MOTION OP THE JUICES. 335 



influenced by its temperature is a plain deduction from 

 the fact that the leaves within were found wilted in the 

 morning, while they recovered toward noon, although the 

 temperature of the air without remained below freezing. 

 The wilting was no doubt chiefly due to the diminished 

 power of the stem to transmit water ; the return of the 

 leaves to their normal condition was probably the conse- 

 quence of the warming of the stem by the sun's radiant 

 heat.* 



One mode in which changes of temperature in the trunk 

 influence the fl.ow of sap is very obvious. The wood-cells 

 contain, not only water, but air. Both are expanded by 

 heat, and both contract by cold. Air, especially, under- 

 goes a decided change of bulk in this way. Water ex- 

 pands nearly one-twentieth ia being warmed from 82° to 

 212°, and air increases in volume more than one-third by 

 the same change of tem2:)erature. When, therefore, the 

 trunk of a tree is warmed by the sun's heat the air is ex- 

 panded, exerts a pressure on the sap, and forces it out of 

 any wound made through the bark and wood-cells. It 

 only requires a rise of temperature to the extent of a few 

 degrees to occasion from this cause alone a considerable 

 flow of sap from a large tree. (Hartig.) 



If we admit that water continuously enters the deep-ly- 

 ing roots whose temperature and absorbent power must 

 remain, for the most part, invariable from day to day, we 

 should have a constant slow escape of sap from the trunk 

 were the temperature of the latter uniform and sufiiciently 

 high. This really happens at times during every sugar- 

 season. When the trunk is cooled down to the freezing 

 point, or near it, the contraction of air and water in the 

 tree makes a vacuum there, sap ceases to flow, and air is 



* The temperature of the air is not always a sure indication of that of the 

 solid todies which it suiTounds. A thermometer will often rise hy e-sposure of 

 the bulb to the direct rays of the sun, 30 or 40° above its indications when in the 

 shade. 



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