ADJUSTMENT TO TEMPERATURE 93 



iving matter does. The temperature of the plant, especially 

 n the case of aerial parts, is not always the same as that of the 

 nedium, for several important reasons. The temperature of the 

 )lant responds more or less slowly to that of the air, and in the 

 3ase of a sudden change it is for a time greater or less than that 

 )f the air. In fact, the temperatures of the plant behave more 

 ike those of the soil than of the air. The plant lags in the change 

 n proportion to its mass and to the character of its surface. 

 Cranspiration probably has some effect upon the leaf surface, 

 rhis effect does not seem to be great, since the leaves of rosettes 

 )ften have a temperature as high as that of the surface of the 

 oil upon which they are found, and, in mountain regions at least, 

 he temperature of sun leaves is higher than that of the surround- 

 ng air. 



In taking internal temperatures of plants it is desirable to use 

 L thermometer with a bulb as small as possible. In the case of thick 

 tems and roots or other fleshy parts, a slit is cut or a hole is 

 nade with a cork-borer, and the entire bulb sunken in the tissue, 

 temperature readings are obtained less easily in the case of ordinary 

 eaves. Fairly satisfactory results have been secured by rolling 

 he leaf while in position tightly about the thermometer bulb. 

 Vhen it is possible, the thermometers are left in place, in order 

 hat readings may be made at various times, thus showing the 

 onnection between the temperature of the plant and of the 

 nedium. 



Experiment 31. Temperatures of plant and habitat. In addition to 

 he temperature readings in the field outlined in Experiment 5, make 

 imultaneous readings of air, soil, and plant temperatures. This can be 

 [one most conveniently in the plant-house, although it is desirable to 

 aake similar observations out of doors in the spring.' A plant with a 

 ieshy root, such as the beet, is the most satisfactory. Bore a hole of 

 he proper size in the root, and sink the thermometer bulb well below 

 he surface. Roll the tip of a leaf closely about the bulb of a second 

 hermometer, folding in the edge of the leaf in such a manner as to 

 over the bulb entirely. Hang one thermometer in the air alongside 

 he leaves but in the sun, and place another in position for making soil 

 eadings. Finally sink another bulb in dead or inactive tissue, such 

 ,s that of an apple or potato. Read the five thermometers as nearly 

 imultaneously as possible at various times during the day. The best 

 esults are obtained by making readings early in the morning, at noon, 



' Research Methods, 88. 



