39] CHAPTER I. GENERAL PHYSIOLOGY. 159 



from the sphere of the metabolic activity, by being either excreted, 

 6r~~iecreted in the insoluble form in special receptacles (see p. 

 96}7~ Of the products of catabolism, carbon dioxide is' the most 

 constant. 



There is one property of living plant-cells which is of such funda- 

 mental importance, particularly in connexion with movement, that 

 it requires special mention. It is this, that the cells tend to take 

 up such large quantities of water, that a considerable pressure is 

 set up in the cell between the cell-sap, on the one hand, and the 

 elastic cell-wall on the other. This state of tension is known as 

 iuryidity or turgescence, and a cell in this state is said to be turgid. 

 The conditions upon which turgidity depends are three : first, the 

 presence of substances in the cell-sap which attract water ; second, 

 the presence of a layer of protoplasm lining the cell- wall ; third, the 

 presence of an elastic cell-wall. With regard to the first of these 

 conditions, the necessity for it is obvious. It appears that the 

 substances in question are especially the organic acids or acid salts, 

 which are abundantly produced in the metabolism of plants. The 

 significance of the second condition is, that the layer of protoplasm 

 prevents, at least within certain limits, the escape of the cell-sap 

 as the pressure in the cell increases, and it is on this account that 

 a high tension can be attained. Finally, the presence of an elastic 

 cell-wall is a necessary factor, for without resistance there can be 

 no tension . 



39. The External Conditions. The functions of the plant 

 can only be carried on under a certain coincidence of favourable 

 external conditions. Thus, an ordinary green plant will onlv 

 flourish when the conditions are such that it is supplied with 

 appropriate food, with water, and with free oxygen for its respir- 

 ation, and is exposed to a suitable temperature and to sufficiently 

 intense light. 



The importance of a supply of food and of water is sufficiently 

 obvious to need no further explanation here. The importance of a 

 supply of oxygen is that without it the normal catabolic processes 

 would either cease, or be SD far suppressed that the plant would no 

 longer manifest its vital phenomena ; for instance, it would cease 

 to grow, and would eventually die. 



Inasmuch as the influence of heat and light is so comprehensive, 

 it may be generally considered now, the detailed consideration of 

 these and other external conditions being relegated to the discussion 

 of the functions which they especially affect. 



