RESPIRATION 



111 



result of the combination, water and carbon dioxide are con- 

 tinually excreted (see note, page 536). 



The amount of oxygen absorbed and of carbon dioxide given 

 off is, however, so trifling compared with the amount of each 

 gas passing in the opposite direction, while starch making is 

 going on in sunlight at temperatures most favorable for photo- 

 synthesis, that under such circumstances it is difficult to observe 

 the occurrence of respiration. 



When the illumination is 

 very feeble, from -^ to ^ 

 that of bright, diffuse day- 

 light, the manufacture of 

 carbon dioxide by respira- 

 tion and its consumption by 

 photosynthesis are equal. 



At high temperatures, 

 such as 104 F. (40 C.), 

 respiration may produce car- 

 bon dioxide more rapidly 

 than photosynthesis can con- 

 sume it, even with brilliant 

 illumination. 



In ordinary leafy plants 

 the leaves, through their 

 stomata, are the principal organs for absorption of air, but much 

 air passes into the plant through the lenticels of the bark. 



In partly submerged aquatics especial provisions are found 

 for carrying the air absorbed by the leaves down to the sub- 

 merged parts. This is accomplished in pond lilies by ventilating 

 tubes which traverse the leafstalks lengthwise. In many cases 

 such channels run up and down the stem (Fig. 120). In the 

 American cypress (Taxodium) the "knees," which rise from the 

 roots, as shown in Plate III, are thought to be for use in respira- 

 tion, obtaining oxygen from the air and carrying it into the 

 roots beneath the water. 



FIG. 120. Cross section of stem of mares- 

 tail (Hippuris), with air passages a 



After Baillon 



