22 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 3 



this hypothesis is fully confirmed by research. The water 

 is absorbed into the plant from the soil through the roots, 

 conducted through the stem, and distributed through the 

 veins to all parts of the chlorenchyma, from which its 

 immediate evaporation is prevented by the waterproof 

 epidermis. As to the carbon, that is known to come 

 not from the soil (for plants can be grown to perfection 



in soils, or even in water, 

 which lack it completely), 

 but from the air, in which 

 it exists in the form of 

 carbon dioxide (C0 2 ), 

 the heavy poisonous gas 

 which is released by com- 

 bustion and also by the 

 respiration of animals. 

 It is true, this gas is 

 relatively scarce in the 

 atmosphere, of which it 

 comprises only about .03 

 per cent (3 parts in 

 10,000) as compared with 

 about 21 per cent of 

 oxygen, and 79 per cent 

 of nitrogen; but even 

 this small amount suffices 

 for the photosynthetic needs of plants, as can be proved in 

 various ways. Thus, one has only to keep a thin-leaved plant 

 for a day or two in the dark to free it of starch : remove two 

 similar leaves and place them in water in two glass chambers 

 exactly alike except that from one all carbon dioxide has been 

 removed by a chemical absorbent : expose them thus a few 

 hours to Hght : blanch them of chlorophyll : and immerse 

 them in iodine, when there follows the result pictured here- 

 with from an actual experiment (Fig. 5). Thus it is clear 

 that a leaf can make starch, and therefore sugar, if the car- 



Fiq. 5. — Leaves treated with iodine 

 after exposure to light in air lacking and 

 possessing, respectively, the usual carbon 

 dioxide ; X^. The black shading represents 

 dark blue in the actual leaf. 



