The writer's studies of anaerobic respiration of several different varieties of fruits and such underground parts as tubers and rhizomes have now been extended to the fleshy structures of cacti. This work was done at the Desert Laboratory of the Carnegie Institution of Washington at Tucson, Arizona, during the winter months of October to January. Although the climate in Tucson is rather mild as winters go, yet it was by no means warm. There were several periods when for one night or several

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... ts in succession the temperature was below freezing. For the greater part of the time there was not much growth of any kind and certainly not in any of the cacti used in these experiments. The temperature of the bath in which the experiments were conducted gradually rose during a period of three and a half months from 25°to 25.5°C ., but during an individual experiment, lasting usually a week, the change was so slight that it could not be detected. For respiration chambers largemouthed pickle jars were used. Otherwise the experiments were conducted as in the past (I). In order to establish an equilibrium, the plants (kept in the dark) were in the respiration chambers, through which air was drawn, usually for about 20 hours before the collection of CO 2 was commenced. Six species of cacti and the leaves from two species of thin-leaved plants were used. The leaves were used to get a comparison between fleshy plants and thin-leaved structures from the same habitat and under identical conditions. The plants used were: Carnegiea gigantea} F erocactus toislieeni, N eomammillaria microcarpa, Echinocereus [endleri, Opuntia enqelmanmi, O. versicolor} and the leaves of Verbena ciliata and Encelia farinosa. The cacti ranged in compactness from the nearly spherical Ferocactus to the flat Opuntia enqelmannii and the long and cylindrical growth of Opuntia versicolor. (The names as here used are those given by Britton and Rose, "The Cactaceae.") To eliminate the activity of bacteria or fungi all plants used were first washed free from soil and other foreign matter and were always healthy and uninjured, except where joints were broken off or roots cut off, and in one experiment with Ferocactus wislizeni in which a fungus infection developed. Sometimes these injured surfaces healed before the plants were introduced into the respiration chambers, or they were coated with vaselin or grafting wax, although in some experiments joints with fresh cuts were placed in