130 INTAKE AND DISTRIBUTION OF GASES 



The Lenticels. — In stems where the epidermis is being re- 

 placed by cork the phellogen or cork cambium (see page 54) 

 instead of producing ordinary cork tissue immediately below 

 the stomata forms loose layers of cells, through the intercell- 

 ular spaces of which gases pass in and out (Fig. 22). These 

 groups of loose cells are called lenticels. As the lenticels be- 

 come larger by additions from the phellogen they press out 

 and burst the epidermis and appear at the surface as rounded, 

 elliptical or oblong excrescences. 



The formation of lenticels outruns that of the cork tissue, so 

 that when the cork has encased the stem and replaced the epi- 

 dermis as a protective tissue the lenticels have taken the place 

 of the stomata and have provided aeriferous channels from 

 the exterior entirely through the periderm. (See page 55.) 



The intercellular spaces of the lenticels are in direct com- 

 munication with the main intercellular channels of the bark 

 and wood. This is demonstrated by sealing one end of a cut- 

 off stem, submerging the stem with the closed end downward, 

 and forcing air into the upper end. Air bubbles then issue 

 from the lenticels. And when a ring of bark is removed from 

 the upper end so that the air pressure tube is connected with 

 the wood only the bubbles issue as before, showing that the 

 lenticels communicate with the intercellular spaces in the wood 

 as well as with those of the bark. Plainly the lenticels are 

 for the aeration of the stem throughout its entire body. 



The radial distribution of gases that enter through the len- 

 ticels is carried on through intercellular spaces running through 

 the medullary rays, and from these the vertical distribution 

 takes place largely through spaces between the cells of the 

 xylem parenchyma (Fig. 67). 



The main use of the lenticels is evidently for the interchange 

 of gases in respiration, namely, for the intake of oxygen and 

 the excretion of carbon dioxide, since they occur on stems where 

 respiration must be going on continuously, but where the con- 

 verse process of taking in carbon dioxide for food building is 

 taking place to a very subordinate degree, if at all. Incidentally 



