SUN RAYS AND PLANT LIFE— JOHNSTON 355 



obtain relative values; the physiologist aims to deteniiine the 

 absolute values of such relationships. 



As a result of observations and studies by plant ecologists and 

 physiologists, many interesting relationships have been found be- 

 tween light and the structure and growth of plants. The brilliant 

 colors of alpine flowers have been attributed to the presence of ultra- 

 violet light in the clear sky of high altitudes. The broad succulent 

 leaf growth within a dense tropical forest can be attributed in part 

 to a reduced light condition. Many interesting structural modifica- 

 tions in desert plants are brought about by a change in moisture 

 and light. Although such observations are interesting, but little 

 quantitative data can be obtained until plants are grown under 

 controlled conditions. 



The plants around us are registering within themselves the total 

 effects of the climate, sunlight being one of the important factors 

 of the climatic complex. Perhaps the most familiar record of the 

 plant's environment is that recorded by trees. The type of rings, 

 their thickness and shape, give to those familiar with the language 

 a story of the climate during the life of the tree. The researches 

 of Dr. Douglass (1932) on tree rings have given us a most instructive 

 picture of the climatic conditions prevailing during the past cen- 

 turies. 



Use has actually been made of the plant as an integrating instru- 

 ment for measuring climatic conditions. Wliile a student at the 

 Johns Hopkins University, I conducted an experiment in which 

 the climatic conditions of a greenhouse for a period of 13 consecu- 

 tive months were recorded by a set of "standard" buckwheat plants 

 (Johnston, 1921). The plants were grown for 4-week exposure 

 periods. A new period began every 2 weeks. Measurements of stem 

 height, dry weight, leaf area, and transpiration were made at regular 

 intervals. Simultaneous measurements of evaporation, radiation, and 

 temperature were also obtained. Two series of tests were conducted, 

 one under ordinary conditions of an unshaded greenhouse, the other 

 within a cheesecloth enclosure in the same greenhouse. Some of these 

 data are summarized in the form of gi'aphs and shown in figure 1. 



The rates of stem elongation, dry weight increase, and leaf-area 

 increase had high summer values and low winter ones. These values 

 increased during the spring and decreased during the autumn. The 

 rates of transpirational water loss varied throughout the year in a 

 similar manner. For the rates of stem elongation, and possibly also 

 for that of transpiration, it appears that a period of low values 

 occurs about the summer solstice. 



The greenhouse climate during this particular year, as measured 

 by these plant processes, appears to have been most favorable to 



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