Scunpson and Allen : physical factors on transpiration. 41 



From the above table it will be seen that loss by transpiration 

 gradually decreases toward midnight. The maximum is reached 

 from ten a. m. to two p. m. From that time on there is a decrease 

 again. 



Light reaches its maximum during the hour that the transpira- 

 tion current is the strongest, while the percentage of humidity grad- 

 ually decreases at that time. The temperature, however, does not 

 reach its highest point until 3 p. m., after which it begins to de- 

 crease. 



In order to correlate the criteria obtained with the hour of day 

 and the physical factors, and to show more in detail the fluctua- 

 tions in individual variation, curves have been plotted for the three 

 series, using the figures given in the preceding table. 



Effect of Light. 



In order to determine the effect of light on transpiration, three 

 shade tents were constructed in the plant house, using different 

 weights of muslin. The plan was to have the darkest shade tent 

 compare, with respect to light, to that of the most dense forest. 

 The tables of physical factors, being an average of a great number 

 of readings taken during the experiment, will give a good idea of 

 the condition within the tents. Plants were measured in full light 

 in the plant house and in the respective shade tents, weighings being 

 made twice daily. The plants used were Helianthus annuns and 

 Ciicumis sativiis. 



From the table of physical factors it will be noticed that light is 

 practically the only variable factor in the tents in question. The 

 response of transpiration to this factor is found to be very pro- 

 nounced.^ A decrease in water loss of from six to eight milligrams 

 is obtained by comparing Series I with Series II, their light in- 

 tensities being .6 and .056 respectively. Series III with a light in- 

 tensity of .0038 shows a decrease of about six milligrams as com- 

 pared with Series II. Series IV with light .0012, a little less than 

 half the intensity of Series III, has a decrease of from two to 

 four milligrams per unit area. 



1 Schlmper, A. F. W. Plant Geography. 1903, p. 4. 



