104 BOTANY OF THE LIVING PLANT 



fall, but at the same time the level will rise in the left-hand tube. It 

 will be found that the gas evolved on the right is approximately equal 

 in volume to that absorbed on the left. Tests show that the right- 

 hand gas is oxygen, while the left-hand gas is still carbon dioxide. 

 What has happened is that as the carbon dioxide in solution in the 

 water is absorbed by the leaf, and oxygen given off, fresh carbon 

 dioxide is withdrawn from the left-hand tube, and it is seen that its 

 volume is approximately equal to the oxygen evolved. 



The process of Photo -Synthesis involves deoxidation, for its first step 

 consists in the breaking up of the stable molecule of carbon dioxide, 

 and the separation from it of free oxygen, given off as the bubbles seen 

 in experiment (Fig. 73). For such deoxidation, and the consequent 

 production of a combustible substance such as starch, energy must be 

 expended. The source of that energy is Light ; and since the rays 

 which are absorbed by the chlorophyll are shown to be those which 

 are active in Photo-Synthesis, it may be concluded that it is the energy 

 of the absorbed rays which causes the deoxidation. It is probable that 

 the process is not a simple one, but includes several steps. In the 

 interchange of gases, and in the visible product, starch, we only see 

 the initial and final results. But starch is not always the first visible 

 product. Oil globules make their appearance in some cases, e.g. 

 Musaceae. In others no visible product is directly observed, as in 

 many Monocotyledons. Since starch is not the invariable product 

 first seen, it seems not unlikely that it is here, as elsewhere in the plant, 

 merely a compact insoluble form in which carbohydrate is put aside. 

 It is derived from sugar, and it is probable that sugar is the substance 

 actually formed in Photo -Synthesis. It has further been suggested that 

 a step in the constructive process is the production of formic aldehyde 

 (CH 2 0). One reason for this conclusion is that if an Alga, such as 

 Spirogyra, be supplied with a weak solution of formic aldehyde in the 

 absence of carbon dioxide, and exposed to the light, it will form 

 starch. In this expeiiment the plant is presumed to be saved one of 

 the constructive steps by receiving the formic aldehyde already 

 formed. A further step might be the polymerisation of the formic 

 aldehyde to form sugar, and so on to the starch which we see. These 

 are, however, matters rather of theory than of demonstration, and 

 it must be admitted that exact knowledge of the steps of the Photo- 

 Synthetic process is still wanting. But none the less is it certain that 

 the construction of new organic material is actually carried out. 



It is well to form some concrete idea of the amount of material 

 gained in the process. It has been calculated that the amount of 



