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 line 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 xerophyte 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 xerophytes, because these periods of abundance 
are not of sufficient length to effect the general adaptations 
of the plants. 
177. Mesophytes.—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 
