MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 315 



The leaves are triangular in cross section and as the pairs emerge 

 from the sheathing bases of the antecedent pair the inner or upper 

 faces are appressed. The upright position implied is held until a 

 half or a third of their length is attained. The tips of a pair were 

 harnessed together and 'being turgid and firm were arranged to press 

 upward on the bearing lever of the auxograph. 



The general features of the daily behavior of this plant were 

 quite similar to those of Opuntia in that elongation accelerated in 

 mid-forenoon, about 9 to ii A.M., and continued until i to 3 P.M., 

 when it was checked and a shrinkage ensued which generally ended 

 at 5 or 6 P.M. or sunset. After this time temperature being favor- 

 able a low rate of growth continued through the night and until the 

 daily acceleration occurred a few hours after sunrise. 



The daily course of transpiration has not been determined, but 

 it is allowable to assume that the imbibition capacity of the growing 

 regions is lessened by acidity as it is in Opuntia. 



Growth of Wheat (Triticiim) and Corn (Zea). 



A great amount of data obtained by the measurement of the 

 elongation of Triticum is available. The figures have been obtained 

 chiefly by the measurement of numbers of organs for a brief period. 

 The so-called critical temperature points have been obtained by 

 taking averages of the performance of several plants. The facts 

 of importance in connection with the present paper are those which 

 have been obtained by analyses of the march of growth from day to 

 day. Similar methods were used with corn (Zea). 



Varieties of these two plants cultivated in the region of the 

 Desert Laboratory were selected, and grains were germinated in an 

 unheated glass house. The temperatures given were obtained by 

 shaded mercurial thermometers and are Fahrenheit scale. 



The bases of the plantlets were fixed in place by layers of plaster 

 poured on the surface of the soil. The tips of leaves which had 

 emerged to a length of 10 to 15 mm. were brought into the field of 

 a horizontal microscope and the variation in length measured at 

 half hour intervals so far as it was possible to do so. The leaves 

 were maintained in a vertical position by a requisite number of 

 horizontal glass rods with a minimum of shading effect. 



