ELEMENTARY VITAL PHENOMENA 



159 



cleavage of carbonic acid ; this takes place in the green plant-cell 

 under the influence of sunlight. The plant gives off oxygen to 

 the outside. As to the fate of the retained.- carbon, microscopic 

 observation gives us information. It shows, namely, that m pro- 

 portion to the destruction of the carbonic acid starch is formed m 

 the chlorophyll-grains themselves, and is laid down in the form of 

 small, highly refractive granules (Fig. 23, p. 81 , and Fig. 56). More- 

 over, by a series of experiments Sachs has shown that as soon as the 

 breaking-up of the carbonic acid ceases 

 in darkness the formation of starch also 

 ceases, immediately to begin again in 

 the light along with the destruction of 

 carbonic acid. Since starch contains, in 

 addition to carbon, only hydrogen and 

 oxygen in the same relative proportion as 

 in water, it can be derived only by syn- 

 thesis from the carbon that is set free 

 and the water that i.s received through 

 the roots. Starch is, therefore, the first 

 assimilation-product to appear. 



" If," says Sachs, ('82)," starch is the first 

 and sole visible product of assimilation, it 

 follows directly that all other organic com- 

 pounds of the plant must originate by 

 chemical metamorphosis from it." It will 

 be remembered that no carbon was present 

 in the artificial nutrient solution in which 

 plants were allowed to grow.'^ If, therefore, 



laterthe plant manufactures other carbohydrates,fats,andfinallypro- 

 teids, all of which contain carbon, it can employ only starch as the 

 starting-point. Of course almost nothing is known concerning the 

 special chemical transformations which starch undergoes further. 

 But an idea can be formed, at least in gross outline, of the further 

 processes of assimilation. The fact that from the starch soluble 

 varieties of sugar can be derived very easily by cleavage with 

 hydration, is at once understood when it is borne in mind that 

 starch is a polymeric molecule of the anhydride of sugar. Hence 

 it can pass into the condition of the soluble carbohydrates, and 

 this is necessary in order to make possible further chemical syn- 

 theses. The formation of fatty oils out of starch can also be 

 directly observed. If unripe seeds of certain plants, e.g., Pceonia, 

 which contain carbohydrates and no fats, be allowed to lie in moist 

 air, it is found after some time that all starch has disappeared, but 

 fatty oil has appeared in its place. But much more complicated 

 is the origin of proteid from carbohydrates. Since in addition to 

 the atoms of carbohydrate proteid contains nitrogen and sulphur, 



' Cf. p. 138. 



Fig. 56. — Starch appearing as 

 transparent scales in chloro- 

 pliyll-bodies. A, Chlorophyll- 

 bodies lying in the cell. B, 

 Chlorophyll-bodies undergoing 

 division. (After Sachs.) 



