500 FIELD CROPS 



seeds of many plants are so light or they are provided with 

 appendages of such a nature that they are easily carried long 

 distances by the wind. The seeds of the milkweed, thistle, 

 and dandelion spread more widely by winds than by any 

 other means. Some plants, 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 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. Bottom 

 lands are ordinarily infested with weeds, as the seeds are 

 washed down from the surrounding higher lands and depos- 

 ited 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 oats, 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 locahze 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. 



