232 PHYSIOLOGY OF NUTRITION. 



tilled water free from dust. The pressure forces the water 

 through the piece of branch, which may be either used with or 

 without removal of the cortex, so that in a short time a consider- 

 able quantity of water niters through. The rate of flow becomes 

 less in time as we readily observe. This happens even if we 

 take care to keep the level of the water in the funnel always at 

 the same height, and the cause of the phenomenon is to be sought 

 in a gradual change (Verunreiniguny) in the cut surface of the 

 wood through which the water enters. 



We now make another experiment with our apparatus, filling 

 it not with clean water but with water containing very finely 

 divided Cinnabar in suspension. A large quantity of distilled 

 water is treated with the best Cinnabar, and the fluid is filtered 

 several times, so that only the very finest particles remain sus- 

 pended in it, which do not settle to the bottom even in the course 

 of several days. The water which filters in the course of one to 

 two days through the cylinder of wood at the lower end of our 

 apparatus is perfectly clear. Examination of the piece of wood it- 

 self shows that only its upper end, to a depth of a few millimetres, 

 is impregnated with Cinnabar. Microscopic examination of deli- 

 cate sections of the wood demonstrates the presence of Cinnabar 

 in the tracheides, and we ultimately come to the following inter- 

 pretation of the results of our experiment. 



Naturally the tracheides at the cut surface of the cylinder of 

 wood used in the experiment have been opened in cutting it. 

 During filtration, water and Cinnabar penetrate into these tra- 

 cheides. No doubt at all now exists that water, even under a 

 minimal pressure, filters through the closing membranes of the 

 bordered pits of tracheides ; all our experiments bring this fact 

 clearly into view. The particles of Cinnabar, however, are not 

 able to pass from one tracheide into another, since they cannot 

 traverse the closing membranes of the pits. At the same time the 

 experiment offers direct proof of the presence of closing mem- 

 branes between the elements of coniferous wood. 1 



The ease with which water filters through wood is of great im- 

 portance in relation to the passage of water in the wood masses of 

 plants. 



1 See Th. Hartig, Botan. Zeitung, 1863, and especially Sachs, Arbeiten d. 

 botan. Instituts in Wurzburg, Bd. 2, p. 296. 



