XXI 



THE EFFECTS OF LIGHT INTENSITY UPON SEED PLANTS 



Hardy L. Shirley 



Lake States Forest Experiment Station, Forest Service, United States 



Department of Agriculture 



Introduction. Influence of darkness on plants: Effect of darkness on growth, form, 

 and structure — Effect of darkness on composition — The influence of light on etiolated 

 plants — Miscellaneous effects of darkness — Theories as to the cause of etiolation. The 

 effect of light intensity upon plants: The "light-growth reaction" — The effect of light 

 intensity upon the growth and development of plants — Light intensity and mineral nutri- 

 tion — The influence of light intensity on flowering — The influence of light intensity upon 

 transpiration, winter hardiness, and resistance to drought — Minimum light requirements. 

 Summary. References. 



INTRODUCTION 



This section is devoted chiefly to the effects of radiation within the 



o 



visible region, X 4000 to 7200 A, or "Hght" upon plants. However, the 

 term "light" is frequently used in a more general sense, i.e., in speaking 

 of sunlight we generally mean total solar radiation, and in speaking of 

 artificial Ught we mean the total radiation of the source in question. 

 Unless otherwise qualified, "light" in this section will be used to designate 

 the total radiation from the source used. A discussion of the methods 

 and instruments available to the worker in the field of radiation, as well 

 as the variability of radiation and other factors are considered in another 

 section of this survey. It should be emphasized here, however, that, in 

 examining the work in this field, the experimental technique available 

 to the worker must be constantly kept in mind and the results weighed 

 accordingly. When the difficulties involved in studying the influence 

 of light on plants are fully appreciated, it is little wonder that we find 

 in the literature many contradictions and many opposing theories. 



INFLUENCE OF DARKNESS ON PLANTS 



Darkness, or zero light intensity, produces in plants an effect known 

 as etiolation, the general features of which are familiar to all plant 

 workers. The typical characteristics of etiolation were described by 

 John Ray in 1686. From his time on, many botanists have studied 

 etiolated plants. The general features of etiolated plants are described 

 in textbooks on plant physiology and morphology, such as Strasburger 

 (102), Benecke-Jost (9), Haberlandt (44), Goebel (39), and Sachs (88). 

 MacDougal (65) made a comprehensive review of the literature dealing 



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