THE MOLECULAR FORCES IN PLANTS. 



207 



(see Fig. 78). The cut surface dips into the water. We place 



the arrangement in a large glass 



cylinder, pour mercury into the long 



limb of the tube, and finally cover 



the cylinder with a glass plate, so 



as to retard as far as possible the 



transpiration of the shoot. After a 



longer or shorter time, drops of water 



escape from the tissue of the leaves. 



I saw this formation of drops of 



water very beautifully in experiments 



performed as described with shoots 



of Impatiens Balsamina. With an 



over-pressure of 35 cm. of mercury, 



drops of water escaped from the 



teeth along the margin of the leaves 



in the course of a few minutes. 1 



According to Pitra's account, it 

 should be easy to make the observa- 

 tion that leaf- bearing shoots, cut off, 

 and almost completely immersed up- 

 side-down in water, yield fluid in 

 considerable quantities from the cut 

 surface of the stem. I have repeated 

 many of Pitra's experiments, but 

 with negative results. My experience, 

 however, does not extend far enough 

 for a critical examination of the ex- 

 periments of this author, and therefore I shall not here consider 

 them further. On the other hand, it is possible to prove with- 

 out difficulty with pieces of the stem of Zea, or Sorghum vul- 

 gare, that not only in the roots, but even in the cells of stem 

 structures, pressures may be set up osmotically, which result in 

 escape of fluid. The pieces of stem, about 10 cm. in length, are 

 taken from vigorous maize plants, and from plants of Sorghum 

 which are beginning to flower, the upper section being made at a 

 point a few millimetres above a node. On immersing them in 

 water with the upper cut surface outside the fluid, and covering 

 with a bell-glass, sap soon escapes from the cut surface. If we 

 dry the surface with blotting-paper, it soon becomes moist again. 



But other causes besides osmotic pressure may give rise to an 



FIG. 78. Apparatus for investi- 

 gating the influence of pressure on 

 the escape of water from plant 

 structures. 



