Elementary Plant Physiology. 



the cutting of the channels which carry food substances from 

 the leaves to the lower part of the stem, although the incisions 

 may be made deep enough to sever the conduits of the up- 

 wardly mov- 

 ing current of 

 water also. If 

 the downward 

 stream only is 

 interrupted in 

 the girdling of 

 a tree, it may 

 live through 

 the season in 

 which the operation was performed, but 

 die soon afterward from starvation of the 

 roots. 



74. Air passages in plants. — The 

 living cells in active tissues in all but the 

 simplest plants are generally more or less 

 separated by spaces filled with atmos- 

 pheric gases. In some instances the vol- 

 ume of these air spaces may comprise as 

 much as one-twelfth of the entire bulk of 

 the plant, a condition prevailing in many 

 aquatics. These spaces and passages fa- 

 cilitate the interchange of gases between 

 the protoplasts and the atmosphere, and are generally most 

 numerous in tissues not provided with ready communication 

 with the air. 



The air spaces in the loosely arranged parenchyma cells of the 

 leaves are generally continuous with those of the cortex of the 

 stems, and these spaces connect at many points with the vessels 

 of the vascular system, which are usually free from liquid con- 

 tents. In addition to the stomata, the lenticels also afford 



Fig. 54. — Girdled 

 shoot of Sambucus. 

 After Oels. 



