LEAF XEROMORPHY IN DICOTYLEDON SPECIES FROM A GYPSUM SAND DEPOSIT

Abstract

THE STRUCTURAL modifications of xeromorphic leaves result from a restricted water supply, excessive water loss (Maximov, 1929), intense light (Clements, 1905; Shirley, 1929; Turrell, 1940) or low nitrogen (Mothes, 1932). Xeromorphic sun leaves in particular are characterized by a decreased ratio of external surface to volume. Shade leaves of Rhuts toxicodendron L. growing in moist dunes of a maple forest are five times the area of those from a sandy level with few trees, and compactness of the spongy mesophyll varies directly with xerophily of habitat (Turner, 1923). Reduced external leaf surface is accompanied by increased thickness, resulting from a greater development of palisade at the expense of a laterally expanding spongy mesophyll (Schimper, 1903; Maximov, 1929). Plants grown in sunlight develop longer palisade cells in more rows than in shade leaves of the same species, forming 77 per cent more cell substance in the leaf mesophyll (Hormann, 1927). Trichomes, a heavy cuticle, and water storage tissue are also especially characteristic of many xeromorphic leaves. Certain of these modifications establish conditions favorable to an increased photosynthetic rate, and often to an accelerated transpiration rate as well (Maximov, 1931). LOCALE.-This deposit of drifting gypsum crystals covers 224 sq. mi. south and west of Alamogordo, New Mexico, at an altitude around 4000 ft. Over an area approximately 28 mi. long and 8-10 mi. wide, white dunes 20-40 ft. high alternate with narrow flats. The southern extremity of the area, including 40 per cent of the dunes (Schaffner, 1948), was set aside in 1933 as the White Sands National Monument.2 Most of the dunes are fairly well stabilized, moving only a few inches to a few feet each year. The gypsum being heavier than other materials present, it is winnowed by the wind and remains in the dune area while darker, lighter weight impurities are blown free of the drifts (Schaffner, 1948). The sand in the dunes is 9697 per cent calcium sulfate (Coville and MacDougal, 1903; Byers, 1936), the water table in the flats lies 2-3 ft. below the surface (Emerson, 1935), and the soil pH is 7.5 (Byers, 1936). Only one of the numerous tests by soil chemists has shown traces of nitrogen in any form (Emerson, 1935). The maximum temperature recorded at headquarters between 1933 and 1948 was 108?F., the minimum -80F., and the average annual temperature was approximately 600F. (Schaffner, 1948).