84 THE VEGETATIVE ORGANS 



important fact, namely, that a green leaf, working under 

 normal conditions, but in air devoid of carbon dioxide, 

 is unable to form starch. In addition to the above condi- 

 tions it is found that the work of a leaf can only proceed 

 at a suitable temperature and when the plant is able 

 to obtain a sufficient supply of water. Thus moisture, 

 warmth, sunlight, carbon dioxide, and chlorophyll are 

 all necessary for the formation of starch in a leaf. 



If we think over the previous experiments we meet 

 with an apparent contradiction. We have just learnt that 

 starch is formed in green leaves only under the influence 

 of sunlight, and not in parts that do not contain chloro- 

 phyll, yet we found an abundance of starch in the 

 cotyledons of the Bean and in the endosperm of the Wheat 

 and other grains ; and it also occurs, as we shall see, in the 

 non-green parts of stems and roots. It is obvious, therefore, 

 that starch arises in more ways than one within the tissues 

 of a plant. 



Now the starch grains formed in green leaves are 

 formed entirely from inorganic substances ; the exact 

 method is not known, but a possible explanation is the 

 following : 



In words, the carbon dioxide and water within the living 

 chlorophyll-containing cells of the leaf, and under the 

 conditions already enumerated, may be split up and their 

 constituent atoms rearranged to form a compound called 

 formaldehyde and also the element oxygen. This will 



