4© DESERT BOTANICAL LABORATORY 



Experiment 3. — A well grown shoot of the tomato with a total 

 surface of 256 square inches was fastened to the potometer in a room 

 in diffuse light with a humidity of 25 to 35 per cent., about the same as 

 in the previous experiments, at a temperature of 79° F. Readings of 

 1,000 milligrams in 32 minutes, and 600 milligrams in 21.2 minutes 

 were made, giving an average rate of about .5 milligram per second 

 and subsequent observations showed no important deviation from this 

 rate. It is to be noted that the conditions differed from those of the 

 desert plant in the lower temperature and the much lower intensity of 

 the light. 



Experiment 4. — Eucalyptus globulus was used, being the shoot of a 

 young plant grown from seed in the greenhouse and having a surface of 

 352 square inches. The test was made at the same time of day (10 to 

 II A. M.), and under approximately the same conditions as experi- 

 ment3. Readings of 500 milligrams in 7.5 minutes, 9.5 minutes, 10.5 

 minutes, 10 minutes, and 10 minutes were made with an average rate 

 of .79 to I.I I milligrams per second. 



The data furnished by the above tests afford a fair means of com- 

 parison of the relations of moisture loving and desert plants to water 

 if due allowance is made for dissimilar conditions. It is to be seen 

 that a given area of surface of Mentzelia at similar temperatures and 

 in a light vastly more intense and in a drier atmosphere, transpires 

 water at a rate slightly less than the tomato and at a rate about a third 

 to a half that of Eucalyptus. The exposure of the two last named 

 species to similar temperatures, insolation, and dryness of the air would 

 doubtless show that the moisture loving plants would take up and lose 

 water at a rate even much greater than the Mentzelia. 



The shrubby Artetnisia was found to use water at a rate per area 

 about one fourth that of the tomato under the dissimilar conditions 

 offered. An increase of the temperature, insolation and dryness of the 

 air affecting the tomato would doubtless increase the ratio many times. 

 The Eucalyptus would offer even greater disproportion. 



Still another interesting suggestion arises from these results. Ment- 

 zelia is an annual that carries on its growth only during the season of 

 maximum humidity, while Artemisia is an example of the perennial 

 shrubby plant which makes no reduction of its surfaces during the dry 

 seasons. The latter therefore must be better protected against the 

 dangers of drought and actually uses only about half the amount of 

 water per area of surface that is needed by Mentzelia., and it sends its 

 roots to enormous depths to ensure a constant supply to keep up a 

 steady but slow rate of transpiration. 



