522 EXPERIMENT STATION RECORD. [Vol.38 



Plants placed in the damper scrub community transpire much less vigorously 

 than others of the same species placed in the prairie. In general there is a 

 correlation between the evaporating power of the air and the amount of trans- 

 piration. 



When sufficient light is available, the humidity of the air and soil are the 

 most important factors affecting the establishment of the different plant com- 

 munities. The progressive increase of the humidity of the habitat causes a 

 corresponding increase of the mesophytism of the plant community. 



The evaporation rates and the amount of soil moisture in the various com- 

 munities of both Minnesota and Nebraska vary in general directly with the 

 order of tlieir occurrence in the succession, the community nearest the climax 

 being the most mesophytic in both respects. 



Bedwoods, rainfall, and fog', W. S. Cooper {Plant World, 20 (1917), No. 6, 

 pp. 179-189, figs. 2). — The studies here noted as carried out during the j-ainy 

 seasons of 1913-14 and 1914-15 in the Santa Cruz Mountains by means of a 

 simple type of rain gauge, described as maliing possible the summation of pre- 

 cipitation for long periods, are said to show that the California redwood (Se- 

 quoia sempervirens) requires a high ratio of water supply to water loss, being 

 unusually sensitive to the danger of rapid transpiration, even when the water 

 supply is ample. During the rainless season the soil becomes dangerously dry, 

 even in the more mesophytic habitats. In regions of deficient rainfall the red- 

 wood can exist only near permanent streams. The full development of redwood 

 forest requires not only heavy winter precipitation but abundant and frequent 

 summer fogs. 



Incipient drying and temporary and permanent wilting of plants, as re- 

 lated to external and internal conditions, B. E. Livingston (Johns Hopkins 

 Univ. Circ, n. ser., No. S (1917), pp. 176-182). — Discussing recent and former 

 work regarding circumstances connected with the phenomenon of diminished 

 water content designated as incipient drying, the author states that the rate 

 of absorption of water by plant roots appears to be determined by two condi- 

 tions, the absorbing power of the roots (internal) and the supplying power of 

 the medium in which they lie (external). The internal condition is at least 

 partially controlled by the degree of incipient drying occurring in the plant, 

 which is in turn partly dependent upon the transpiration rate. 



Incipient drying of leaves is due to inadequate water supply to these parts, 

 due to causes either internal or external to the plant. The data reported are 

 considered to show that incipient drying, temporary wilting, and even perma- 

 nent wilting of most of the leaves may occur without any resistance to water 

 absorption by the roots and even in the presence of a relatively low atmometric 

 index. In some cases noted, at least, the inadequacy of absorbing power appears 

 to be due to conditions internal to the plant. None of the three stages of 

 incipient drying is necessarily related to soil-moisture conditions. 



The vapor tension deficit as an index of the moisture condition of the 

 air, B. B. Livingston (Johns Hopkins Univ. Circ, n. ser.. No. 3 (1917), pp. 170- 

 175). — This is an analysis of the index of the air conditions which influence 

 water loss from aerially exposed organisms into its two components, namely, 

 velocity of the air movement or circulation and the moisture condition of the 

 air. It is considered important to give serious attention to the latter, which 

 is defined as that factor in atmospheric evaporating power that is independent 

 of the rate of air movement. The difference between the tendency for water to 

 evaporate into air in contact with its surface (vaporization pressure) and to 

 deposit liquid on the evaporating surface (condensation pressure) is the vapor 

 pressure deficit, and is the factor of atmospheric evaporating power that is not 



