476 REPORT — 1881. 



The effect of interposing a mere thin sheet of clear glass between the 

 plants and the sonrce of electric light was most striking. On placing 

 such a sheet of clear glass so as to intercept the rays of the electric light 

 from a portion only of a plant — for instance, a tomato plant — it was 

 observed that in the course of a single night the line of demarcation was 

 most distinctly shown upon the leaves. The portion of the plant under 

 the direct influence of the naked electric light, though at a distance from 

 it of nine to ten feet, wa.s shrivelled, whereas that portion under cover of 

 the clear glass, continued to show a healthy appearance, and this line of 

 demarcation was distinctly visible on individual leaves — not only the 

 leaves but the young stems of the plants soon showed signs of destruction 

 when exposed to the naked electric light, and these destructive in- 

 fluences were perceptible, though in a less marked degree, at a distance 

 of twenty feet from the source of light. 



A question here presents itself that can hardly fail to excite the 

 interest of the physiological botanist. The clear glass does not appar- 

 ently intercept any of the luminous rays, which cannot therefore be the 

 cause of the destructive action. Professor G. Gr. Stokes showed, how- 

 ever, in 1853, that the electric arc is particularly rich in highly refrangible 

 invisible rays, and that these are largely absorbed in their passage 

 through clear glass ; it therefore appears reasonable to siippose that it is 

 those highly refrangible rays beyond the visible spectrum that work 

 destruction on vegetable cells ; thus contrasting with the luminous rays 

 of less refrangibility, which, on the contrary, stimulate their organic 

 action. 



Being desirous to follow up this inquiry a little further, I sowed a 

 portion of the ground in the experimental conservatory with mustard 

 and other quick-growing seeds, and divided the field into equal radial 

 portions by means of a framework, excluding diffused light but admitting 

 light at equal distances from the electric arc. The first section was 

 under the action of the naked light, the second was covered with a pane 

 of clear glass, the third with yellow glass, the fourth with red, and the 

 fifth with blue glass. The I'elative progress of the plants was noted from 

 day to day, and the difierences of effect upon the development of the 

 plants was sufficiently striking to justify the following conclusions : viz., 

 under the clear glass the largest amount of and most vigorous growth 

 was induced; the yellow glass came next in order, but the plants, though 

 nearly equal in size, were greatly inferior in colour and thickness of stem 

 to those under the clear glass ; the I'ed glass gives rise to lanky growth 

 and yellowish leaf, while the blue glass produces still more lanky growth 

 and sickly leaf. The uncovered compartment showed a stunted growth, 

 with a very dark and partly shrivelled leaf in the case of mustard, 

 whereas tender-leaved plants, such as cress and salad, were completely 

 destroyed. 



It should be observed that the electric light was kept on from 5 p.m. 

 till 6 a.m. every night except Sundays during the experiment, which took 

 place in January 1881, but that diffused daylight was not excluded during 

 the intervals, also that circulation of air through the dividing framework 

 ■was provided for. 



These results are confirmatory of those obtained by Dr. J. W. Draper 

 in his valuable researches on plant-life in the solar spectrum in 1843,' 

 which led him to the conclusion, in opposition to the then prevailing 

 ' See Scientijic Memoirs by J. W. Draper, M.D., LL.D. — Memoir X. 



