346 



NATURE^STUDY 



simplest treatment of the seed by the parent is to let it 

 fall upon the ground and trust that it will roll away or be 

 blown or washed farther. But in this way the young plants 

 are apt to come up too thickly at the base of the parent plant 

 and thus choke each other. Some plants, however, have 

 pods or capsules arranged in such a manner that the seeds 

 cannot drop out directly, but are cast out to some distance 



Fig. 142. Russian " Thistle "—A Tumble Weed. 



by the swaying of the elastic stem of the plant. This is seen 

 in the garden pinks and poppies. 



The wiad is the chief agency in seed dispersal for a great 

 variety of plants. The Russian "thistle," the tumbleweed, 

 and others break oiT as a whole close to the root and are 

 carried by the wind over the hills and plains, scattering hun- 

 dreds of seeds at every bound. This mode of dispersal 

 accounts for the rapid spread of the Russian thistle since its 

 introduction into this country a few years ago. Examine 



