MORPHOLOGY OF HIGHER PLANTS. 221 



with the denser liquid. This process is known as osmosis, ^nd 

 when a liquid passes outward through a membrane or cell-wall 

 it is called exosmosis, and when inward it is called endosmosis. 

 The soil is made up of minute earth particles, each of which is 

 surrounded by a thin film or envelope of water, and it is this por- 

 tion of the soil liquid that is absorbed by the root hairs. The root 

 hairs come into close contact with these soil particles ; in fact, 

 appear to grow fast to them, and the cell-liquid in the root hairs 

 being denser than that surrounding the soil particles, the latter 

 passes through the wall into the root hairs. 



If, on the other hand, the water supplied to the roots of plants 

 should contain an excess of soluble material, the plant will be 

 injured. In this case exosmosis ensues and the plant loses some 

 of its own liquids or cell-sap and will show signs of wilting. It 

 is well known that if cultivated plants are supplied with strong 

 solutions of fertilizer the plants will be injured rather than 

 benefited. 



Root Pressure. — The distribution of the water absorbed by 

 the roots to other parts of the plant is influenced by a number of 

 factors, which are commonly spoken of together as root-pressure. 

 Among these are osmosis within the plant, due to unequal density 

 of the liquids in different cells ; the changes in the equilibrium of 

 the cell-liquids, due to chemical changes, and the transpiration 

 of water from the leaves, thus establishing a flow of liquids from 

 the roots upward, which is usually spoken of as the ascent of 

 SAP. The cell-sap passes upward through the xylem for the most 

 part carrying constituents obtained from the soil to the growing 

 parts, where they are combined wjth the products of photosyn- 

 thesis, and through a series of reactions protoplasm is finally 

 built up. 



Oxidation. — The free oxygen taken in by plants through the 

 stomata and lenticels serves the same purpose in plants as that 

 inhaled by animals, namely, the oxidation of certain compounds, 

 whereby part of the energy necessary for vital activity is lib- 

 erated. Oxygen is required by all parts of the plant. When the 

 roots of plants, such as those of Zea Mays, are surrounded by 

 water so as to exclude the air the plants will become yellow. 

 Germinating seeds consume a large amount of oxygen, but not 



