294 Plant Physiology 



through the more rapid growth of certain regions, and the 

 rupture of cells adjacent, as in the case of certain grasses, 

 and of many other plants generally possessing a loose 

 central pith during the early stages of growth. In the 

 more solid or woody stems, intercellular spaces constitute 

 some part of the structure, and the better aerated cortex 

 may be provided with lenticels, or special areas of loose 

 tissue permitting gas exchange.' These lenticels are 

 interruptions of the more or less continuous corky enve- 

 lope constituting an essential part of the true bark of 

 many woody plants. 



Roots may, in general, obtain some oxygen in solution, 

 but the cortical parts of these organs exhibit, as a rule, 

 rather large intercellular spaces, so it is evident that this 

 special type of diffusion mechanism for aeration is impor- 

 tant likewise in subterranean organs. In fact, in many 

 roots there may be found special tissues, apparently 

 insuring a surplus of air, and such may be designated 

 air-storage tissues. Certain plants inhabiting stagnant 

 water are provided with special roots, or root branches, 

 which seem to be important in aeration. To these organs 

 the term "hydathodes" has been applied. 



The relation of leaf-stalk and blade to air or the con- 

 tinuity of the aerating tissues may be very well empha- 

 sized by the experiment shown in Figure 74, in which, 

 when suction is applied to the tube, air passes through the 

 leaf, and is given off in bubbles from the petioles below the 

 surfaces of the water in the bottle. 



170. Anaerobic respiration. It has been clearly 

 demonstrated that respiration may proceed for a time, in 

 most tissues and cells, when no free oxygen is available. 



