530 FIELD CROPS 



as the tumbleweeds and the Russian thistle, retain their 

 seeds in the capsules for several months after they ripen. 

 The stems are broken off by the winter winds and the plants 

 are driven across the fields, distributing their seeds as they 

 go. Some seeds which are not readily carried by wind alone 

 are carried by drifting snow. 



Water is an efficient agent in the spread of weeds. The 

 seeds may float on its surface, or they- may be carried along 

 with soil or driftwood. The roots or branches of weeds 

 may be washed out by sudden freshets or the cutting away 

 of stream banks and be carried to new locations. The carry- 

 ing power of water accounts for the fact that bottom lands 

 are ordinarily infested with weeds, as the seeds are brought 

 down from the surrounding higher lands and deposited on 

 the bottoms. 



Some plants are provided with special mechanisms which 

 aid in their dissemination. The vetches and some members 

 of the pea family have pods that twist suddenly when they 

 open, throwing the seeds in different directions. Others, 

 like the wild oat, have twisted awns or appendages which 

 coil or uncoil with changes in the weather and aid in burying 

 the seeds. Various other plants are provided with special 

 means of distribution of this nature, but these are seldom 

 efficient in spreading the plant for any distance. 



Another means by which plants spread, but which tends 

 to localize them unless assisted by water, animals, or man, is 

 by the extension of their own growth. Quack grass, Canada 

 thistle, Johnson grass, and many other plants increase by 

 means of running rootstocks, which send up shoots at inter- 

 vals and form new plants. Others, like crabgrass, have 

 creeping or running stems which root at the joints and may 

 form new plants if broken off. 



700. Animals. Wild and domestic animals both aid in 



