SEED DISPERSAL 



219 



the ground "right side up," i.e., with the radicle of the embryo 

 downward. In the dandelion the flower stem elongates just as the 

 seed is ripening, so 

 that the head is lifted 

 up where the cur- 

 rents of air readily 

 reach it. The bris- 

 tles of the pappus in 

 many composites, at 

 first straight, turn out 

 at nearly right angles, 

 like the spokes of a 

 tiny wheel, so that it 

 is more effective as a 

 float. The so-called 

 "tumble weeds" are 

 rolled on the ground 

 by the wind to great 

 distances, and the 

 seeds are scattered 

 by the way. Some 

 of these are the light, 

 much branched 

 grasses, which when 

 ripe and dry are 

 broken off by the 

 wind and swept 

 along on the ground. 

 The "resurrection" 

 plant (Lycopodium) 

 is another example of 

 a plant which is dis- 

 tributed by the wind. 

 As it dries up during droughts it curls into a rounded mass, the 

 roots are torn from the ground, it rolls along in the wind and 

 with the advent of rains takes root and grows again. 



Fig. 171. 

 Lactuca scariola. 



