REPRODUCTION AND DISPERSAL 907 



weed (Datura), the chestnut, and certain gooseberries (as Ribes Cynos- 

 bati), the fruits are spinescent. 



The prickly pear {Opuntia) is especially interesting from this viewpoint, since 

 the unpleasant bristles of the young fruits fall off as the fruit ripens, from which it 

 has been inferred that the young fruit is protected from the fruit-eating animals 

 which later scatter the ripe seeds. Such views are misleading in their implications, 

 since most young fruits are not especially attractive to animals. Their unpalata- 

 bility is a sign of immaturity rather than of protection. 



The protective structures of mature seeds. — Seeds as a class are the 

 most xerophytic of plant structures, since not alone in xerophytes, but 

 also in mesophytes and even in hydrophytes, they generally are covered 

 with hard and impermeable coats. So universal is the xerophytism of 

 the seed that usually it is impossible to determine from its structure the 

 habitat in which it grew. This xerophytism consists in three features: 

 the thick and impermeable coat, the compactness of the tissues within 

 the testa, and the small amount of water. The testa, or seed coat, com- 

 monly is single, being derived from the ovule integument (from the outer 

 integument, in case there are two) through thickening, hardening, and 

 other modification. In some seeds there is a second coat within the 

 testa, and in others there is a structure outside the testa, which is known 

 as an aril (e.g. in the water lily). The testa at maturity usually is hard 

 and bony, being composed of several or more layers of cells with greatly 

 thickened walls; in the hickory nut it is made up of a number of layers 

 of sclereids. Sometimes the testa is so hard that it is difficult to cut it 

 with a knife, as in Gymnocladus. In most one-seeded fruits, such as the 

 grains of cereals (fig. 121 1) and the achenes of the composites, the fruit 

 wall or pericarp remains closed about the seed at detachment, and often 

 is the chief protective layer, especially where it is hard and bony (as in 

 Lithospermum). In some instances seeds are essentially without a pro- 

 tective outer layer; this is the case particularly in the Amaryllidaceae, 

 where the outer integument or the endosperm may become fleshy and 

 green (as in Hymenocallis and Crinum). 



The advantages of seed protection. — The chief dangers which beset 

 seeds are premature germination, loss of viability, and destruction by 

 herbivorous animals. Adequate protection is especially important in 

 monocarpic species, above all in annuals, since the maintenance of the 

 species depends absolutely upon the viability of its seeds. For months 

 at a time annuals may be non-existent over vast tracts of country 

 except in the form of seeds. While most trees, as the pines, spread 



