140 A TEXT-BOOK OF GRASSES 



divided into four groups — mesophytes, xerophytes, halo- 

 phytes and hydrophytes. It should be understood that 

 there is no sharp Une between these groups. There are 

 transitions in all directions. It is impossible to define 

 in exact terms the limits that circumscribe these groups. 

 One cannot, except approximately, say that plants 

 growing upon soil containing certain definite limits of 

 moisture shall be classed as mesophytes and that between 

 other limits the plants shall be called xerophytes. Many 

 other conditions modify the effect. One must judge 

 rather by the sum total of the effect upon the plant, that is, 

 the reaction to environment. If the plant shows general 

 adaptations that aid it in resisting loss of moisture, the 

 plant is a xerophyte. However, it often happens that the 

 soil may contain sufficient moisture a part of the time and 

 a deficiency at other times. So far as the plant is con- 

 cerned the critical period is the growing season. A beech 

 tree is a mesophyte in summer and a xerophjrte in winter. 

 In the summer there is sufficient moisture for its broad 

 thin leaf-blades. In winter the ground freezes, the 

 branches and twigs may freeze, moisture can not be sup- 

 plied to so great a surface, and the surface is reduced by 

 casting off the leaves. Nevertheless the beech is classed 

 as a mesophyte. On the other hand, desert regions are 

 visited occasionally by heavy rains and for a short time 

 the soil may be saturated. But the plants of these regions 

 are called xerophjrtes, because these periods of abundance 

 are not of sufficient length to effect the general adaptations 

 of the plants. 



177. Mesophjrtes. — ^As the name indicates, this group 

 includes those grasses that thrive under medium condi- 

 tions of moisture. They are not water plants on the one 

 hand, and on the other hand are not especially adapted to 



