12 THE PLANT CELL. 



essential to the continued activity of protoplasm in plant-cells 

 is the presence of oxygen, either as a gas or in a compound, 

 whereby, just as in the animal cell, the protoplasm is oxidised, 

 giving rise to the evolution of heat ; many of the bye-products 

 formed in cells are the result of oxidation processes, whereby, 

 finally, complex compounds are broken down into carbon dioxide 

 and water (see also Chap x.). In addition to the water of 

 constitution mentioned above, protoplasm requires an extra 

 supply of water (in which certain essential salts are dissolved) for 

 vital processes, and this it derives from the soil, air, or water 

 surrounding the cells of a plant; and here a very important 

 point arises — viz., the question of "turgidity," — by which term 

 is meant an equilibrium between the sap inside a cell and the 

 fluids outside, this balance being known physically as osmotic 

 equilibrium. Turgidity has been shown to favour growth, and 

 it is a common experience that slack or withering parts of a 

 plant soon cease to live (see Plasmolysis, Chap. ii.). 



To recapitulate then, it may be said that the following con- 

 ditions are necessary to the continued activity of protoplasm : — 



(a) A certain temperature, which, in most plants, is something 

 above zero Centigrade. 



(6) Access to moisture. 



(c) The presence of oxygen. 



(rf) A requisite degree of turgidity in the case of an enclosed 

 protoplast, and, in addition, 



(e) Protoplasmic continuity in the case of a cell-community 

 between the living cells of the same plant. This factor 

 is important, and will be considered more fully later ; and 



(/) Tlie presence of certain assimilable food-materials and 

 mineral salts (see Chap. x.). 



Note. — Protoplasm is soluble in dilute caustic potash and also in 

 solutions of sodium or potassium hypochlorite : the nucleus also being 

 dissolved. The living substance (cytoplasm) is also dissolved by solutions 

 of pepsin or trypsin ; the nucleus (diromatin) resists pepsin, but dissolves 

 in trypsin solution. At a certain temperature (between 70° and 80° C. ) 

 protoplasm passes into a condition known as "heat-rigor," when all 

 functions cease, the living substance being killed (coagulation). 



