222 PLANT PHYSIOLOGY 



atal openings. Even the cells buried most deeply in the leaf are 

 thus seen to be in direct communication with the outside air, with 

 the result that carbon dioxide can reach them freely. 



Lenticels and Air Channels. — The cortex of the stem contains 

 many living cells and is in similar contact with the outer air through 

 the lenticels, which permit gases to diffuse in and out. Water 

 plants, such as the water lily and lotus, have special air channels 

 which connect the leaves floating on the surface with the stem 

 lying at the bottom of the stream or lake. These air channels are 

 so large that they can be easily seen with the naked eye. What 

 child who knows them has not used them for soda straws or for 

 blowing bubbles? In marshes are found plants like the cat-tail 

 (Tijpha) and umbrella rush {Cyperus), which also have large air 

 spaces connecting the parts above the soil with the roots growing 

 in their muggy, airless environment. The underground portions 

 of plants like the bald cypress (Taxodium) and the water willow 

 {Jussixa) which grow in swampy, poorly aerated soils have spe- 

 cial outgrowths which come up into the air from the roots. These 

 structures have spongy tips, which readily permit air to enter 

 and thus act like ventilation pipes conducting air to the regions 

 below. The pneumathodes of plants like the oil palm can be 

 induced by excessive moisture and serve a similar purpose. The 

 living cells of the roots are in this way kept in constant connection 

 with abundant sources of oxygen. 



Work of Molisch and Wiesner — While the gases may diffuse 

 freely into the intercellular spaces between the cells, that does not 

 explain how they get into the protoplasm. Between the air cham- 

 ber and the interior of the cell are two barriers— the cell wall and 

 the plasma membrane — which must be passed. The most im- 

 portant work on the movement of gases through cell walls has 

 been done by Wiesner and Molisch (1890) who experimented 

 with various kinds of tissues fastened to the end of a glass tube. 

 The tissue was sealed to the tube so that the margins were air- 

 tight and was supported in such a way that no cracks or breaks 

 would occur. The tube was then partly or entirely filled with 

 mercury, and the open end was closed with the finger while the 

 tube was inverted and the lower end placed in a vessel of mer- 

 cury. After the tube was thus arranged in an upright position, 

 the height of the mercury column was measured and the time 

 noted for the mercury to drop in the tube when the upper surface 



