100 PHYSIOLOCY 



branches of the Plant Sciences, In a greenhouse as is ordinarily passed 

 Agricultural Colleges, in Experiment through an air-conditioned auditorium, 

 Stations, in special Research Institu- the outgoing air is still 4-5 C. warmer 

 tions there are hundreds or thousands than the incoming air. But as soon as 

 of Plant Pathologists, Entomologists, the sun disappears behind clouds the 

 Agronomists, Plant Physiologists, Ge- heat-load on the greenhouse may be- 

 neticists and other scientists who are come negative and the incoming air 

 investigating all factors of the internal may require heating, therefore the air- 

 and external en\ironment of the plant conditioning equipment of a green- 

 with the ultimate aim of better pro- house must be highly flexible and must 

 duction of plants. Tliere is however be instantaneously adjustable to chang- 

 one group of scientists which is hardly ing conditions of radiation, 

 represented. These arc the Plant Cli- Completion in 1939 of the Clark 

 matologists, dealing with the aerial en- Greenhouses and in 1949 of the Ear- 

 vironment of the plant. Yet, the aerial hart Plant Research Laboratory, both 

 environment or climate is at least as at the California Institute of Technol- 

 important in connection with plant ogy, provided for the first time large- 

 production, as the soil, pests, diseases, scale growing facilities for plants under 

 growing methods or gcnctical back- controlled conditions. The following 

 ground are. article deals with the results obtained in 



There are many good reasons why experiments carried out in these labo- 



the field of Plant Climatology has been ratories. . . . 



so neglected by investigators. In the One of the most important results 

 first place it is difficult to define climate obtained in these air-conditioned green- 

 clearly and accurately, partly because of houses and growing rooms is the extent 

 the variability of the weather from day to which biological variability can be 

 to day and from year to year, and reduced. This variability usually is the 

 partly because weather is an integration greatest handicap in biological experi- 

 of non-comparable values. In the sec- mentation and it reduces the reliabil- 

 ond place we have not had methods for ity of conclusions based on quantitative 

 growing plants under strictly controlled responses. The uncertainty in conclu- 

 environments or synthetic climates. sions reached in biological experimen- 



Mainly through the efforts of Dr. tation has led physicists and chemists 



H. O. Eversole and Dr. L. C. Marshall to distinguish themselves as working 



control over the plant environment has in the "exact" sciences, 



become possible, after they had eluci- Tliere are several sources of variabil- 



dated the principles upon which green- ity in biological material. The first is 



houses can be air-conditioned. Also in the hereditar}^ background. The greater 



growing plants we need high light in- the genetical differences between indi- 



tensities, which are most conveniently viduals, the greater their phenotypie 



available in the form of sunlight. But variability will be. On the other hand, 



in greenhouses, using the sun as a light well-selected seed material, or cuttings 



source, the problem of temperature made from the same plant, has a po- 



control is critical. On a sunny day at tential uniformity which far surpasses 



noon one calorie is absorbed per square the homogeneity of reagent-pure chem- 



eentimeter of greenhouse per minute, icals. 



The heat thus generated has to be re- The second source of variabilit}' in 



moved by air renewal, which requires biological material is the inhomoge- 



large volumes of air. Even when 20-30 neity of the environment during 



times as much air is circulated through growth. Tlic extent to which this factor 



