CHIEF ENVIRONMENTAL CONDITIONS. 135 



IV. LIGHT. 

 1. GENERAL NATURE OF LIGHT. 



The relations of water and temperature to plants, which have 

 already been considered, involve variations only in intensity, there 

 being no qualitative differences involved; the amount of moisture 

 and the degree of temperature in the plant-body are all that need to 

 be specified in order that these conditions be defined. Light, however, 

 may vary not only in amount, that is, in intensity, but also in quality, 

 and herein lies a most serious complication. Furthermore, making 

 matters still more difficult, it is impossible, excepting on purely arbi- 

 trary grounds, to distinguish the radiant energy that we term fight 

 from radiant heat on the one hand and from the ultra-violet rays on 

 the other. It thus appears that the term ''light " itself is nothing more 

 than an arbitrary term, denoting a range of different sorts of radiant 

 energy, the range being characterized by certain wave-lengths. This 

 range is extended on either side, beyond the arbitrarily limited region, 

 by still other wave-lengths which are not included under the term 

 "light." 



Light is usually understood to mean radiant energy that is capable 

 of affecting the human eye, having a range, then, of wave-lengths 

 from about 400 to about 750 mifiionths of a millimeter. The sun's 

 spectrum, however, extends to wave-lengths of about 293 jUju, where 

 the opacity of the earth's atmosphere to these ultra-\dolet rays brings 

 its range to a rather abrupt limit.^ Radiant energy with wave-lengths 

 greater than about 750 ju/x, and extending beyond 2,400 ju/x (Nutting, 

 1912, page 202) are termed heat. 



The study of the characteristics of radiant energy has been facilitated 

 in certain aspects, and perhaps retarded in others, by the fact that 

 all these various radiations may be mostly transformed, on being 

 allowed to fall upon a blackened surface, into the molecular vibrations 

 of matter. It has thus come about that the intensity of all these 

 forms of radiant energy is usually measured by converting them into 

 molecular heat and by determining the temperature acquired by the 

 heated body. It is thus that the intensity of light, or other^ radiant 

 form of energy transfer, is commonly measured and described in terms 

 of calories received per square centimeter of the cross-section of the 

 impinging beam per unit of time. 



The quality of light is defined by its range of wave-length and by 

 the relative intensities for the different portions of the range. The 

 range of wave-length may be determined through the use of a properly 

 constructed spectroscope. The plant never receives light of just a 

 single wave-length; it always receives a mixture of wave-lengths with 

 a more or less broad range. Since light i s most easily perceived by us 



1 Nutting, P. G., Outlines of applied optics, Philadelphia. 1912, p. 2. 



