IN¥LtJENCE OF EXTERNAL CONDITIONS ON GROWTH 259 



Plants differ with respect to their light requirements and they may be 

 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 ph'otolepsy) of a plant is said 

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

 place whereits Hght 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 ex- 

 pressed in photometric units; he used the Bunsen-Roscoe unit.^ 



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

 natural habitats at Vienna are shown below: 



Buxus sempervirens (box) i-Hoo (o.oio) 



Fagus syhatica (beech) i-Mo (0-013) 



Betula verrucosa (birch) i-J^ (o. iii) 



Larix decidua (larch) i-M (o ■ 200) 



, 1 Wiesner, igop. [See note r, p. 244.] 

 ' 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 I'adaptationde la plante 

 a I'intensitd de la lumiere. Compt. rend. Paris 138 : 1346-1349. 1904. Idem, 1907, p. 5 

 (see note i, p. 244).] Whatever may be the pros and cons with reference to these two words, 

 ft is dear 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 Hght 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. Of the light actually reaching the plant surface 

 only a part is absorbed, of course; much is directly reflected at the periphery and some usually 

 passes through or is reflected from internal surfaces. — Ed. 



