182 PHYSIOLOGY OF NUTRITION. 



escape from the vessels. It is finally very instructive to fill g 

 with Carbon dioxide, and / with clear lime-water, and then pro- 

 ceed as before. It is found that the Carbon dioxide forced into 

 fhe stomata escapes from the cut surface, and renders the lime- 

 water turbid. 



We now proceed to acquaint ourselves in detail with the facts 

 which are known concerning the negative pressure of the air in 

 the wood- vessels. It is certain that at particular times, when the 

 transpiration is feeble, the vessels of the wood contain larger or 

 smaller quantities of water, viz. in spring, and also in summer at 

 night-time. When the transpiration becomes energetic, the water 

 is used up, and since, as we have seen, wood substance is extremely 

 impermeable to air, a negative gaseous pressure will be set up in 

 the vessels, i.e. the air in the vessels will be at a lower pressure 

 than that of the atmospheric air. The negative pressure varies 

 considerably in amount. It is even possible that the gaseous 

 pressure in the vessels may at times be extremely small. To 

 prove that the air in the vessels is actually at a negative pres- 

 sure, we may perform the following experiments : 



We bore a hole to the centre of a birch tree, and fix in it, air- 

 tight, one limb of a glass tube, bent at right angles, the other 

 limb being allowed to dip into water. We shall soon see that the 

 water rises in the tube. 



We put together an apparatus such as is represented in Fig. 66. 

 A cut shoot (I experimented with Lonicera, and especially with 

 willow twigs) is placed with its lower end in water. The bent 

 tube G is in air-tight connection with the branch, say at a, by 

 means of rubber tubing, its other end dipping into water or 

 mercury. Owing to the rarefaction of the air in the vessels, 

 brought about by transpiration, the fluid soon rises in the tube, 

 as in the previous experiment. 



For further experiments we may use shoots of Ampelopsis, Vitis, 

 Clematis, Aristolochia, Phaseolus, Helianthus, Quercus, Bobinia, 

 or Juglans, without, however, removing them at first from the 

 parent plants. The bean plants may be grown in flower pots. 

 In experiments with Ampelopsis, Quercus, and Juglans we test 

 the behaviour of branches several years old. We bend these 

 down, dip them into an aqueous solution of eosin at a point about 

 50 or 100 cm. from the end of the branch, and then sever them 

 at this point, under the solution, with shears. The cut surfaces 

 are left in the solution for two minutes longer. We then carefully 



