160 THE PLANT CELL. 



to take place in the substance of small structures comparable to 

 plastids, knuwu as elaioplasts ; in these bodies glycerine and a 

 fatty acid are combined to form the oil or fat. 



The deposition of starch in the plastids and chloroplasts is in 

 the main a process of secretion ; the sugar, which is first formed 

 during photosynthesis (see infra), being utilised for the purpose 

 of starch-formation (storage) ; this process is thus katabolic in 

 nature. 



The cell obtains energy for the purposes of elaboration of 

 food from several sources, viz. : — 



a. Light. 



/'. External heat. 



t. Internal heat liberated during oxidation." 



The influence of heat upon vital activity iu a cell increases 

 up to a certain point, the so-called optimum temperature, after 

 which it again decreases. 



With regard to the relation between heat and chemical 

 action, the following reservations must be made; — Some 

 reactions require for their completion heat from outside or from 

 the cell itself, and these are known as exothermic reactions, 

 whilst others evolve heat during their progress, and are called 

 endothermic reactions. In the former case the cell loses a 

 certain amount of energy, whilst in the latter energy is gained. 

 Occasionally reactions occur which may be exothermic or endo- 

 thermic according to circumstances, and these are known as 

 reversible reactions. In the case of the energy of light rays 

 (radiant energy) it will be seen further on that the chloroplasts 

 are able to transform the radiant energy into energy of chemical 

 action (actinic), and in this manner the chloroplast is enabled to 

 form starch (or sugar) from the raw materials COj and HgO 

 supplied to it. 



Occasionally the energy of chemical action (oxidation) is 

 intense enough in plant-cells to cause luminosity (certain 

 Bacteria). This phenomenon is, however, not so frequent in 

 plants as in animals (see infra). 



The absorption of water by germinating seeds is often 

 attended with a considerable evolution of heat, due partly to 



* A large part of the energy of a plant is derived frona the oxidation of 

 carbohydrates during respiration. 



