210 W. A. CANNON 



The three species referred to were grown for experimental 

 purposes in the glass house of the Desert Laboratory. Opuntia 

 had been in the house for over one year and had been given water 

 frequently. The specimens of Fouquieria, transplanted young 

 plants, had also been in the glass house for several months. 

 They were growing in glass tubes of different sizes, and, in order 

 to bring about the formation of leaves, Fouquieria also was given 

 water in fairly large amount. The specimens of Peganum, grow- 

 ing in glass tubes also, were raised from seed which the writer col- 

 lected in southern x^lgeria in 1910-1911. At the time of the tests 

 here reported the seedlings were about eight months old. The 

 treatment of Peganum was somewhat different from that of the 

 other species in that as little water was given as possible consist- 

 ent with appreciable growth. For this reason the conditions of 

 growth of this form may have approached, or possibly equaled, 

 the aridity to which the species is subject in the proper habitat. 



The general method employed in determining the approximate 

 density of the cell sap of the plants used was to strip portions of 

 the epidermis quickly from the leaves and plunge them at once 

 into KNO3 solutions of different densities. In case of the roots 

 the procedure was of necessity somewhat different. The entire 

 root-systems were washed out of the soil by the KNO3 solution 

 to be used in this particular test. This method, although ap- 

 parently necessary, sharply restricted the number of tests. In 

 both root and 'shoot the effects of the plasmolyzing solution were 

 looked for in the epidermal cells. In all tests the temperature 

 of the solutions was about 25°C. 



Opuntia discata 



In the latter part of May, when the tests were made, the speci- 

 mens of 0. discata, which were growing in soil beds of the glass 

 house, were in leaf. The leaves of the species are ephemeral and 

 of a surprisingly delicate structure. They wilt and fall away 

 with the end of abundant moisture, as, in nature, at the close of 

 the summer rains. 



In a 0.25 N KNO3 solution the leaves after ten minutes' 

 immersion showed an occasional plasmolyzed epidermal cell. 



