REPRODUCTION AND DISPERSAL 



921 



FIGS. 1215, 1216. Winged fruits: 

 1215, a samara or key fruit of the moun- 

 tain maple (Acer spicatum); 1216, a 



number of disseminules carried. As 

 seeds are much larger than spores, the 

 distances covered are much less, though 

 in the case of small seeds, as in the 

 orchids, it is possible that the dis- 

 tances may be very great. Among 

 the commonest of wind-scattered dis- 

 seminules are those with wings, as in 

 the seeds of the catalpa, and in the 

 fruits (known as samaras) of the 

 maple (fig. 1215), hop tree (fig. 1216), 

 and elm. Such disseminules com- 

 monly are much flattened, and hence samara of the hop tree (Ptdea trifoliate); 

 are unlikely to fall rapidly to the ' Wi 



ground; furthermore, the wings are light, often containing air spaces of 

 considerable size. The wings may be terminal, as in the ash and the 

 maple, or they may form a margin about the seed-bearing portion, as in 

 the hop tree, elm, and bugseed. Similar winged disseminules are found 



in the pines and birches. In the linden 

 there are relatively heavy, globular, in- 

 dehiscent fruits, but they are borne on 

 a large, membranous bract attached to 

 the peduncle. 



Many wind-scattered disseminules are 

 crowned with hairs. Perhaps the most 

 representative of these are found in the 

 composites, especially in those with milky 

 juice; in the latter, at maturation, the 

 involucre falls back once more as at an- 

 thesis, exposing the achenes, with their 

 crowns of hairs (known as the pappus) 

 spread out in such a way that the entire 

 structure resembles a parachute (fig. 1217) ; 

 as in parachutes, also, the resistance to 

 the air in falling is considerable, so that 

 wind currents are apt to scatter the 

 achenes for some distance. Dispersal is 

 facilitated still further, if there is a long, 

 slender process (known as the beak) sepa- 



FIG. 1217. A fruiting head of 

 the prickly lettuce (Lactuca scari- 

 ola), from which all of the mature 

 fruits but one have been blown 

 away; note the reflexed involucral 

 scales (s) and the achene (a), 

 which is prolonged into a beak (6) 

 and is crowned with pappus com- 

 posed of capillary bristles (/>); r, 

 receptacle; P', peduncle. 



