Ill] OF THE INFLUENCE OF LIGHT 241 



with short intervals of drought, during the eighteenth century ; then, 

 after 1790, longer droughts and shorter spells of rainy seasons*. 



It has been often remarked that our common European trees, 

 such as the elm or the cherry, have larger leaves the farther north 

 we go ; but the phenomenon is due to the longer hours of dayhght 

 throughout the summer, rather than to intensity of illumination or 

 diJBFerence of temperature. On the other hand, long dayhght, by 

 prolonging vegetative growth, retards flowering and fruiting; and 

 late varieties of soya bean may be forced into early ripeness by 

 artificially shortening their dayhght at midsummer f. 



The effect of ultra-violet hght, or any other portion of the 

 spectrum, is part, and perhaps the chief part, of the same problem. 

 That ultra-violet hght accelerates growth has been shewn both in 

 plants and animals f. In tomatoes, growth is favoured by just such 

 ultra-violet hght as comes very near the end of the solar spectrum §, 

 and as happens, also, to be especially absorbed by ordinary green- 

 house glass II . At the other end of the spectrum, in red or orange 

 light, the leaves become smaller, their petioles longer, the nodes 

 more numerous, the very cells longer and more attenuated. It is 

 a physiological problem, and as such it shews how plant-hfe is 

 adapted, on the whole, to just such rays as the sun sends; but it 

 also shews the morphologist how the secondary effects of chmate 

 may so influence growth as to modify both size and form^. An 

 analogous case is the influence of hght, rather than temperature, 

 in modifying the coloration of organisms, such as certain butterflies. 



* Le chene Zeem d'Ain Draham, Bull, du Directeur General, Tunisie, 1927. 



t That the plant grows by turns in darkness and in light, and has its characteristic 

 growth-phases in each, longer >or shorter according to species and variety and 

 normal habitat, is a subject now studied under the name of "photoperiodism," 

 and become of great practical importance for the northerly extension of cereal 

 crops in Canada and Russia. Cf. R. G. Whyte and M. A. Oljhovikov, Nature, 

 Feb. 18, 1939. 



I Cf. Kuro Suzuki and T. Hatano, in Proc. Imp. Acad, of Japan, in, pp. 94-96, 1927. 

 § Withrow and Benedict, in Bull, of Basic Scient. Research, iii, pp. 161-174, 1931. 



II Cf. E. C. Teodoresco, Croissance des plantes aux lumieres de diverses longueurs 

 d'onde, ^WTi. Sc. Nat, Bat. (8), pp. 141-336, 1929; N. Pfeiffer, Botan. Gaz. lxxxv, 

 p. 127, 1929; etc. 



II See D. T. MacDougal, Influence of light and darkness, etc., Mem. N. Y. Botan. 

 Garden, 1903, 392 pp.; Growth in trees, Carnegie Inst. 1921, 1924, etc.; J. Wiesner, 

 Lichtgenuss der Pflunzen, vn, 322 pp., 1907; Earl S. Johnston, Smithson. Misc. 

 Contrib. 18 pp., 1938; etc. On the curious effect of short spells of light and dark- 

 ness, see H. Dickson, Proc. E.S. (B), cxv, pp. 115-123, 1938. 



