474 



BIOLOGY AND HUMAN LIFE 



Many fruits are covered with spines ; others have hard or 

 tough coverings or shells; still others contain bitter or acrid 

 substances, which probably prevent their being eaten. Seeds 

 that become separated from the fruit are frequently tough- 

 skinned or are covered with some other protective layers. 



To be in a position to 

 perform its functions in 

 the propagation of the 

 species a seed must get 

 out and get away — and 

 the farther away the 

 better, in most cases. 



In many common 



plants the seeds escape 



from the parent through 



the splitting open of the 



ripe fruit along definite 



lines or through definite 



openings. The pods of 



the bean family and the 



evening-primrose family 



illustrate this dehiscence, 



and the poppy furnishes 



a good example of the 



formation of pores (see 



Fig. 197). 



Fleshy fruits often drop off, carrying the seeds with them, 



and the seeds escape when the fleshy part of the fruit is eaten by 



some animal or rots (that is, is eaten by some plant). 



Many fruits, however, do not permit the seeds to escape ; the 

 fruit and the seed are so closely united that they constitute a 

 structure which acts as a whole, as in the grains, the nuts, and 

 the nutlets of the dandelion family (Fig. 196). 



350. Seed distribution. In their dehiscence many fruits open 

 so suddenly that they shoot the seeds to a distance of a yard or 

 more. This shooting is commonly brought about by the rapid 



Fig. 195. Mechanical protection of seeds 



I, bitternut (Hicoria minima), of the walnut 

 family; 2, chestnut oak {Quercus prinus)', 3, 

 sweet gum {Liquidambar styracifliia), of the 

 witch-hazel family; 4, table-mountain pine 

 (Pinus pungens) 



