44 THE PLANT LIFE OF MARYLAND 



meability of the rock to water and through its inclination or dip. A 

 highly important factor in determining the water content of soils is 

 I heir physical texture, for this determines both their capacity to re- 

 tain water which falls upon them as rain and to lift water by capil- 

 larity from the region of the ground water level. Coarse gravels and 

 sands retain but a small percentage of the water which falls upon 

 them while a high percentage is held by such soils as the clays and 

 finer loams. Not only are clays and other tine soils capable of 

 retaining water hut also of lifting it by capillarity from lower levels 

 as it is removed from the surface by evaporation or plant absorption. 

 Gravels and coarse sands are incapable of thus lifting water by capil- 

 larity from the lower levels of the soil. The water table is not then 

 of such great importance to plant-life in itself as it is in its office as a 

 reservoir from which water may be supplied to the upper layers of 

 the soil. The roots of upland plants are distributed through the soil 

 chiefly in the region above the water table, where water is secured by 

 the root hairs from the capillary films which surround the solid 

 particles of the soil. Indeed, below the water table the soil is in a 

 condition of saturation, which is hostile to the activity of the roots of 

 upland plants through the exclusion of the air. In the beds of 

 small valleys the water table often approaches the surface, being sup- 

 plied by the movement of the soil-water from the neighboring hills, 

 and in this manner springs arise and streams are fed. Along streams 

 the heigh: of the water table will depend upon the elevation of the 

 banks. If these are low the earth immediately adjacent to the 

 stream may be kept in a saturated condition or at least more moist 

 than elsewhere. Percolation of stream water into the adjacent soil 

 will account for the occurrence of trees along the streams of the semi- 

 arid plains of western Kansas and Nebraska, where the soil moisture 

 is elsewhere so low, because of the low rain-fall, as not to support 

 tree growth. Broad level stretches adjoining streams may be so 

 nearly of the same level with the stream as to be inundated at the 

 time of high tides or freshets. Such levels are built up gradually bv 

 the deposition of silt at the time of their inundation. The soils of 

 these flood plains may he of varying degrees of water content accord- 

 ing to their elevation and the frequency of their inundation, but com- 

 monly vary from high percentages of water to saturation. 



