22 



A TEXTBOOK OF BOTANY 



[Cn. Ill, 3 



this hypothesis is fully confirmed by research. The water 

 is absorbed into the plant from the soil through tr^e rooia^ 

 conducted through the stem, and distributedthrouglTThe 

 veins to all parts of the ^h[nrppchyma,. from which its 

 immediate evapur a, llo"lT~Ts ^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 wilich 

 it exists in the form of 

 carbon dioxide (CO 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 



FIG. 5. Leaves treated with iodine , , ,. 



after exposure to light in air lacking and % about 21 per Cent Ot 



possessing, respectively, the usual carbon oxygen, and 79 per Cent 

 dioxide; XT- The black shading represents ,. .. 



dark blue in the actual leaf. f nitrogen; but even 



this^Bftftil^mount sufhctes 



forjii^photosynthetic needs of plants, as canptTprov^d in 

 vajjmis^ 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 light : 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- 



