236 PLANT BIOCHEMISTRY 



one part of oxygen. A thick covering of compact soil may bring about 

 such a condition. The thickness of the soil cover which is best for 

 germination depends upon the physical condition and the moisture 

 content of the soil as well as the kind of seed. 



An interesting example of delayed germination due to a lack of 

 oxygen is found in the cocklebur. Each cocklebur contains two seeds; 

 the lower of the two germinates the first year after ripening, whereas 

 the upper seed may germinate a year or more later. A thin membrane 

 surrounding the upper seed prevents sufficient oxygen from reaching 

 the embryo and results in delayed germination. This membrane must 

 be made more permeable to oxygen before germination will take 

 place. Freezing, decay, or high temj^eratures will accomplish this. 

 Other seeds are also known that lie dormant in the soil because of 

 lack of oxygen, for, although ihey absorb moisture readily, they 

 possess membranes which reduce the oxygen supply to the embryo. 

 Viable seeds respire during storage, but such respiration is very slow, 

 for there are records of seeds that were still alive after many years 

 of storage. Thus the seeds of Nelumbium (a species of large water 

 lily) were alive after storage for 250 years. Since the respiration rate 

 of such long-lived seeds is very low, only minute amounts of oxygen 

 are required during their storage. 



Light 



The effect of light on germination varies considerably with seeds 

 of different species. For the seeds of the onion and many other 

 members of the lily family the presence of light has an inhibitory 

 effect on the germination process. The seeds of several common 

 agricultural plants, such as corn, small grains, clover, and beans, 

 germinate equally well in the presence or the absence of light. For the 

 seeds of many grasses, the evening primrose, and mullein, light is a 

 desirable adjunct to the germination process. Finally, seeds of the 

 strangling fig, mistletoe, and some other epiphytes fail to germinate 

 unless exposed to light. 



PHYSIOLOGICAL FACTORS IN SEED GERMINATION 



Age of the Seed 



Seeds degenerate with age, although a certain percentage of various 

 seeds will germinate after storage for many years. The reason for 

 this loss of vitality is probably not the depletion of stored foods, for 

 even aged seeds contain large amounts of reserve food material. How- 



