THE MOLECULAR FORCES IN PLANTS. 175 



the comparatively small pressure produced by mere blowing with 

 the mouth is insufficient to force the water out of these stomata. 



In experiments respecting the passage of air through the 

 stomata, it is often suitable to adopt the following method of 

 procedure. We fix air-tight in the shorter limb of a bent glass 

 tube, a leaf-stalk bearing an uninjured lamina, or a stem bearing 

 leaves (I used, e.g., the end of a Camellia shoot bearing a terminal 

 bud and a single leaf). The closure is made air-tight in different 

 ways according to circumstances. Frequently it is sufficient to 

 seal up with a mixture prepared by melting together equal parts 

 of yellow wax, olive oil, and melted mutton suet. In other cases 

 we use a piece of rubber tubing ; or we first fit the tube with a 

 perforated cork, and pass the leaf-stalk or shoot through this, 

 and theo complete the operation by smearing carefully with wax 

 mixture. If we pour mercury into the longer limb of the bent 

 glass tube, and place the apparatus in a cylinder filled with water, 

 the compressed air escapes from the stomata, and bubbles of air 

 of various sizes pass from the leaf-blade and rise through the 

 water. 3 



1 See Weiss, in Pringsheim's Jahrbiicher, Bd. 4. 



2 Literature : Mohl, Botan. Zeitung, 1856 ; Schwendener, Monatsberichte d. 

 Berliner Akademie d. Wiss., 1881; Leitgeb, Mittheilungen d. botan. Institnts zu 

 Graz, Bd. 1, Jena, 1886; Schaefer, Jahrbiicher f. wissensch. Botanik, Bd. 19. 



3 With reference to what is here said, see especially Sachs, Handbuch der 

 Experimentalphysiologie d. Pflanzen, 1865, p. 252. 



68. Positive and Negative Gaseous Pressure in Plants. 



The air in the intercellular spaces of submerged plants, in 

 which, as is well known, stomata are usually absent, is often at a 

 positive pressure. This excess of pressure may be brought about 

 in various ways, but the assimilatory activity of the green parts of 

 the plants, under the influence of sunlight, is of special importance. 

 If we place shoots of Elodea in spring water, and expose them to 

 direct sunlight, a stream of bubbles will spring from the cut end 

 of the stem, as we have proved in 11. This evolution of Oxygen 

 ceases almost at once if we prevent access of light. If uninjured 

 plants of Elodea or Ceratophyllum, or shoots of these plants (I 

 experimented with shoots of Elodea) whose cut surfaces have been 

 smeared with wax, are exposed under water to the influence of 

 direct sunlight, there is no escape of bubbles of gas. The Oxygen 



