MOVEMENT OF MATERIALS IN THE PLANT 131 



on the upper surface. If water happens to lie upon the upper leaf surface 

 gas bubbles are observed to be given off rapidly on sunny days, these bubbles 

 arising from the stomata and from any chance openings in the surface of the 

 petiole. This evolution of gas is so violent at times that the water appears to be 

 boiling. This phenomenon is unrelated to life processes, since it occurs also with 

 dead leaves. A similar elimination of gas may be artificially produced by a 

 special arrangement. This consists of a cylindrical porous clay cell filled with 

 finely powdered chalk, or simply with air. A glass tube is inserted through a 

 stopper closing the open end; this tube corresponds to the petiole of the Nelum- 

 bium leaf, while the cell corresponds to the leaf-blade. The porous cell is first 

 dipped in water and is then supported obliquely, the tube ending in a vessel of 

 water below. When the clay cell is heated, gas is given out in large quantities 

 from the open end of the glass tube. This gas is air, practically saturated with 

 water vapor. Frequently the volume of gas thus eliminated is as much as 

 forty times as great as that of the cell itself, so that gas must enter the cell 

 through the porous wall during the experiment. This phenomenon is caused 

 by unequal heating, both in the case of the porous clay cell and in that of the 

 Nelumbium leaf." 



The underground portions of many plants growing in submerged, swampy, 

 or poorly aerated soils, 6 possess root outgrowths that grow upward into the air 



Ohno found the pressure under which gas escapes from Nelumbo leaves to rise sometimes 

 to more than 40 mm. of a mercury column. The explanation is somewhat complicated. The 

 gas pressure outside the clay chamber is due to a large partial pressure of oxygen and nitrogen 

 and a very much smaller one of water vapor, the magnitude of the latter depending upon 

 the humidity of the air. The conditions are reversed on the inside, where the partial pressure 

 due to water vapor is larger than in the other case and that due to the other gases of the air is 

 smaller. The wet porous clay wall, being permeable to the other gases as well as water, move- 

 ment takes place in both directions; water moves outward and evaporates, and nitrogen and 

 oxygen diffuse inward. Since there is an excess of liquid water, the partial pressure of water 

 vapor on the inside remains constant in spite of the outward movement. Also, the water vapor 

 that evaporates from the external surface of the porous clay is quickly removed from the vicin- 

 ity by air currents, so that the partial gas pressure due to water vapor on the outside also 

 remains nearly constant. The external partial pressure of nitrogen and oxygen is also constant, 

 in spite of the inward diffusion, for there is here an excess of these gases and the whole atmos- 

 phere is available. But, as these gases diffuse into the chamber they raise the partial pressure 

 of non-aqueous gases within, and so increase the total gas pressure on the inside. Since the 

 chamber opens to the outside through the tube, this internal gas pressure can never rise much 

 above what it was at the start, for bubbles escape from the open end of the tube. The arrange- 

 ment is a sort of osmometer, with a concentrated solution of water vapor in the other gases on 

 the inside and a very dilute solution of the same sort on the outside, the wet wall being more 

 permeable to nitrogen and oxygen than to water vapor. A relatively large amount of water 

 vapor is contained in the gas that exudes from the tube. The heating of the tube seems to 

 accelerate the process partly because it tends to remove the water vapor as it evaporates from 

 the tube, so as to keep the external partial pressure of the other air gases near its original high 

 value. It thus acts like a stirrer in an osmometer cell, which keeps the internal solution from 

 becoming too much diluted next to the membrane. Also, at higher temperature the vapor pres- 

 sure of water inside the chamber is higher. — Ed. 



6 These structures (called "knees") are characteristic of Taxodium distichum (bald cypress), 

 of the swamps of the southeastern United States. For an excellent photograph showing these 

 see: Schimper-Fisher, 1903. [See note k, p. 100.] Fig. 48, facing p. 74. — Ed. 



