TKANSACTIONS OF SECTION B. 519 



In connection with this question of the characters of growtli under the influence 

 of continuous light, compared with those developed with alternate light and dark- 

 ness, the recent experiments of Dr. Siemens on the influence of electric light on 

 vegetation are of considerable interest. 



In one series of experiments, he kept one set of plants entirely in the dark, a second 

 he exposed to electric light only, a third to daylight only, and a fourth to daylight, 

 and afterwards to electric light from 5 to 11 P.M. Those kept in the dark acquired 

 a pale yellow colour, and died ; those exposed to electric light only, maintained a 

 light green colour, and survived ; those exposed to daylight were of a darker green 

 colour, and were more vigorous ; and, lastly, those submitted to alternate daylight 

 and electric light, and but a few hours of darkness, showed decidedly greater 

 vigour, and, as he says, the green of the leaf was of a dark rich hue. He concluded 

 that daylight was t^^-ice as effective as electric light ; but that, nevertheless, ' elec- 

 tric light was clearly sufficiently powerful to form chlorophyll and its derivatives 

 in the plants.' 



In a second series of experiments one group of plants was exposed to daybght 

 alone ; a second to electric light during eleven hours of the night, and was kept in 

 the dark during the day ; and a third to eleven hours day, and eleven hours electric 

 light. The plants in daylight showed the usual healthy appearance ; those in 

 alternate electric light and darkness Avere for the most part of a lighter colour ; 

 and those in alternate daylight and electric light far surpassed Jhe others in dark- 

 ness of green and ^-igorous appearance generally. 



I have carefully considered these general descriptions with a view to theii" 

 bearing on the question whether the characters developed under the influence of 

 electric light, and especially those under the influence of almost continuous light, 

 are more prominently those of assimilation or of groA\-th ; but I have not been able 

 to come to a decisive opinion on the point. From some conversation I had AA-ith 

 Dr. Siemens on the subject, I gather that the characteristics were more those of 

 dark colour and vigour than of tendency to great extension in size. The dark green 

 colour we may suppose to indicate a liberal production of chlorophyll ; but if the 

 depth of colour was more than normal, it might be concluded that the chlorophyll 

 had not performed its due amount of assimilation work. In regard to this point, 

 attention may be called to the fact that Dr. Siemens refers to the abundance of the 

 blue or actinic rays in the electric arc, conditions which would not be supposed 

 specially to favou/assimilation. On the other hand, the \"igour, rather than charac- 

 teristic extension in size, would seem to indicate a limitation of what is technically 

 called growth, under the influence of tlie almost continuous light. 



Among the numerous field experiments made at Kothamsted, we have many 

 examples of great variation in depth of green colour of the vegetation growing on 

 plots side by side under knov.u diflerences as to manuring ; and we have abundant 

 evidence of difference of composition, and of rate of carbon-assimilation, coincidently 

 with these different shades of colour. One or two instances wiU strikingly illus- 

 trate the point under consideration. 



There are two plots side by side in the series of experiments on permanent grass 

 land, each of which received dm-ing six consecutive years precisely the same amount 

 of a mixed mineral manure, including potass, and the same amount of nitrogen in 

 the form of ammonia salts. After those six years, one of the two plots was still 

 manured in exactly the same way each year ; wliilst the other was so, with one 

 t'xception — namely, the potass was now excluded from the manure. Calculation 

 .■;hows that there was a great excess of potass applied during the first six years ; ■ 

 and there was no marked diminution of produce during the five or six years .suc- 

 ceeding the cessation of the application. But each year subsequently, up to the 

 present time, now a period of fourteen years, or of nineteen since the exclusion of the 

 potass, the falling off in produce has been very great. 



The point of special interest is, however, that all but identically the same 

 amount of nitrogen has been taken up by the herbage growing with the deficiency 

 of potass as by that with the continued supply of it. The colour of the vegetation 

 with the deficiency of potass has been very much darker green than that with the 

 full supply of it. "Nevertheless, taking the average of the eight years succeeding.: 



