72 CARNEGIE INSTITUTION OF WASHINGTON. 



between perennials of unlike habit but occurring under similar conditions, 

 were observed at Matjesfontein in Eucalyptus globulus and in Cotyledon 

 paniculata, both of which had been introduced, subsequently abandoned, and 

 not irrigated. On October 4, the index of transpiring power of Cotyledon, 

 between 7^ 50™ a. m. and 5^ 50™ p. m. ran from 0.013 in early morning to 0.0089 

 at 2^ 08™ p. m., rising afterward to 0.02 near evening. In the case of the 

 Eucalyptus, circular but mature leaves of the juvenile type and elongated 

 leaves of the adult type were studied. In the former the index varied from 

 0.32 in the morning to 0.1 in midafternoon and 0.36 in early evening. In the 

 adult type of leaf, however, the index varied from 0.08 to 0.1 during the 

 course of the day. The maximum index of the elongated form of leaf thus 

 was the minimum of the juvenile type. But the minimum index of the adult 

 leaf was very much greater than the maximum index for Cotyledon growing in 

 this instance under apparently similar environmental conditions. 



A single series of studies on the transpiring power of Welwitschia mirabilis 

 was carried out early in July and merits especial notice because of the great 

 interest of the species. The habitat where the tests were made is on the Namib 

 plain immediately south of the Swakop River and about 50 kilometers east of 

 Swakopmund. It lies on the eastern edge of the coastal fog-belt, where, because 

 of the fog, as shown in another place in this report, the rate of evaporation 

 is frequently low. There are few specimens of Welwitschia at the place which 

 are scattered over an area 1 kilometer or more in diameter. The specimens 

 especially studied were about 2 meters from tip to tip of the diverging leaves 

 which lie freely on the surface of the ground. The ends are frayed by the 

 wind, but the basal half, more or less, is intact. At the tips the leaves are 

 brownish, but for the most part they are green, which assumes a bright grass- 

 like hue at the base. The leaves are apparently devoid of pubescence, but 

 a double epidermis^ and deeply sunken stomata prevent rapid loss of water 

 from the surface. The tests of the transpiring power were confined to the 

 dorsal, upper surface, immediately above the base of the leaf. The study 

 was begun about 3 hours after sunrise and was continued about an hour. 

 Several readings were made. The average length of time required to com- 

 plete the color change from dark blue to light blue was found to be 120 

 seconds, which gave 0.0138 as the index of transpiring power. When this 

 index is compared with those of other species from arid regions, as from the 

 Karroo, which were obtained at the same season of the year, it is seen that 

 that of Welwitschia is relatively and actually very low, which ranks the 

 species among the most marked of xerophytes. 



Seasonal Changes in Water Relations of Desert Plants, by Edith B. Shreve. 



With a desire to discover the responses by which desert plants meet the 

 conditions that approach the critical point of the physiological limits of their 

 endurance, the daily march of the processes that are dependent upon water- 

 condition has been investigated for selected typical desert plants and, for 

 the sake of comparison, the same experiments have been performed upon a 

 cultivated plant which will thrive during certain seasons. The plants used 

 were Encelia farinosa (a perennial), Streptanthus arizonicus (a spring annual), 

 Amaranthus palmeri (a summer annual), and Phaseolus sp. (Papago bean). 



^M. G. Sykes, The anatomy and morphology of the leaves and inflorescences of Welwitschia 

 mirabilis, Phil. Trans. Roy. Soc. of London, ser. B, vol. 201, p. 180. 



