42 



DESERT BOTANICAL LABORATORY 



The temperatures of a number of plants were obtained by Mr. Mac- 

 Dougal by thrusting the bulbs of small mercurial thermometers into 

 the fleshy stems, and shading the exposed portion of the instrument 

 from the sun's rays. The following data were recorded from tests of 

 this character with an Optintia (probably O. engelmannii') on July 

 17, 1898. 



The flattened fronds of the cactus were in an upright position with 

 the edges in the plane of the meridian, so that the angle of the incident 

 rays of sunlight decreased with the altitude of the sun. As a conse- 

 quence of this insolation the resulting temperatures rise until about 10 

 A. M., and then decrease until the sun once more comes into a posi- 

 tion where the rays might strike the surface at or near a right angle, 

 reaching a second maximum at 2 P. M., though Mr. MacDougal's 

 observations on this point were somewhat obscured by the daily cloud- 

 ing at the time of the experiments. In the thermometry of globular, 

 decumbent or cylindrical forms of fleshy plants such as Cereus 

 temperatures of 113° to 115° F. were often found with the air at a 

 temperature of 93° to 100° F. It is to be seen that plants in this 

 region are subject to the action of a fierce insolation and to an atmos- 

 phere of low relative humidity. As a result of such insolation the 

 body of the plant and the surface layers of the soil are raised to very 

 high temperatures. The increase in temperature of the shoot aided by 

 the direct action of the light upon the transpiratory mechanism would 

 tend to increase the amount of water given off by the shoot. At the 

 same time however the temperature of the soil undergoes a correspond- 

 ing increase, thereby increasing the osmotic processes of absorption so 

 that the two processes, absorption and transpiration, automatically 

 equalize each other, provided the maximum temperature of proto- 

 plasmic activity is not passed. 



It is to be noted that the amount of evaporation or transpiration of 

 water from a leaf must be sufficient to keep the temperature of the 

 leaves and other green organs below the critical point of chlorophyl, 

 or the temperature at which it begins to suffer chemical decomposition 

 as a result of heat. This critical temperature is usually given as 1 13° F., 



