252 A TEXT-BOOK OF BOTANY. 



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 with 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 

 all the energy formed is used by the plantlet, much of it escaping 

 as heat, as in the germination of barley in the preparation of malt. 

 Those plants dependent upon the presence of free atmospheric 

 oxygen are called A ERODES, while those which are not thus de- 

 oendent, as certain fungi and bacteria, are called ANAEROBES. 



METABOLISM. Processes of construction and destruction are 

 going on simultaneously in the plant, and these are all grouped 

 under the general name of metabolism. The processes whereby 

 complex substances are built up from simpler ones, as in photo- 

 synthesis, are together spoken of as CONSTRUCTIVE METABOLISM 

 (anabolism), while those which involve the breaking down of 

 complex compounds into simpler ones, either through oxidation 

 or other chemical action, as when sugar is changed into carbon 

 dioxide and water, are grouped under the head of DESTRUCTIVE 

 METABOLISM (catabolism) . 



Inasmuch as the carbon dioxide of the atmosphere and the 

 water taken up by the roots together with the mineral salts which 



