THE MOLECULAR FORCES IN PLANTS. 



177 



shorter limb of a bent glass tube, a few millimetres in diameter. 

 On pouring mercury into the longer limb no escape of air took 

 place through the cork disc, even when there was a considerable 

 difference in the level of the mercury in the two limbs of the tube, 

 and the position of the mercury even after some time was the 

 same as at the beginning of the experiment. 

 Similarly, as Wiesner showed, all other 

 kinds of tissue refuse to permit filtration 

 of gas under pressure, if they are com- 

 pletely closed on the outside. In many 

 cases it is convenient, in making experi- 

 ments on this subject, to have the bent 

 glass tube (5-6 mm. in diameter) provided 

 with a metallic attachment, such that a 

 suitable object can be inserted into it, and 

 fixed air-tight by screwing on a perforated 

 cap. To protect the tissue it is laid be- 

 tween rubber washers. Suitable objects 

 for investigation are the skin of an apple, 

 the seed-coats of peas and beans, and 

 pieces of living or dried ivy-leaf, whose 

 upper surface as is known is free from 

 stomata. 



Before going on to consider the negative 

 pressure of the gases in the elements of 

 the wood, we will here proceed to make a 

 few experiments regarding the permeability 

 to gases of the lumen of the wood vessels, 

 and also of the membranes of the elements 

 of the wood. If pieces of twig, about 

 6 cm. long and 8 mm. thick, provided 

 with cortex, are fixed by means of rubber tubing in the shorter 

 lirnb of a bent glass tube (see Fig. 63), and we then pour mer- 

 cury into the longer limb of the tube, and place the apparatus 

 in a cylinder of water so that the upper cut end of the stem is a 

 few centimetres below the surface of the water, we see numerous 

 bubbles of air rise from the smooth cut surface, indicating that 

 the lumen of the vessels is permeable to air. The mercury sinks 

 more and more in the longer limb, rising in the shorter one, till 

 the difference in level in the two limbs is reduced to, say, one 

 or two centimetres. When the mercury has come to rest, the 



P.P. N 



Fia. 63. Apparatus for 

 determining the permea- 

 bility of the wood vessels to 

 air. 



