THE INTERNAL TEMPERATURES OF COTTON BOLLS

RECENT STUDIES (Kerr, 1937; Anderson and Moore, 1937) have demonstrated that temperature changes exert an important effect upon the structure of the wall of cotton fibers. The process of wall thickening in cotton hairs begins about 16 or 18 days after flowering and continues for 20 or more days. It is in this period of wall thickening that the influence of temperature changes becomes apparent. Whenever the temperature falls much below 200C., the rate of cellulose deposition is so greatly decreased that it is difficult to detect the amount deposited by means of stains when sections are swollen. When the temperature remains above 250C., cellulose deposition occurs rapidly, and the wall formed is sufficiently dense to stain conspicuously even when greatly enlarged by swelling agents. The diurnal variations in temperature common during the growing season result, therefore, in more or less regular variations in the rate of cellulose deposition. The consequences of these periodic differences in the rate of cellulose deposition become strikingly apparent when cross sections of the cotton hair are swollen. The swollen sections of the hair exhibit a conspicuous concentric zonation of the cellulose. Densely aggregated zones of cellulose resulting from the rapid deposition of the cellulose during the warmer daylight hours alternate with more porous open bands produced by the less rapid deposition of cellulose during the cool hours of the night (Kerr, 1937). When cotton plants are grown under conditions in which light and temperature are continuously maintained at an approximately constant level, all evidence of zonation in the walls of the fibers disappears (Anderson and Moore, 1937). Furthermore, it has been demonstrated repeatedly that the conspicuous zonation of the wall may be controlled at will by regulating the temperature of the air around the plants at the time of wall thickening (Kerr and Anderson, unpublished work). Cotton hairs undergo their complete development while enclosed within the thick, tough carpel wall of the cotton boll. An enormous number of rapidly growing hairs are tightly compressed within this covering which isolates them more or less completely from the external environment. Because of these conditions it seems probable that the temperature within the cotton boll will be different from that of the air outside. Commonly cotton bolls are exposed to the full intensity of solar radiation for periods of varying length during the day. At other times they are shaded by leaves. At night some bolls are exposed to the sky while others are shielded by leaves. 1 Received for publication October 5, 1939. A part of the joint cotton research program of the Bureau of Plant Industry and the North Carolina Agricultural Experiment Station. It is a pleasure to acknowledge the helpful suggestions of Professor E. K. Plyler of the Department of Physics of the University of North Carolina in interpreting the experimental data and the assistance of Dr. J. 0. Beasley in making measurements in the field. A number of investigators have shown that exposure to full solar radiation produces a very rapid and marked increase in the temperature of leaves, and Curtis (1936) has called attention to the importance of radiation in the long infra-red as a factor influencing the loss of heat from leaves exposed to a clear sky. It seems very probable that the internal temperatures of cotton bolls will also be influenced by these factors. In view of the significant influence of temperature upon the wall structure of the cotton hair it seems important to know something about the temperatures within the boll and of the relation that these internal temperatures bear to the air temperatures outside the boll. The investigation described in this paper was undertaken with these objectives. Although the temperatures of leaves have been carefully and extensively studied under a wide range of environmental conditions, the temperature variations that occur within bulkier tissues have received much less attention. Grainger and Allen (1936) have studied the internal temperature of the buds of some fruits. Using thermocouples, they found that the internal temperatures of apple buds were usually higher than the air during the day and lower than the air temperature at night. The lower night temperatures were attributed chiefly to evaporation, but losses by radiation were considered to be an "important secondary factor." When the buds were shaded by means of a metal screen, their internal temperatures were found to be lower than the air temperatures during both the day and the night. The night temperatures of shaded buds were found to be slightly higher than those of unshaded buds. At night the internal temperatures of the shaded buds closely approximated those of the surrounding air. Temperatures within the buds were not found to differ widely from the air temperatures under any of the conditions studied, the variations usually being less than 30C. The internal temperatures of apple fruits have been measured by Brooks and Fisher (1926). Apples in the shade had internal temperatures that closely approximated those of the air, but apples exposed to the direct radiation of the sun had temperatures much above those of the surrounding air. Tissues about 1/4 of an inch below the exposed surface of apple fruits were found to have temperatures as much as 10?C. above the air. On the shaded side of exposed apples the temperatures at similar depths were about 50 or 60C. above the surrounding air. Harvey (1923a) recorded temperatures at the exposed surface of apples as much as 180C. above the air temperature and some 120C. above the air on the shaded side of exposed apples. Harvey (1923b) also measured the temperatures at the cambium layer in apple and plum trees during periods of low air temperatures. The results indicate that cambium temperatures of 100 or 120 C. above those of the air are