d 



REPRODUCTION AND DISPERSAL 929 



Seeds that pass through the alimentary tracts 

 of large animals, such as cattle, are planted most 

 advantageously in their excrements, where, upon 

 germination, the young seedlings find an excellent sup- 

 ply of food materials. Nuts buried by animals, if they 

 chance to escape being eaten, often are favorably placed 

 for germination; it is to be recalled also that some fruits 

 mature in the ground (as in the peanut and the violet), so that 

 favorable planting is sure to result. Some seeds and fruits 

 have features enabling them to remain attached to their posi- 

 tion on the ground, notably in such hooked fruits as those of 

 the cocklebur and the burdock; in the seeds of flax and mustard 

 the outer layer becomes mucilaginous when moistened, facili- 

 tating adherence to the substratum. 



A remarkable seed-planting mechanism is seen in certain 

 hygroscopic fruits, notably in the porcupine grass (Stipa, fig. 

 1224). Here the fruit is prolonged below into a sharp spine 

 that is clothed except at the tip with hairs that point upward, 

 while above there is a long awn whose basal portion coils into a 

 close spiral when exposed to desiccation, and uncoils when 

 moistened, the tissues being so constructed that the evaporation 

 and the absorption of water are unequally distributed. If the 

 spine-tipped base sticks into the ground, the repeated twisting 

 and untwisting of the awn serve to bury it deeper and deeper 



in the soil, the upward-pointing hairs preventing any move- 

 _ c ment in the reverse direction. These fruits 



FIG. 1224. A mature are suc h efficient penetrating mechanisms that 

 fruit of the porcupine grass they work readily through clothes or through 



(Stipa spartea), showing the enyelopes in which they are stored and pene . 

 seed-bearing portion (d) and . .... 



the long, spirally twisted awn trate even into the flesh of grazing animals. 

 (a); the basal portion or When the fruits of Stipa lie horizontally on 

 callus (c) is stiff and sharp, the ground c h a nges of moisture result in a 



and is clothed with bristles . 



(6) which point upward. slow creeping movement along the surface. 



Hygroscopic fruits similar in character to 



those of Stipa are found in various grasses (as Aristida and Avena) and 

 in Erodium, a relative of the geraniums. 



