238 COLLOIDS IN BIOLOGY AND MEDICINE 



The Movement of Water in Plants. 



The evaporation of water in plants proceeds more rapidly than 

 in animals. The enormous development of surface in the shape of 

 leaves and needles underlies a great transpiration which requires 

 replacement, so that a stream of water moves upward through the 

 roots and vascular bundles to the leaves. On bright summer days 

 (see W. PFEFFER* 2 loc. cit. I, p. 233) 1 to 10 gm. water are evaporated 

 from 1 cm. 2 of leaf surface. On very hot days the loss by transpiration 

 from big trees exceeds 400 kilos; on rainy days, however, it may be 

 reduced to a few kilos. To explain the upward movement of the 

 water, the most varied theories have been advanced, and usually 

 abandoned. Explanation by means of osmotic pressure has proved 

 thoroughly unsatisfactory, and mere capillary imbibition is of no 

 greater use. We may well understand that the colloids of leaves 

 suffer a loss of water by evaporation, and that, in swelling, they are 

 able to lift a great column of water from the ground to the tree top. 

 Experiments of E. STRASSBURGER showed that in poisoned trees, 

 water may rise to a height of 22 meters, so that pure capillary forces 

 do not suffice for the explanation of the phenomena. More recent ex- 

 periments (P. A. ROSHARDT,* E. REINDERS*) show that in the living 

 plant, living elements assist in pumping up the water. Since no pre- 

 vious explanation of this has been given, I believe that I am justified 

 in formulating the following hypothesis. In my opinion, the living 

 cells of plants assist in the elevation of the sap by their respiration. 

 With respiration, not only does CO 2 develop, but also great quantities 

 of organic acids. Both cause a swelling or attraction of water, which 

 is liberated to the extent that C0 2 disappears, and the other acids are 

 removed in any one of the many possible ways. This would fit in 

 with the fact that the breathing in fully developed leaves and 

 branches, in which the need for water is also diminished, is less than 

 in the developing shoots. The dead leaf, whose breathing has ceased, 

 withers. 



Circulation of Crystalloids. 



The circulation of crystalloids is also largely governed by the factors 

 of diffusion and osmotic pressure, with certain limitations due to 

 the colloid media. Although between two aqueous solutions, sep- 

 arated by an easily permeable membrane, unrestricted mixing occurs 

 as a result of diffusion, this does not hold for a jelly-like medium 

 (see H. BECHHOLD and J. ZiEGLER* 1 )- In order to bring about a 

 mixture in such a case an excess of osmotic pressure is required 

 (see p. 57). It even seems that with equal osmotic pressure, acid 



