56 



PHYSIOLOGY OF NUTRITION. 



17. The Stomata and Assimilation. 



Recent researches of Stahl clearly indicate that the Carbon 

 dioxide required in assimilation enters the leaf under normal cir- 

 cumstances through the stomata (the case is different if the 

 quantity of Carbon dioxide in the air sur- 

 rounding the plants is increased), and that at 

 most only traces of the gas find their way 

 into the interior of the tissues through the 

 cuticularised membranes. I repeated Stahl's 

 experiments with cut twigs of Lonicera 

 tatarica, and pot plants of Phaseolus multi- 

 florus. The material was first kept for 

 twenty-four hours in the dark ; the leaves 

 had by this time become free from starch. 

 Now a few of the leaves, without being cut 

 off, were painted on one half of the lower 

 side with a warmed mixture of 1 part wax 

 and 3 parts cacao butter, in order to close 

 the stomata, and then at once exposed to 

 the sunlight. It is advisable to leave on the 

 twigs or rooted plants only a few leaves so 

 made from material sub- that they transpire feebly and do not wither. 

 After 4 to 6 hours the leaves are dipped into 



^.A.lLGr &L3/D1.J A * 



cold spring water, the crust of fat is dis- 

 solved from their under sides, and after extraction with alcohol 

 they are submitted to the Iodine test. The portions of the leaves 

 whose stomata were sealed, have formed no starch, while the 

 assimilation in the other halves of the leaves has taken place 

 normally (see Fig. 20). 



If the upper surface of the leaves is coated with the cacao wax, 

 the assimilation is not much reduced, since large quantities of 

 Carbon dioxide pass into the interior of the leaf through the 

 stomata of the lower surface. This experiment at the same time 

 teaches that the presence of the coating of wax does not in itself 

 in any way injure the leaf. 



We cut twigs of Lonicera tatarica, Syringa vulgaris, or Sambucus 

 nigra, and place them for twenty- four hours in the dark to deprive 

 them of starch. Small fragments are removed from some of the 

 leaves, and it is determined that starch is absent. When this is the 

 case we cut a few leaves from the twigs, and divide them into halves, 



FIG. 20. Young leaf of 

 Prunus padus. The right 

 half has been smeared 



