604 



SCIENCE. 



[N. S. Vol. XVIII. No. 462. 



SHORTER ARTICLES. 



PHOTOTEOPISM UNDER LIGHT-RAYS OF DIFFERENT 



WAVE-LENGTHS. 



The effect of lateral incidence of liglit upon 

 ormophytes is of such a nature as to produce 

 a tendency in the plant to arrange its axis 

 parallel to the direction of the incident ray. 

 This response to the stimulus of light is quite 

 general with regard to higher plants, and has 

 long been known under the name heliotropism. 

 The term phototropism — from its literal 

 meaning more appropriate — has recently been 

 introduced to displace the older term. 



The relative phototropic efEects of rays of 

 different wave-length have been given by 

 Wiesner * to be greatest between ultra- 

 violet and violet rays, diminishing gradually 

 over to the yellow, where it disappears, then 

 beginning in the orange and reaching a small 

 secondary maximum in the ultra-red. Guille- 

 man'sf results resembled those of Wiesner 

 excepting with respect to the yellow. He 

 concluded that curvature takes place under 

 all the rays except the least refrangible heat 

 rays. Sachs himself states that under blue 

 light curvature takes place as in ordinary 

 daylight, and that no curvature whatever 

 takes place, behind a ruby-red glass; and he 

 agrees with Wiesner that no curvature takes 

 place behind a yellow screen. 



With regard to the decoloring effect upon 

 a fresh alcoholic solution of chlorophyll by 

 rays of different wave-length, the present posi- 

 tion is practically that expressed by Vines::}: 

 * Sachs and Wiesner have ascertained that the 

 rays of low refrangibility are more active in 

 forwarding it than those of high refrangi- 

 bility.' 



Some investigation of these subjects has 

 been made by the writer, and the results are 

 here given because they differ materially from 

 those referred to, and because it is thought 

 the methods here used to test the matter are 

 less open to objection and more complete than 

 those of the authors mentioned above. 



The following named glass plates (each nine 

 *'Die Heliotrop. Er.schein. im Pflanzenreich.' 

 t Ann. de Sci. Nat., IV.; 7; 1858. Both referred 

 to by Saehs, ' Plant Physiol.,' p. 696. 

 t' Lectures on Plant Physiol.,' p. 266. 



by twelve inches) were secured from BauscU 

 and Lomb and are what are commonly called 

 standard ' colors ' — violet, blue, green, yellow, 

 red — several of each. Window-glass was used 

 to admit daylight, and sheet iron for the 

 opaque screen. These colored plates were 

 examined by the writer with a view to getting 

 the particular spectrum of each, because col- 

 ored glass can scarcely be represented accu- 

 rately by simply naming the ' color ' ; for cer- 

 tain colored screens allow other ' colors ' to 

 pass than that which would seem from ordi- 

 nary observation. The curve for each ' color ' 

 is plotted approximately in the accompanying 



figure — 1, red; 2, yellow; 3, green; 4, blue; 

 5, violet. The letters A, B, C, D, E, F, G, H 

 are the ordinary significant points (Fraun- 

 hofer lines) of the solar spectrum. It will 

 be seen that there is none of them, with the 

 exception possibly of the red, which can be 

 considered entirely a pure 'color.' However, 

 they are fairly close approximations towards 

 simple 'colors,' and as such are of some sig- 

 nificance in regard to the subject under dis- 

 cussion. 

 In order to test the relative phototropic 



