374 HOW CROPS GROW. 



from the fact that the leaves within were found wilted 

 in the morning, while they recovered toward uoon, al- 

 though 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 consequence of the warming of the stem by 

 the sun's radiant heat.* 



One mode in which changes of temperature in the 

 trunk influence the flow 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, undergoes a decided change of bulk in this 

 way. Water expands nearly one-twentieth in being 

 warmed from 32 to 212, and air increases in volume 

 more than one-third by the same change of temperature. 

 When, therefore, the trunk of a tree is warmed by the 

 sun's heat, the air is expanded, 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 tempera- 

 ture 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- 

 lying 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 

 sufficiently 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 sucked in through the spile ; as the trunk 



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

 of the solid bodies which it surrounds. A thermometer will often rise 

 by exposure of the bulb to the direct rays of the sun, 30 or 40 above its 

 Indications when in the shade. 



