THE DESCENDING SAP 



133 



In the leaves are manufactured the 

 starch-grains, proteids, etc., of which 

 some account has been already given, 

 and which are required, not only for 

 the present growth of the plant, but 

 also as reserve food material. But 

 these substances are solid and insoluble 

 in water, a circumstance which pre- 

 vents their passage from cell to cell ; 

 and they have therefore to undergo 

 further changes before they are dis- 

 missed from the leaves. Starch, for 

 example (C H 10 5 ), is converted, by 

 means of a ferment called diastase, into 

 the soluble substance sugar* (C 6 H 12 6 ), 

 which, becoming part of the assimilated 

 nutrient sap, is distributed through 

 the plant, to be again fixed in the 

 form of starch at particular places, as 

 in the grains of cereals, the tubers of 

 the Potato, etc. The proteids are also 

 changed, the agents in their trans- 

 formation (known as proteolytic enzymes) 

 being pepsin and the various trypsins. 



But we spoke of respiration. What 

 is it ? If the term is a misnomer as 

 applied to the evolution of oxygen 

 from plants, in what does true respira- 

 tion consist? The question may be 

 answered by a simple experiment. Soak 

 in water for twenty-four hours, to 

 induce germination, a quantity of peas, 

 then place them in a jar, disposed in 

 single layers between pieces of moist 

 blotting-paper. The mouth of the jar 

 is closed by a tightly fitting cork, which 

 is withdrawn after an interval of a 

 few hours. Now take a lighted taper 

 and plunge it into the vessel. In- 

 stantly the flame is extinguished. You 

 guess the cause ? While confined in 

 the jar the peas have been evolving carbon dioxide, and in carbon dioxide 



* I.e. glucose or grape-sugar. The formula for canose or cane-sugar, sometimes called 

 sucrose, is Cn 



Photo by\ [. Step. 



FIG. 168. Monstera deliciosa. 



The fruit-spike of this Mexican plant. The seeds "are 



embedded in a luscious pulp, which has a flavour 



similar to that of pine-apple. The leaves have large 



perforations in their tissues. 



