THE SEED 



25 



voluntary agency of man. Dispersal by this means, whether 

 intentional or accidental, is purely artificial, and except in the 

 case of a few annuals like horseweed, bitterweed, ragweed, 

 goosefoot, and other field pests that have adjusted their sea- 

 son of growth and flowering to the conditions of cultivation, 

 is not correlated with any special modification of the plants 

 for self-propagation. On the contrary, many of the most 

 widely distributed weeds of cultivation, such as the ox-eye 

 daisy, the rib grass, mayweed and bitterweed, possess very 

 imperfect natural means of dispersal, and are largely depend- 

 ent for their propagation on the involuntary agency of man. 

 23. Use of the fruit in dispersal. It will be seen from the 

 foregoing observations that the fruit plays a very important 

 part in the work 

 of dispersal, most 

 of the adapta- 

 tions for this pur- 

 pose being con- 

 nected with it. 

 In cases where a 

 number of seeds 

 are contained 

 in a large pod 

 that could not 

 conveniently be 

 blown about by 

 the breeze, 

 adaptations for 



wind dispersal are attached to the individual seeds, as in the 

 willow, milkweed, trumpet creeper, and paulonia; but as a 

 general thing, adaptations of the seed are for protection, the 

 work of dispersal being provided for by the fruit. In the case 

 of the large class of plants known as " tumbleweeds, " the 

 whole plant body is fitted to assist in the work of transporta- 

 tion. Such plants generally grow in light soils and either 

 have very light root systems, or are easily broken from their 



FIG. 43. A fruiting plant of 

 winged pigweed (Cycloloma), 

 showing the bunchy top and FIG. 44. Panicle of 

 weak anchorage of a typical "old witch grass," a com- 

 tumbleweed. mon tumbleweed. 



