82 ILLINOIS STATE ACADEMY OF SCIENCE 



A COMPARISON OF THE TRANSPIRATION 



RATES OF CORN AND CERTAIN 



COMMON WEEDS 



Helen A. McGinnis and W. B. McDougall, 

 Univeesity of Illinois 



That the presence of weeds in a corn field is detrimen- 

 tal to the intake of moisture, the reception of light and 

 the manufacture of food by the corn plant has been 

 demonstrated by experiment (15). Such experiments, 

 however, do not show either the amount or the rate of 

 removal of water from the soil by the weed invaders. 

 It is the purpose of the present paper to present data 

 concerning the relative rates of water loss by transpira- 

 tion from the leaves of corn and of corn field weeds 

 growing under the same environmental conditions. 



The study of transpiration from the leaves of grow- 

 ing plants is by no means new. Trelease and Livingston 

 (19), for example, measured the relative transpiring 

 power of a number of plants. These authors were inter- 

 ested, however, in the diurnal fluctuations of this trans- 

 piring power rather than with the differences between 

 different plants. Bakke (1) also measured the index of 

 transpiring power of various plants and the same might 

 be said of several other authors. A fairly complete bib- 

 liography of the subject is given by Kiesselbach (10). 

 No historical resume of the literature will be attempted 

 here but it may be pointed out that most of the writers 

 have been concerned with the relative transpiring power 

 of plants as compared with evaporation from a free water 

 surface, the latter, rather than any living plant, being 

 the standard for comparison. 



In the experiments described in the present paper the 

 corn plant, rather than a free water surface, is the stand- 

 ard for comparison. The data presented do not give 

 quantitative information as to the rate of water loss but 

 they do show comparative rates of transpiration from a 

 given area of corn leaf and equal areas of weed leaves, 

 thus demonstrating which transpires more rapidly area 

 for area. The experimental work was carried on by Miss 

 McGinnis mostly in the botanical greenhouses at the 



