Sept. 16, 1800] 



NA TURE 



475 



of light, cell-divisiou, cell-multiplication, increase in the structure 

 of the plant, in other words, what, as distinguished from assimi- 

 lation, vegetable physiologists designate as groivlh, takes place, 

 at any rate chiefly, during the night ; and is accompanied, not 

 with the taldng up of carbonic acid and the yielding up of 

 oxygen, but with the taking up of oxygen and the giving up of 

 carbonic acid. This evolution of carbonic acid during darkness 

 must obviously be extremely small, compared with the converse 

 action during daylight, coincidentally with which practically the 

 whole of the accumulation of solid substance is accomplished. 

 But as the product of the night action is the same as in the 

 respiration of animals, this is distinguished by vegetable physio- 

 logists as the respiration of plants. 



I suppose I shall be considered a heretic if I venture to suggest 

 that it seems in a sense inappropriate to apply the term grcnuth 

 to that which is associated with actual loss of material, and that 

 the term respiration should be applied to so secondary an action 

 as that as the result of which carbonic acid is given off from the 

 plant. It may, I think, be a question whether there is any 

 advantage in thus attempting to establish a parallelism between 

 animal and vegetable processes ; rather would i: seem advanta- 

 geous to .keep prominently in view their contrasted, or at any 

 rate complementary chai-act eristics, especially in the matter of 

 the taking up of carbonic acid and the giving up of oxygen on 

 the one hand, and the taking in of oxygen and the giving up of 

 carbonic acid on the other. 



But it is obvious that in latitudes where there is comparatively 

 continuous daylight during the periods' of vegetation, the two 

 actions — designated respectively assimilation and growth — must 

 go on much more simultaneously than where there is a more 

 marked alternation of daylight and darkness. In p.arts of Nor- 

 way and Sweden, for example, where during the summer there 

 is almost continuous daylight, crops of barley are grown with 

 only from six to eight weeks intervening from seed-time to 

 harvest. And Prof. Schiibeler, of Christiania, after making 

 observations on the subject for nearly thirty years, has recently 

 described the characteristics of the vegetation developed under 

 the influence of short summers with almost continuous light. 

 He states that, after acclimatisation, many garden flowers in- 

 crease in size and depth of colour ; that there is a prevailing 

 tinge of red in the plants in the fjelds ; that the aroma of fruits 

 IS increased, and their colour well developed, but that they are 

 deficient in sweetness ; and that the development of essential 

 oils in certain plants is greater than in the same plants grown in 

 other latitudes. Indeed he considers it to be an established fact 

 that light bears the same relation to aroma as heat does to 

 sweetness. 



In connection with this question of the characters of growth 

 under the influence of continuous light, compared with those 

 developetl with alternate light and darkness, the recent experi- 

 ments 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 1 1 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 vigo- 

 rous ; 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 twice as effec- 

 tive as electric light ; but that, nevertheless, "electric light was 

 clearly sufficiently powerful to form chlorophyll and its deriva- 

 tives in the plants." 



In a second series of experiments one group of plants was 

 exposed to daylight alone ; a second to electric light during 

 eleven ,hours of the night, and was kept in the dark during 

 the day ; and a thu-d 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 were 

 for the most part of a lighter colour ; and those in alternate 

 daylight and electric light far surpassed the others in darkness 

 of green and vigorous appearance generally. 



_ I have carefully considered these general descriptions with a 

 view to their bearing on the question whether the ch.iracters 

 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 gi-owth ; but I have not 



been able to come to a decisive opinion on the point. From 

 some conversation I had with 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 w-e may suppose to indicate a liberal produc- 

 tion 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 favour 

 assimilation. On the other .hand, the vigour, rather than 

 characteristic extension in size, would seem to indicate a limita- 

 tion of what is technically called growth, under the influence of 

 the almost continuous light. 



Among the numerous field experiments made at Rothamsted, 

 we have many examples of great variation m depth of green 

 colour of the vegetation growing on plots side by side under 

 known differences 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 will strikingly illustrate the point imder 

 consideration. 



The point of special interest is, however, that all but identi- 

 cally 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. Tiie colour of the vegetation with 

 the deficiency of potass has been very much darker green than 

 that with the full supply of it. 



An equal amount of nitrogen was taken up in both cases, 

 chlorophyll was abundantly produced, but the fuU amoimt of 

 carbon was not assimilated. In other words, the nitrogen was 

 there, the chlorophyll was there, there was the same sunlight for 

 both plots ; but the assimilation-work was not done where there 

 was not a due supply of potass. 



It may be stated generally that, in comparable cases, depth of 

 green colour, if not beyond a certain limit, may be taken to 

 indicate corresponding activity of carbon assimilation ; but the 

 two instances cited are sufficient to show that we may, so far as 

 the nitrogen, the chlorophyll, and the light are concerned, have 

 the necessary conditions for full assimilation, but not correspond- 

 ing actual assimilation. 



It cannot, I think, fail to be recognised that in these consider- 

 ations we have opened up to view a very wide field of research, 

 and some of the points involved we may hope will receive 

 elucidation from the further prosecution of Dr. Siemens's ex- 

 periments. He will himself, I am sure, be the first to admit 

 that what he has already accomplished has done more in raising 

 than in settling important questions. I understand that he pro- 

 poses to submit plants to the action of the separated rays of his 

 artificial light, and the results obtained cannot fail to be of much 

 interest." But it is obvious that the investigation should now 

 pass from its present initiative character to that of a strictly quan- 

 titative inquiry. We ought to know not only that, under given 

 conditions as to light, plants acquire a deeper green colour, and 

 attain maturity much earlier than under others, but how much 

 matter is assimilated in each case, and something also of the 

 comparative chemical characters of the products. As between 

 the action of one description of light and another, and as 

 between the greater or less continuity of exposure, we ought to 

 be able to form a judgment whether the proper balance between 

 assimilation on the one hand, and growth and proper maturation 

 on the other, has teen attained ; whether the plants have taken 

 up nitrogen and mineral matter and produced clilorophyll in a 

 greater degree than the quantity and the quality of the light have 

 been able to turn to account ; or whether the conditions as to 

 light have been such that the processes of transformation and 

 growth from the reserve material provided by assimilation have 

 not been normal or have not kept pace with the production of 

 that material. 



But one word more in reference to Dr. Siemens' results and 

 proposed extension of his inquiries. Even supposing that by 

 submitting growing crops to continuous light by the aid of the 

 electric light during the night, they could be brought to maturity 

 within a period shorter than at present approximately in propor- 

 tion to the increased number of hours of exposure, the estimates 

 of the cost of illuminating the vegetation of an acre of land cer- 

 tainly do not seem to hold put any hope that agriculture isjikely 

 to derive benefit from such an application of science to its needs. 

 If, however, the characters of growth and of maturation should 



