38 DESERT BOTANICAL LABORATORY 



TRANSPIRATION AND TEMPERATURES. 



Mr, D. T. MacDougal, in 1898, made a series of experiments at 

 Turkey Tanks on the western edge of the malpais or lava desert near 

 the Little Colorado river east of Flagstaff, Arizona. In these attention 

 was chiefly given to the amount of transpiration and to the tempera- 

 tures of the soil, of the air, and of the bodies of succulent plants, the 

 results of which are now given for the first time. 



Measurements of transpiration were made by means of a potonieter 

 of the form described in the Bota7tical Gazette (24 : 1 10, 1897). This 

 apparatus consists of a long calibrated tube of small internal diameter 

 supported in a horizontal position and fitted with a Y extension at one 

 end. The tube is filled with water and the excised shoot of a plant 

 fitted to one end of the Y by means of a tightly wired section of rub- 

 ber tubing. The other end of the Y is closed by a stopcock, which 

 may be opened to admit water when necessary. The rate at which 

 water is taken into the shoot is noted by the progress of an air bubble 

 in the horizontal portion of the tube. It is to be borne in mind that 

 the rate at which water may be absorbed by the basal portion of an 

 excised shoot in contact with water may not, and probably does not, 

 represent the exact rate at which transpiration actually takes place, but 

 it offers a very valuable method of comparison of the capacities of 

 shoots of various types to take up and throw off water under similar 

 conditions. 



Experit7ient i. — Me7itzelia pu7nila is a representative of a class of 

 plants which, annually growing from seeds, produce flowers and seed 

 during the season of greatest humidity, and then die, the species sur- 

 viving through the resting season in the form of seeds. It is a marked 

 example of the xerophytic species which have a weakl}^ developed 

 root system consisting of a number of thin branching fibrous roots 

 which extend chiefly laterally through the upper layers of soil and do 

 not penetrate beyond a depth of a few inches. The aerial shoot has a 

 roughened cylindrical stem about 16 inches long and a number of 

 lateral branches of equal length giving the entire leafy shoot a globoid 

 outline, a form characteristic of many desert plants. The specimen 

 used was furnished with 18 branches and bore about 900 irregular 

 narrow roughened leaves and 200 yellow flowers. The entire surface 

 of the portion of the plant exposed to the air might be estimated at 

 about 800 square inches. The plant was taken from the soil after the 

 above facts had been ascertained, and the root system was cut away 

 from the base of the stem before attachment to the potometer as above. 

 Several minutes were allowed to elapse before observations were taken 



