THE MOLECULAK FORCES IX PLANTS. 229 



April) we bore a hole into the trunk of a birch tree, not far above 

 the ground, and by means of rubber tubing or sealing wax fasten 

 into it, air-tight, one limb of a glass tube bent at right angles, 

 it will be found, especially during the night, that a considerable 

 quantity of fluid will be forced out of the tree owing to root 

 pressure. If we repeat the experiment in summer, e.g. in June, 

 not a drop of sap flows out, and indeed water may even be sucked 

 up into the stem, as may easily be determined by dipping the free 

 end of the tube into water. This phenomenon is the result of 

 transpiration. 



If a transpiring plant, the elements of whose wood at first con- 

 tain large quantities of water, has not at its disposal a good supply 

 of fluid and this will especially be the case in summer then the 

 water gradually disappears from the lumina of the wood vessels 

 and tracheides. 



As was shown earlier, air only passes with difficulty from the 

 intercellular air system into the cellular system, and the quantity 

 of air which the water present in plants holds in solution, and 

 under certain conditions can give off to its surroundings, is not 

 considerable. Therefore, during strong transpiration in summer, 

 the wood elements do not contain water alone, but in addition 

 there is present a large quantity of moist rarefied air, a fact 

 which has already been referred to on p. 182. 



If we cut out a piece of wood from a transpiring tree, and throw 

 it into water, it will float on the surface. If the lumina of the 

 wood elements had been completely filled with water, it would 

 have sunk, since the specific gravity of wood substance in itself is 

 greater than that of water. A piece of fresh wood, however, 

 thrown into water gradually sinks deeper in the fluid, and at the 

 same time it increases in weight, a fact which can only be due to 

 absorption of water. 



The following investigation also teaches that while under some 

 conditions the lumina of the wood elements are completely filled 

 with water, this can certainly not be the case always. 7 Two 

 vigorous specimens of Cucurbita, or of some species of Begonia, 

 are decapitated and provided with tubes in the manner shown in 

 Fig. 73, one of the plants having previously been allowed to tran- 

 spire for a day, while the other has been protected from consider- 

 able loss of water by covering with a bell-glass. The latter plant 

 at once yields sap, which under the influence of root pressure 

 rises in the tube. The former at first does not yield sap ; it even 



