156 



PHYSIOLOGY OF NUTRITION. 



particular plant had, at half-past ten in the morning, a tempera- 

 ture of 23 C. By half-past two it had risen to 4O5 0. The 

 temperature of the air at this time, in the shade, was 24*5 C. 

 The same plant, during the afternoon of another day, reached a 

 temperature of 45*5 C. To obtain information as to the tem- 

 perature within the trunks of trees, we bore to the centre and 

 introduce thermometers into the holes. By slipping short pieces 

 of rubber tubing over the thermometers, the holes can be closed 

 perfectly air-tight. The temperature inside trees is, of course, 

 not the same at points close to the ground as at higher levels, 

 and it is also obviously not a matter of indifference whether or 

 not the plant is exposed during the day to direct sunlight. If 

 we work with moderately thick trunks, e.g. 40 cm. in diameter, 

 into which, therefore, the thermometers will penetrate 20 cm., 

 we shall find in general that the temperature is lower than that 

 of the atmosphere during the day, but higher at night, and that 

 the daily maximum of temperature in the tree is attained con- 

 siderably later than that of the air. 



By a simple experiment which, moreover, is easily performed 

 in lecture we can demonstrate the fact that dry wood conducts 

 heat more rapidly in a direction parallel to the axis of the trunk 

 than in one at right angles to it. A smoothly planed piece of 

 lime, birch, or oak, is smeared over with a thin layer of wax, by 



means of a brush dipped into melted 

 wax. We now warm a wire, and press 

 its heated end perpendicularly against 

 the wood. A zone of fusion appears, 

 $, Fig. 52, which has the form of an 

 ellipse whose major axis runs parallel 

 with the wood fibres. We can measure 

 the length of the major and minor axes 

 of the ellipse, and so calculate the ratio 

 between the rates of conduction of heat 

 in the wood longitudinally and trans- 

 versely. If instead of the block of 

 wood we use for the experiment a sheet 

 FIG ,52 -Block of wood smeared f j smea red with a thin layer of 



with wax. S, zone of melting. _ J 



wax, the zone fusion which we obtain 



is not elliptical but circular, because the conductivity of the glass 

 is equal in different directions. 



Since the thermal state of the soil exerts a considerable in- 



