FUNCTIONS OF FRUITS AND SEEDS 



327 



(dispersal) is accomplished. (See page 63.) In very 

 many cases it is the seeds alone which are dispersed. Just 

 as frequently, it is the fruits containing the seeds which are 

 dispersed, while in some cases, as in tumbleweeds, whole 

 shoots of the 

 parent plant are 

 scattered about. 

 (See Figure 142.) 



In some plants 

 it is not simple 

 to tell whether 

 the part scattered 

 is seed or fruit. 

 Thus in the dan- 

 delion or in the 

 sticktight (Bi- 

 dens) or in others 

 of the Composites 

 it is common to 

 think of the scat- 

 tered parts as being seeds, whereas they are more than 

 that. They are fruits, one-seeded fruits, in which the 

 fruit wall (pericarp) grows closely around the seed wall 

 (testa) so that they are practically one structure. One- 

 seeded fruits of this kind are called, in the dicotyledons, 

 akenes, and they are particularly characteristic of the 

 Composite. Among monocotyledons they are called grains, 

 and they are particularly characteristic of the grass family 

 (Graminea), to which wheat, corn, and the other cereals 

 belong. 



The principal agencies by which dissemination or seed 

 dispersal is accomplished are wind, water, and animals. 



FIG. 142. Mature plants of the winged pigweed, which 

 is one of the tumbleweeds. Tumbleweeds are plants 

 which, upon maturity, break from the soil and " tum- 

 ble" along before the wind. Wherever they lodge, 

 they drop seeds. Plants of this kind are usually 

 frequent in treeless regions. The Russian thistle and 

 the amaranth are other well-known examples of 

 tumbleweeds. 



