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- 

 ment in the reverse direction. These fruits 

 are such efficient penetrating mechanisms that 

 they work readily through clothes or through 

 envelopes in which they are stored, and pene- 

 trate even into the flesh of grazing animals. 

 When the fruits of Stipa lie horizontally on 

 the ground, changes of moisture result in a 

 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. 



FIG. 1224. A mature 

 fruit of the porcupine grass 

 (Stipa spartea}, showing the 

 seed-bearing portion (d) and 

 the long, spirally twisted awn 

 (a); the basal portion or 

 callus (c) is stiff and sharp, 

 and is clothed with bristles 

 (6) which point upward. 



