468 BOTANY 



become completely dried up during the summer, but absorb water 

 and again become active with the first rains. A similar absorption 

 by aerial organs is seen in many Liverworts and Mosses ; and some 

 epiphytic plants, like Bromeliaceae, often have upon their leaves 

 scales which absorb water from the atmosphere. So also the aerial 

 roots of many Orchids can absorb atmospheric moisture. 



The absorption of water by ordinary foliage leaves is usually pre- 

 vented by the character of the epidermis, which often has a waxy 

 coating that sheds water. Fluid water is necessary to restore tur- 

 gidity to the cells, but sometimes this may be obtained by the con- 

 densation of watery vapor from a saturated atmosphere. This is 

 seen in such Lichens as Ramalina reticulata. 



Properties of the Soil 



The soil serves a twofold purpose to the roots. It is both a means 

 of attachment to hold the plant in place, and the source of the greater 

 part of the food of the plant aside from carbon-dioxide. Air is pres- 

 ent in the interstices of the soil, and this is necessary for the healthy 

 action of the roots. 



Ordinary soils consist of two kinds of elements, both of which are 

 important to the plant. The first of these comprise mineral detritus, 

 sand, clay, etc., but these are mixed with a certain amount of organic 

 matter humus derived largely from the decomposition of vege- 

 table tissues. The humus seems to be especially important in the 

 retention of moisture in the soil. 



The root-hairs push through the interstices in the earth, and 

 apply themselves very closely to the solid particles composing the 

 soil. These particles are, in ordinary soils, invested with a film of 

 water, and the air in the spaces is also saturated with water. Soils 

 abounding in humus are more retentive of moisture than those which 

 have little or no humus. 



Within the soil, through the agency of water, or water combined 

 with C0 2 , the soluble food constituents of the soil are dissolved, and 

 thereby put in such form that they may be absorbed by the roots. 

 The water in the soil is a weak saline solution, containing substances 

 washed into the soil by rain, or formed from the dissolving of the 

 solid mineral particles of the soil. The decomposition of the humus, 

 through the agency of Fungi or Bacteria, also furnishes some food 

 materials. The great importance of certain Bacteria in fitting the 

 soil for the growth of the higher plants has only recently been fully 

 recognized. 



The roots themselves, by the evolution of C0 2 , and in a lesser 

 degree by excreting various organic acids and salts of these acids, 

 exercise a solvent action upon the mineral constituents in the soil. 



