THE LEAF 131 



This substance, however, is always found in some quantity 

 in tJie fresh extract of green tissues, which fact suggests that 

 at all times a breaking down and a new formation of chloro- 

 phyll is taking place. 



The less soluble xanthophyll forms in the above-mentioned 

 liquids a yellowish non-fluorescing solution ; it does not con- 

 tain any nitrogen, and is less readily decomposed than chloro- 

 phyll. It makes its appearance in the cells before the green 

 colouring matter, and it remains longer in the cells at the 

 approach of autumn. Hence the leaves become yellow coloured 

 In the autumn, and in spring the young shoots and leaves are 

 yellowish-green. 



As soon as the chlorophyll corpuscle is fully formed, it 

 begins to assimilate, forming in most cases starch, more rarely 

 drops of oil (in many Monocotyledons). It is certain that other 

 substances are contained in the chlorophyll corpuscle (in 

 some cases protein crystals and tannin masses have been 

 observed), but a micro-chemical demonstration of the same has 

 not as yet been possible. Speaking generally, however, of 

 the production of organic matter in the cell, we find that the 

 first products of assimilation are not very rich in oxygen, and 

 that in the further changes they become more and more 

 oxidised, until finally oxalic acid is formed, and then it returns 

 again to carbonic acid, which is set free and given off. 



So we have in the life of the leaf a marvellous cycle of 

 changes. The leaf forms, under the influence of light, its 

 chief formative substance (chlorophyll), and the latter starts 

 at once its assimilation of substances, poor in oxygen, by 

 combining the raw solution of the soil with the carbonic 

 acid of the air. This gas, absorbed from the atmosphere, is 

 split up in the vegetable cell, the carbon being rapidly used 

 up, the oxygen being liberated. In this way sugar, starch, 

 and cellulose are formed. But the untiring oxygen exerts 

 its powers on the newly-formed substance. It unites with 

 these new bodies and forms more highly oxidised com- 

 pounds, and acids, such as oxalic acid, become more numerous. 

 The leaf renders the otherwise injurious acid harmless by 

 combining it with lime, and thus forming insoluble crystals of 

 oxalate of lime. These have either the form of needle-shaped 



