932 



ECOLOGY 



incapable of germination for several years. There are some plants (as 

 the cocklebur, red clover, and black locust) in which some of the 

 seeds appear ordinarily to require a longer time than do others before 

 they are capable of germination. It is probable that in most of these 

 cases delay in germination is due to the impermeabihty of the testa (see 

 below). Yet it is conceivable that in seeds, as in buds, various maturing 

 processes take place after the attainment of apparent maturity ; detach- 

 ment from the carpel and apparent rest may not mean the cessation of 

 maturing activities. Possibly the delayed germination of the hawthorn 

 {Crataegus) is to be thus explained, since the removal of the testa and 

 exposure to good germination conditions seems for a certain period in- 

 effective.' The most remarkable of all cases of delayed germination is 

 afforded by the spores of Lycopodium, which seem 

 to require a rest of three to fifteen years before they 

 are able to develop. 



The relation of the testa to delayed germination. — ■ 

 The common cocklebur {Xanthium canadense) has 

 two seeds in each fruit, differing somewhat in 

 shape and in position (fig. 1228), and it has been 

 found that the seed nearest the base usually germi- 

 nates the first spring after maturation, while the 

 upper seed commonly does not germinate until 

 the second spring. In many species some seeds 

 germinate long before others, and it is not unlikely 

 that in some cases the seeds of a given crop may 

 germinate over a period of three or more years. 

 Such a condition seems advantageous, especially in 

 annuals, since it insures the persistence of a species, 

 even though certain seasons prove unfavorable for 

 seed development. In Xanthium, it has been shown 

 that the delayed germination of the upper seed is 

 due to the fact that its testa is less permeable to 

 oxygen than is that of the lower seed. In nature 

 the lower seed is exposed first to good germinative 

 conditions, because that end of the fruit disintegrates first. In various 

 plants (as Abutilon, Iris, and Axyris) the testa (or endosperm) delays 

 germination because it excludes the necessary water. The upper 



' Even in Crataegus, germination has been brought about in two months through the 

 removal of the testa, though in natural conditions it usually requires a year and a half. 



Fig. 1228. — A coc- 

 klebur fruit {Xan- 

 thium) in longitudinal 

 section, showing the 

 position of the two 

 seeds ; note that the 

 lower seed (/) is larger 

 than the upper seed 

 (m) and better placed 

 for germination, since 

 the fruit begins to de- 

 cay at (c) ; pj hooked 

 prickles which aid in 

 dispersal. — After 

 Crocker (drawn from 

 a photographic repro- 

 duction). 



