

180 PHYSIOLOGY OF NUTRITION. 



'means of the apparatus above described we can at a considerable 

 pressure, or even at a comparatively small one, force air through 

 pieces of coniferous wood and this result we do sometimes obtain 

 this always indicates the presence in the object either of very 

 long tracheides (e-g. in the medullary sheath) or of intercellular 

 spaces, which, indeed, as Russow has shown, are not completely 

 absent even in coniferous wood. In an experiment with a piece 

 of Taxus baccata stem, deprived of its cortex, and about 50 mm. 

 long and 6 mm. in diameter, I found that air escaped from the 

 upper cut surface at a pressure of 20 cm. of mercury, but with 

 a pressure of 15 cm. there was no longer any escape of air. 



We may also employ the poroscope of Christiani to demonstrate 

 that the tracheides of the wood are impermeable to air under a 

 certain pressure. The apparatus, which is made of glass, is shown 

 in Fig. 64, and, as we see, consists essentially of two manometers. 

 If a short peeled piece of a shoot of Taxus or Abies is inserted 

 between a and 6, and fixed air-tight by means of sealing-wax, 

 and we now blow into the end of the tube Sch, the height of the 

 mercury in the manometer M is of course considerably changed, 

 but not that of the mercury in the manometer M', since no air 

 passes through the wood. If we now experiment with the 

 vascular wood of a dicotyledonous plant, the height of the mer- 

 cury is naturally at once altered in both manometers on blow- 

 ing, since the vessels provide an open communication between 

 a and b. 



Another important fact, which can be demonstrated in lecture 

 by the following experiment, is that there is no open communi- 

 cation between the gases in the cellular and intercellular spaces. 

 We experiment with branches of Cornus mas, Philadelphus, and 

 Syringa, using pieces 30 cm. long and 1 cm. thick, and provided 

 with numerous lenticels. The object is fixed in the shorter limb 

 of a bent glass tube, so as to project into it for about two-thirds 

 of its length. The end which is to be introduced into the tube is 

 first of all made air-tight with sealing-wax. We now place the 

 apparatus in a large glass cylinder full of water, and pour mercury 

 into the longer limb of the tube. If we do not employ too high 

 a mercury pressure, say about 20-30 cm., air escapes at the upper 

 cut surface of the object only from the cortex, proving that no 

 free communication exists between the intercellular spaces on the 

 one hand and the vessels on the other. If we raise the pressure, 

 air escapes also, especially after a little time, from the wood, since 



