShuU and Davis (1923) show that the enzyme catalase 

 increases at germination. In nature germination takes 

 place, curiously, at different times in the two fruits of each 

 bur. The coat of the upper seed is the more resistant and 

 the seed germinates two years after formation, while the 

 lower seed germinates a year earlier. 



When dormancy is due to the structure of the seed coat 

 or fruit wall, germination can take place only after some 

 alteration in these. This may be spontaneous, as in 

 Xanthium, probably, as Crocker (1916) thinks, through slow 

 changes in the colloids in the cell wall. External agencies 

 may also be active. There is the possibility of bacterial 

 action. Freezing is important ; in many seeds, as Kinzel 

 (1913, 1915, 1920) has shown, germination in nature takes 

 place only after freezing. The seeds oi Menyanthes trifoliata, 

 Teucrium Chamcedrys, Gentiana lutea, G. nivalis, Adoxa 

 Moschatellina, require increasing sharpness of frost in the 

 order given. The last two must be exposed to temperatures 

 of —20° C. If the coat is water-saturated and freezes, it 



