SEED DISPERSAL 



201 



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 currents 

 of air readily reach 

 it. The bristles 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 

 andsweptalongonthe 

 ground. The " res- 

 urrection " plant (Ly- 

 copodium) is another 

 example of a plant 

 which is distributed 

 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. 162. 

 Lactuca scariola. 



