SOME INFRA-RED EFFECTS ON GREEN PLANTS 849 



emission from the leaf-cellophane system, while the vaselin coating did 

 not decrease transpiration sufficiently to produce any considerable rise 

 in temperature. Leaves enclosed in cellophane and exposed for 15 min. 

 developed large necrotic areas as a result of the treatment when the 

 plants were placed in a greenhouse for two days. No visible signs of 

 injury could be observed when the leaves w^ere first removed from the 

 envelope. However, a distinct aromatic odor was given off by the treated 

 leaves when first removed. This temperature was above the thermal 

 death point of the leaf. At higher radiation values, up to 1.6 gm. cal., 

 the leaf temperature when permitted to transpire freely, was found never 

 to exceed 107°F. under either visible or infra-red alone conditions. The 

 leaves were found to hold this temperature by increasing the water loss. 

 The tendency to maintain a higher leaf temperature under infra-red as 

 compared with the visible region of the same energy value is believed to 

 be due to the lower transpiration rate under infra-red. 



The conclusion seems well estabUshed that plants can and do eliminate 

 most of the excess energy received in both the visible and infra-red regions 

 by the evaporation of water. Whether the transpiration stream through 

 the plant serves another essential purpose besides the cooling effect in 

 the life processes of plants is beyond the scope of this discussion. In so 

 far as the transpiration stream is useful in plant processes, the infra-red 

 region can be considered as of value to the plant in maintaining this 

 stream. Indirectly it serves to heat the soil and the surrounding air 

 so as to produce a more nearly optimum temperature for plants. This is 

 also true of visible radiation. It is perhaps significant that plants do not 

 grow well in desert countries where radiation and temperature are high 

 but the water supply is hmited. In order to conserve the available water, 

 desert plants have a decreased leaf area exposed to radiation and this in 

 turn decreases photosynthesis and growth. Plants produce most 

 abundantly only w'here an adequate transpiration stream can be main- 

 tained to supply large areas of leaves exposed to radiant energy. 



INJURY FROM INFRA-RED 



o 



Burns (5) recently found that infra-red radiation longer than XI 1,000 A 

 was slightly detrimental to photosynthesis when the plant is at a suffi- 

 ciently high temperature. His method was to expose potted seedlings of 

 white pine or spruce sealed in bell jars to the total output of four 1000- 

 watt lamps placed at the four corners of a rectangle. The plants were 

 placed in the center of the rectangle and the amount of carbon dioxide 

 decomposed in a 2-hr. observation period when exposed to the total output 

 was compared with the amount decomposed when exposed similarly 

 except that the output of each lamp was filtered through 1 in. of water 

 plus two layers of plate glass, each 3 mm. in thickness. The total energy 

 used in the two types of exposure was of the order of 3 to 1. The efii- 



