408 Plant Physiology 



morning, usually the coldest period of the day, the heaters 

 are lighted. It has been found possible at an average cost 

 of about $20 per acre to raise the temperature of the or- 

 chard as much as from 5 to 14 F. above that of the nor- 

 mal air, and this often in the face of considerable wind. 

 The practice has recently assumed unexpected impor- 

 tance, and seems to have superseded the relatively ineffec- 

 tive smudge methods. 



246. The temperature of the plant. The temperature 

 of the plant is in general the temperature of the environ- 

 ment. Twigs, branches, and even trunks of trees will show 

 during cold weather changes of temperature more or less 

 in accordance with that of the air. In the case of large 

 branches or trunks some time will be required in order that 

 the minimum of the air may be registered by the tree, and 

 there will be, therefore, a very definite temperature lag. 



In the sunshine dark buds, branches, or trunks may ab- 

 sorb heat to such an extent that the internal temperature 

 will be greater than the external. In the same way, green 

 leaves exposed to sunlight show a temperature from two or 

 three to fifteen degrees higher than the air, depending upon 

 the intensity of the light. This latter point has received 

 careful attention by Blackman, who has employed in the 

 work very delicate electro-thermometric methods. The 

 ordinary method of wrapping the bulb of a thermometer 

 with one or more thicknesses of a leaf will not afford ac- 

 curate indications of the actual leaf temperature. 



247. Adjustment of structure. There are few or no 

 protective structures in plants which are of direct service 

 against injurious temperatures. As will be shown later, 

 both high and low temperatures act upon the plant cell to 



