. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. Fig. 1.—Bisnaga or barrel cattus (Echiiwcactus emoryi). One-ninth natural size. 1 to 8,023. A specimen of bisnaga or barrel cactus {EchInocactibS emoryi)^ in the conservatories of the Department of Agriculture at Washington, weighing 77,000 grams (170 pounds) and without leaves, has a green stem surface of 1,032,320 square millimeters, with a ratio of 1 to (fig. 1). Thus for each gram of tissue the coffee plant, representing the ordina
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. Fig. 1.—Bisnaga or barrel cattus (Echiiwcactus emoryi). One-ninth natural size. 1 to 8,023. A specimen of bisnaga or barrel cactus {EchInocactibS emoryi)^ in the conservatories of the Department of Agriculture at Washington, weighing 77,000 grams (170 pounds) and without leaves, has a green stem surface of 1,032,320 square millimeters, with a ratio of 1 to (fig. 1). Thus for each gram of tissue the coffee plant, representing the ordinar}^ vegetation of a humid climate, has a green surface 599 times greater than that representing a gram of tissue in the cactus; or in physiological terms, the coli'ee plant, other factors being equal, is provided with means for the transpiration of 6oO times as much water as the cactus. Not onl}^ is the green surface of desert plants very much restricted in extent, but it has such a struc- ture as greatl}" to reduce the amount of moisture transpired through it. The structure of an ordinary trans- piration pore ill a plant of humid habitat is shown in tig. 2. Through the courtesy of Dr. R. E. B. McKenney the structure of a pore of Kch'uiocactus emoryi is presented for comparison (tig. 3). It is to be noted that the cuticle of the latter is excessively thickened. Beneath the epidermis is a deep layer of hypodermis with very thick walled cells and small cell cavities. It can scarcely be doubted that, except at the pores, the epidermal struc- ture is impervious to moisture even under the extreme desiccating conditions of the desert. Beneath the minute opening of the pore is an air chamber, the lower contracted end of which is made up of the walls of the green, moist interior cells of the plant. The portion of the walls of this chamber which lie within the hypodermis. Doctor McKenney has discovered, are cuti- nized, so as to be impervious to moisture. The cushion of air in th
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