OR "PHOTOSYNTHESIS 211 



that this compound is the first step in the formation of carbon 

 compounds from carbon dioxide and water is nothing more than 

 a hypothesis. 



What is certain is that sugars are soon formed in the 

 cells of the leaf-parenchyma after the green leaves of plants 

 absorb carbon dioxide from the air, and the brilliant investiga- 

 tions of Brown and Morris point to the conclusion that cane- 

 sugar is the first sugar to be manufactured, and that subsequently 

 dextrose, levulose and maltose sugars make their appearance in 

 leaves in consequence of the action of enzymes upon the 

 previously-formed cane-sugar and starch. 



In a great many plants when the accumulation of sugar within 

 the cells of the leaves reaches a certain point the chloroplasts 

 form starch-grains from it ; the starch-grains appear within the 

 substance of the chloroplasts and are the first visible products of 

 'carbon-fixation.' 



The total amount of carbohydrates produced by leaves of 

 the same area depends upon internal vital peculiarities of the 

 different species of plants ; for example, in a given time a 

 sunflower leaf produces more than a leaf of a dwarf-bean of the 

 same area. The amount manufactured by a sunflower during 

 twelve hours on a moderately bright day was found by Brown and 

 Morris in one instance to be a little more than 12 grains of 

 carbohydrates per square metre of leaf-surface. 



2. The manufacture or synthesis of carbohydrates in the 

 manner indicated above is dependent upon various conditions, 

 of which the following are the most important : — 



(i) The plants must be living. 



(ii) Carbon dioxide must be present in the air surrounding 

 their leaves. 



(iii) The leaves must contain chloroplasts. 



(iv) A certain intensity of light is essential, and 



(v) an adequate degree of temperature is necessary for the 

 process. 



