igS 



NUTRITION 



They are poorer in oxygen than are carbohydrates. They 

 will not dissolve in nor mix with water. 



Formation of Starch at the expense of the carbonic acid 

 absorbed by leaves. — If we cultivate a bean-seedling with the 

 aid of the inorganic culture-solution mentioned on page 191, 

 but keep it constantly in absolute darkness, we shall find that 

 the organic substance in the seedling is not greater in quantity 

 than it was in the seed. This is due to the fact that, in the 

 absence of light, the plant cannot absorb carbonic acid. The 

 plant is starving. If we now pluck some of its leaves, put them 

 in methylated spirits to decolorise them, and iinally place them 



in a solution of iodine, the leaves 

 will assume a yellow colour. They 

 contain no starch. If we now 

 expose the plant to the light for 

 several days the leaves become 

 green, and soon absorb carbonic 

 acid ; and when we treat these 

 leaves as we did the others, they 

 assume a deep blue colour (appear- 

 ing black) in iodine. Therefore 

 these green leaves contain starch. 

 In the bean-plant the absorption 

 of carbonic acid by green leaves 

 exposed to the light causes starch to 

 be manufactured. If we expose 

 only the roots to the light, starch 

 will not appear in the starved 

 plants: this illustrates the fact 

 that chlorophyll is essential for 

 the formation of starch at the 

 expense of carbonic acid. Again, if 

 we expose the leaves of the starved 

 plant to the light, but remove the 

 carbonic acid from the air sur- 

 rounding the plant, no starch 

 will appear. This proves that 

 it is the carbonic acid which 

 supplies the carbon essential to build up the starch. The 

 same experiments may be performed on a green bean-plant 

 which has been grown in the presence of light but subse- 



