CHAPTER I. GENERAL PHYSIOLOGY. 669 



limits, the escape of the cell-sap as the pressure in the cell in- 

 creases, and it is on this account that the high degree of turgidity 

 of plant-cells is attained. Finally, the presence of an elastic 

 cell-wall is obviously a necessary factor, for without resistance 

 there can be no pressure. 



It commonly happens, as will be subsequently pointed out in 

 many instances, that an escape of cell-sap from turgid cells may 

 take place without any rapture or injury of the cells ; this is 

 termed the escape of cell-sap by filtration under pressure. 



It is clear, since the plant can only absorb dilute solutions of 

 solids dissolved in water, that a much larger quantity of water 

 than is necessary for the immediate wants of the plant, must be 

 absorbed ; and it is necessary that this excess of water should be 

 got rid of. It is got rid of mainly in the form of watery vapour 

 which is exhaled into the air. This exhalation of watery vapour 

 is not simply physical evaporation, but is a vital phenomenon 

 controlled by the living protoplasm ; it is termed transpiration. 



2. METABOLISM. This term refers to all the chemical changes 

 which go on in living protoplasm itself, and which it induces in 

 other substances. 



a. Anabolism. It has been already stated that the metabolic 

 processes included under this head, are those in which complex 

 substances are formed from simpler ones. The most complex 

 substance of all being protoplasm, it may be stated that the end of 

 the anabolic processes is the construction of protoplasm. 



The anabolic process which is most fundamentally important, 

 and which is most characteristic of plants as opposed to animals, 

 is the construction, by the green parts of plants under the in- 

 fluence of light, of organic substance from carbon dioxide 

 absorbed from the air, and water absorbed from the soil. This is 

 the first step in the process of assimilation as carried on in 

 plants containing chlorophyll. 



b. Catabolism. Under this head are included the dissociations 

 which the molecules of protoplasm undergo, and those which it 

 induces in other complex substances, either directly, or by means 

 of certain metabolic substances known as enzymes or unorganised 

 ferments. 



In most cases the catabolism of the plant is accompanied by a 

 gaseous interchange between the plant and the air, of this nature 

 that the plant absorbs oxygen gas and gives off carbon dioxide. 

 This gaseous interchange is known as respiration. 



