GROWTH FORMS OF PASTURE AND HAY PLANTS 89 



to the question is not the same everywhere, for various factors 

 may cause a decline in the yield of forage. A declining yield 

 may be due to an excessive amount of weeds, to abnormal cli- 

 mate, to bad handling of the stock, or to various alHed con- 

 ditions. 



Natural succession, or a normal tendency in nature to a 

 rotation of crops, as on timberlands, is going on all the time. As 

 choke cherry or aspen are the natural invaders of a cut-over 

 or burned fir type, so useless weeds may occupy grasslands if 

 no effort is made to check them. 



Abnormal changes in autumn or winter weather, alternate 

 freezing and thawing, and exposure due to the absence of the 

 usual blanket of snow are injurious to many forms of grasses 

 and other vegetation. 



Drought injures forage plants to a very considerable extent. 

 A sudden decline in the water content of the soil may cause the 

 vegetation to wilt beyond recovery with the root system quite 

 intact, or the soil may break and crack and tear the roots asunder. 

 Deep-rooted grasses are usually less liable to injury from drought 

 than are shallow-rooted forms. 



Faulty methods of grazing, as pointed out, may seriously thin 

 the stand, particularly of bunchgrass vegetation. The plants 

 are sometimes literally starved to death by too early or close 

 cropping. Then, too, excessively close grazing in the fall de- 

 stroys the protection otherwise afforded by the forage during 

 the winter. Furthermore, injudicious grazing may change the 

 texture of the soil through heavy packing. The crop-producing 

 power of heavily packed soils falls rapidly until those soils prac- 

 tically cease to support plant life. 



Growth Forms of Pasture and Hay Plants. — The habit of 

 growth of the underground parts makes possible a division of 

 forage plants into two distinct forms. Those that spread by 

 rootstocks or rhizomes and form a continuous, more or less ex- 

 tensive sod, like Kentucky bluegrass, are the most desirable and 

 are largely confined to the humid and semi-humid regions. 

 Those whose growth is confined to a tuft or bunch, like slender 

 wheatgrass, and whose perpetuation and spread is dependent 



