500 DESERT PLANTS AS A SOURCE OF DRINKING WATER. 



Fig. 1. — Bisnaga or barrel cattus (Echiiwcactus 

 emoryi). One-ninth natural size. 



1 to 8,023. A specimen of bisnaga or barrel cactus {Ech InocactibS 

 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 13.4 (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 the chamber is therefore 

 slowly receiving moisture at its lower end 

 from the interior water supply of the plant 

 and slowly giving it off into the outer air when- 

 ever the two guard cells open the narrow slit 

 between them. The whole sti'ucture is evidently well adapted to the 

 maintenance of a transpiration current at an exceedingly attenuated 

 rate adapted to the plant's limited supply of moisture.^ 



The interior of the plant consists chiefly of water-storage cells (tig. 

 4). These are globular in form, devoid of green coloring matter, 

 and with walls somewhat thickened but possessing thinner sieve-plate 



Fig. 2. — Tran.spiratiou pore of 

 Tradescantia virc/inica. a, Ep- 

 idermis; b. outer wall of epi- 

 dermal cell, d, cavity of epi- 

 dermal cell; /, green interior 

 tissue; g, guard cells of the 

 transpiration pore: /;. trans- 

 piration chamber. Much en- 

 larged. After Strasburger. 



