[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 393 



nate immediately after being shed, but are soon killed by desiccation/ 

 The chances of growth of such seedlings and their survival are there- 

 fore greater on wet banks, stream and pond margins, and in unoccupied 

 wet lands. The seeds of silver maple rarely germinate if their water 

 content falls below 30 per cent. 



Oxygen deficiencv most frequently results from the burial of seeas 

 too far below the soil surface, especially of tight clay loams. Flooding 

 of soils by continuous rains mav so compact the soil and fill the spaces 

 between particles that aeration becomes almost zero. Oxygen deficiency 

 is a constant condition in the ooze at the bottom of lakes, ponds, and per- 

 manent marshes and swamps. Here the diffusion of oxygen from above 

 not only is very slow, but the mud and ooze have an enormous popu- 

 lation of bacteria and other organisms that use it as fast as it diffuses 

 from the water. These bottom organisms also release CO- in large quan- 

 tities; and when the CO- accumulates about seeds it may of itself be- 

 come a cause of delayed germination. 



Most seeds will germinate in either light or darkness, but a long list 

 of species has been published in which better germination was obtained 

 in light. Among these plants are bluegrass, certain varieties of tobacco, 

 mullein, carrot, mistletoe, evening primrose, loostrife, and willow herb. 

 On the other hand, light interferes with the germination of some seeds, 

 among which are species of Phacelia, 'Ni^ella, waterleaf, and onion. 

 After a period of suitable storage some of these seeds germinate in either 

 light or darkness. There is evidence that light may alter seed coats in 

 some seeds, and in certain others may affect the embryos or the endo- 

 sperms. Some of these seeds will germinate when alternating periods of 

 higher and lower temperatures are substituted for light. Of the light 

 rays in the visible spectrum, in general the red to yellow rays seem to 

 facilitate germination, while the blue to violet rays retard it. 



Germination delayed by seed and fruit coats. There are many seeds 

 with embryos enclosed by very hard and tough coats: either seed coats 

 or fruit coats. The common pigweed, the water plantain, Christmas 

 holly, and the bloodroot have coats so resistant to mechanical pressure 

 that expansion of the embryo cannot occur until the coats are partially 

 destroyed by soil organisms or by some other means. These coats may 



^ The seeds of some of these plants can be dried and kept ahve for a time under special 

 experimental conditions. Willow seeds, for example, may sur\'ive for several months in a 

 refrigerator if they are placed over 50 per cent H^SO, in a small vessel. The relative 

 humidity in the vessel would be almost 13 per cent. Similarly sugar cane seeds live longer 

 in an atmosphere of carbon dioxide kept dry with calcium chloride. 



