394 TEXTBOOK OF BOTANY 



delay germination in nature for months or years. The embryos of Christ- 

 mas holly do not become dormant, and most of them perish before the 

 surrounding coats are sufficiently weakened by decay. Under natural 

 conditions only about one holly embryo in ten million germinates. In 

 horticultural practice the best germination of holly seeds has been ob- 

 tained by keeping the seed bed above 70° F. 



Seed and fruit coats may often exclude water from the endosperm 

 and embryo. These coats may be impervious to water because of the 

 presence of wax, suberin, resin, and certain structural features. Embryos 

 thus entombed germinate in nature only when the coats have been partly 

 digested by bacteria and other organisms. Seeds of red clover and 

 numerous other legumes have seed coats impervious to water. In lotus 

 and certain stone fruits, it is the fruit coat that is impermeable. 



In horticultural practice such seeds may be treated for a few minutes, 

 or for longer periods, with strong sulfuric acid until the impervious layer 

 of the coat is altered or removed. These seeds may also be scarified by 

 means of sandpaper, sharp sand, and special scarifying machines until 

 the coats are partially removed. Often only the two outermost layers of 

 cells need to be broken. Legumes are noted for the great variation in 

 the "hardness" of the coats of the seeds from a single harvest. 



Oxvgen likewise may be excluded from embryos by impermeable seed 

 and fruit coats. An example of delayed germination due to oxygen defi- 

 ciency has been discovered in cocklebur. There are two seeds in each 

 bur, the lower of which is near the surface and germinates freely after 

 the fruit is shed. The upper embryo, however, is surrounded by tissue 

 that is rather impervious to oxygen, and it does not germinate when the 

 concentration of oxygen is low, even if the coats are destroyed. Both 

 these conditions delay germination for additional months or years, as 

 compared with the lower embryo. Certain grasses and some of the plants 

 of the sunflower family also have seed and fruit coats that are not very 

 permeable to oxygen. 



Intermittent germination of seeds of a species over a period of years 

 is probably an advantage to a plant in nature. In horticulture, forestry, 

 and agriculture it is uneconomical because it results in uneven-aged 

 stands of crop plants, and delays the starting of nursery stock. 



Germination delayed by conditions within the embryo. When the seed 

 of the ginkgo tree falls to the ground its embryo may consist of only a 

 few cells, and sometimes fertilization has not yet occurred. During ensu- 



