918 ECOLOGY 



ment are likely to be spherical, while crowded seeds commonly are 

 angular. In the two-seeded fruits of Xanthium and Cakile, each seed 

 differs considerably in shape and in size from the other. In certain 

 composites the achenes of the ray flowers and of the disk flowers differ 

 strikingly in shape. In the parasitic Scrophulariaceae it has been dis- 

 covered that vigorous plants give rise to larger seeds than do weak plants, 

 and that the large seeds give rise in turn to more vigorous plants than do 

 the smaller seeds; furthermore, the larger seeds are more likely than are 

 the others to grow into autophytic individuals, while the plants coming 

 from small seeds in order to thrive, apparently must be parasitic. 1 

 The achenes of hemp vary considerably in size and in weight, those pro- 

 duced in moist habitats being larger and heavier than those produced 

 in dry habitats. The larger achenes germinate more quickly than do 

 the others, forming stronger plants. Similar differences have been ob- 

 served in the seeds and seedlings of tobacco. In a crowded groip of 

 natural seedlings such a difference in size might be of great significance, 

 since the stronger seedlings would tend to crowd out the others. The 

 influence of grafting upon the character of fruits has been noted elsew'iere, 

 but it may be recalled that changes in the size and in the flavor of c ulti- 

 vated fruits often result from the reciprocal influence of the stock and 

 the scion. Pollination may affect the character of the fruit; for example, 

 when the flowers of watermelons are pollinated by cucumber pollen, 

 the resulting fruit is very poor in sugar. 



Seed variations manifested in behavior. Seeds of the same species, 

 though apparently alike in structure, in reality may be very different. 

 Perhaps the best instance of this is seen in a comparison of seeds raised 

 in different climates. Farmers in the United States have long known 

 that northern-grown seeds produce crops that ripen earlier than do crops 

 raised from seeds grown farther south. It appears as if the progeny of 

 the northern plants have inherited from them their short maturation 

 period, thus furnishing evidence in favor of the theory of the inheritance 

 of acquired characters (p. 947). After a few years, however, it is neces- 

 sary once more to use northern seeds, since the progeny of northern- 

 grown plants come to have the same period that is characteristic ol the 

 climate to which they are transferred. 



1 In this connection it is of interest to note that various parasites (as Hydnora, Rafflesia, 

 and Balanophora) and mycophytes (as Monotropa and the orchids) have minute seeds 

 with rudimentary un differentiated embryos and almost no food, nutritive dependence 

 upon other plants being necessary very early, in most cases even in the earliest stages of 

 germination. 



