BOTANY. 



Britton, N. L., and J. N. Rose, U. S. National Museum. Studies of the 

 Cactaceae. (For previous reports see Year Books Nos. 11-16, 21.) 



The work of the past year has been mainly devoted to the fourth and last 

 volume of the Cactaceae, a monograph of the Cactus family upon which 

 Doctors Britton and Rose have been engaged since 1912. This work has 

 been based upon an exhaustive re-study of this large and difficult family after 

 extensive field work in both North and South America and after growing most 

 of the plants under glass either in Washington or in New York. 



As treated in this monograph, the Cactus family is composed of 3 tribes. 

 The first and second tribes are taken as units, but the third is composed of 

 8 subtribes. The number of genera recognized is 124 and the number of 

 species is 1,235. The plant names, mostly synonyms, number more than 

 10,000. The colored plates have been made from paintings by the plant 

 artist, M. E. Eaton. 



The four volumes will contain 1,246 printed pages, 137 full-page plates, 

 mostly in color, and more than 1,100 text-figures. 



Livingston, Burton E., Johns Hopkins University, Baltimore, Maryland. 

 Studies upon the influence of solar radiation on the rate of transpirational 

 ivater-loss from plants. 



These studies, begun a number of years ago at the Desert Laboratory, are 

 part of a more general investigation aiming to develop methods and expres- 

 sions by which the rate of transpirational water-loss from a plant may be under- 

 stood in terms of the internal and external conditions that together constitute 

 its control. The health and vigor of a plant depend in great measure on 

 the water-content of its tissues, and the water-content is itself clearly a 

 function of the rates of water-intake and water-loss. For this reason, among 

 others, a knowledge of the conditional control of the transpiration rate must 

 be essential to an understanding of the physiology of higher plants in general. 

 Since the beginning made with the work reported in Publication No. 50 of 

 the Institution, considerable progress has been made in these studies, to 

 which many investigators have contributed. Several aspects of the general 

 problem still require quantitative study, however, and the influence of solar 

 radiation is one of these with which the writer has been specially engaged 

 for several years. 



The plant environment acts directly to influence the transpiration rate, 

 only through those variable aerial and meteorological conditions that affect 

 the rate of vaporization of water. These are the conditions that determine 

 the rate of evaporation from a water-surface or the rate of diffusion of water- 

 vapor. For convenience of approach, the aerial conditions here dealt with 

 may be considered in two categories, the evaporating power of the air (the 

 combined influences of air temperature, air humidity, and air movement) 

 and the evaporating power of impinging radiation. The former group of 

 conditions operates on ordinary plants at practically all times, by night as well 

 as by day. The evaporating power of the air is generally positive but becomes 

 negative sometimes, when plant tissues absorb water from moist air, or when 

 dew is formed on plant surfaces. On the other hand, the influence of radia- 

 tion is practically confined to the daylight hours, since solar radiation is the 

 only sort that needs generally to be considered for plants growing in nature, 



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