ii ASCENT OF SAP IN STEMS 39 



cipitated from the atmosphere ; this is done by the loose 

 metal ring surrounding the object-glass, and packed round 

 with a little cotton wool. The thermometer enters by a 

 tubulure in front ; its bulb appears in cross-section at t in 

 the figure. The regulation of the temperature is very 

 simply effected by retarding or accelerating the current of 

 cold liquid (brine) by means of the pinch-cock. 



The section of the wood to be examined is cut, and 

 with the addition of as little water as possible, is luted up 

 between the cover-glasses, so that it is surrounded by air, 

 but contains water within its substance. The close 

 proximity of the section to the upper window, some 

 T5 millimetres, allows of considerable magnification. 



The cold cell, after the introduction into it of the section 

 sealed up between the cover-glasses, is placed on the stage 

 of a microscope, and then, by the arrangement already 

 described, the temperature is caused to fall gradually, 

 while the water within the section is carefully observed. 



The phenomena attending freezing were perfectly definite, 

 the clear liquid in the lumina assuming the aspect of solid 

 paraffin. In two experiments in which the reduction of 

 temperature was effected very gradually, the freezing- 

 point was found to lie between - 10 and - 11 \ Freezing 

 spread with great rapidity all over the field in both wide 

 and narrow lumina. Air-bubbles present exhibited im- 

 mediate reduction of volume, and often distortions of 

 shape, and it was important to observe that an exudation 

 of sap occurred upon bare cell-walls, which, appearing in 

 drops, instantly turned to rough-shaped ice-crystals. 



1 Of course, this is not the true freezing-point of the liquid in the wood, 

 but the temperature which it attains by supercooling before solidification takes 

 place. It thaws consequently at a higher temperature, which again is not the 

 true freezing point, but one which approximates to the eutectic point. It is 

 evident that much variation may be expected in the temperature at which 

 crystallisation occurs in the supercooled liquid. 



The true freezing-point of the sap in the tracheae of other plants is much 

 higher than either of these temperatures, and there is no reason to believe 

 that the sap of Taxus is peculiar in this respect (see page 45). 



