INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 289 



classified by this criterion, into shade plants and non-shade plants. In this 

 connection the work of Wiesner has brought his term Lichtgenuss of plants 

 into considerable prominence. ; 



By relative Lichtgenuss, Wiesner means the light income of the plant in 

 question, expressed as a fractional part of the total sunlight intensity that might 

 reach it if it were not shaded at all in its habitat. It is clear that the light 

 income of different parts of the same individual, and of different individuals 

 in the same natural habitat, is not a constant, but ranges between certain limits; 

 either the maximum or minimum requirement of light intensity may be of inter- 

 est (the limits of the range of light incomes under which a given species may 

 thrive), or the average light income of an individual or group may be studied. 

 The range of light intensities that a plant can bear, with which the work of Wies- 

 ner was most concerned, is a quantitative expression of the degree of the plant's 

 adaptation for growing under various light conditions; it tells something of the 

 internal conditions or properties of the plant as far as its light requirement is 

 concerned. If the relative light genuss (relative photolepsy) of a plant is said 

 to be 0.25, it is to be understood that that particular plant is growing in a shaded 

 place where its light income is approximately 0.25 of what that income would be 

 if all the shade were removed. Wiesner also employed what he terms the abso- 

 lute genuss (absolute photolepsy, absolute light income, etc.), which is 

 expressed in photometric units; he used the Bunsen-Roscoe unit. 1 



The ranges of the relative light incomes of several plants growing in their 

 natural habitats at Vienna are shown below: 



Buxus sempervirens (box) : 1-H00 (0.010) 



Fagus sylvatica (beech) i-^io (0.013) 



Betula verrucosa (birch) i-^i (o . 111) 



Larix decidua (larch) i~}i (o . 200) 



1 Wiesner, 1909. [See note 1, p. 274.] 



> Although Wiesner expresses the hope that the term Lichtgenuss may eventually come to 

 be an international technical word, it seems hardly probable that this hope will be realized. 

 As an alternative he has suggested photolepsy. [Wiesner, J., Sur l'adaptation de la plante 

 a l'intensite de la lumiere. Compt. rend Paris 138: 1346-1340. 1904. Idem, 1907, p. 5 

 (see note 1, p. 274).] Whatever may be the pros and cons with reference to these two words, 

 it is clear that neither one of them can ever have quite the same suggestiveness that Licht- 

 genuss has in German. In that language the word itself is familiar to every one and the tech- 

 nical meaning given it by Wiesner is derived from the ordinary meaning. To the non-techni- 

 cal German, Lichtgenuss carries a meaning very similar to that employed by Wiesner, while 

 neither Lichtgenuss nor Photolepsy has any meaning at all to such a reader in most other 

 languages. It therefore seems desirable to employ a simple and straightforward English 

 word or phrase for non-technical purposes. Light income and optimal light intensity may be 

 used. Neither of these has as much teleological implication as has the word Lichtgenuss in 

 German. Light income means simply the amount of light actually impinging upon the plant 

 in question. The optimal light intensity denotes the amount of light that must impinge upon 

 the plant in order that it grow best, or most rapidly, etc. The light requirement of a given 

 species is the range of light intensity within which that species can thrive, etc., being limited 

 by a maximum and a minimum requirement. Measurements of light intensity should be 

 recorded in absolute terms, not as percentages of the intensity of unobstructed sunshine 

 at the given time and place, which itself varies greatly and rapidly at any locality and is 

 often not at all the same for different places at the same time. Of the light actually reach- 

 ing the plant surface only a partis absorbed, of course; much is directly reflected at the 

 periphery and some usually passes through or is reflected from internal surfaces. — Ed. 

 19 



