684 GENERAL CONDITIONS OF PLANT-LIFE. 



blue solution all the more refrangible rays from the green to the ultra-violet. Those 

 fluids also are of great use which transmit the whole spectrum with the exception 

 of a few groups of rays as sharply limited as possible. If certain phenomena occur 

 when plants are exposed to light transmitted through these solutions, it is certain 

 that they are not caused by rays of that particular refrangibility which are absent, 

 and 'vice 'versa. It is obvious that absorbent media are of use in experiments only 

 when the spectrum of the light that passes through them is accurately known. 

 Glass plates are employed as windows in dark boxes closed on all sides in which plants 

 are placed ; coloured fluids can also be employed for the same purpose by enclosing 

 them between parallel plates of glass and using these as a window. When it is not 

 necessary to allow light to fall in parallel rays upon the plant, the most convenient use 

 of coloured fluids is to fill with them the space between the two walls of a double 

 glass bell which is then placed like an ordinary bell-glass over the plants to be observed. 



For microscopic observations in coloured light I employ boxes like that represented 

 in Fig. 445 (p. 658); only that instead of the colourless plate of glass, a double window 

 is used, the space betw^een the two panes being filled with coloured fluids. 



(3) Coloured Flames — i.e. the light of bodies in a finely divided state heated to 

 incandescence in a flame which is itself non-luminous — have not hitherto been employed 

 for accurate observations on plants. I know only of one statement by Wolkoff"^ ; 

 that etiolated seedlings of Lepidium sati-vum became green when placed for seven or 

 eight hours at eight inches distance from a non-luminous gas flame in which sodium 

 carbonate had volatilised and become incandescent. This light, as is well known, 

 consists only of rays which correspond to Fraunhofer's line D. The red light of 

 the flame of lithium or the blue light of that of indium &c., may be employed in 

 the same manner as this yellow flame, if sufficient intensity and the necessary perma- 

 nence can be attained with these flames-. 



[The foregoing account would be incomplete without some statement of the results 

 attained on this subject by Mr. H. G. Sorby. The following is a brief abstract, sup- 

 plied by him, of investigations which will be found reported in detail in his published 

 papers-^: — 



Vegetable colouring-matters may be divided into two principal classes, fundamental 

 and accidental. The fundamental are those which are essential to the healthy growth of 

 the plant ; and by carefully studying the position of the absorption-bands in living leaves 

 these substances are often found in a free and solid state, even when they are soluble 

 in water, or could easily combine with the closely associated oils or wax. When set 

 free by boiling in water or by decomposition, they dissolve according to their pro- 

 perties in this respect in water, or combine with oil or wax if these be present. 

 The petals and other portions of the organs of reproduction often contain some of 

 the fundamental colouring-matters of the leaves, but frequently others are developed. 



Accidental colouring-matters are those which may be present or absent without 

 apparently interfering wMth the healthy growth of the individual plant, and are often so 

 conspicuous as to make mere colour of very little importance if it depend upon them, 

 and not on the diff'erence in the kind or relative proportion of the fundamental colouring- 

 matters. These non-essential substances are far more common in the petals than in 

 the leaves, and if of any use to the plant, are only indirectly advantageous, as, for in- 

 stance, in attracting insects. It is doubtful to which of these two divisions certain 



^ Wolkoff, Jahib. fur wiss. Bot. 1866, vol. V, p. 11. 



^ [The most recent researches on the spectrum-analysis of the green colouring matter of plants 

 is by Chautard in Ann. de Chim. et de Physique, Sept. 1874. — Ed,] 



^ [Proceedings of the Royal Society, vol. XV. 1867, p. 433. — Quarterly Jonraal of Microscopical 

 Science, vol, IX, 1869, p, 358; vol. XI, 1871, p. 215. — Monthly Microscopical Journal, vol III, 

 1870, p. 229; vol. VI, 1871, p, 124.— Proceedings of the Royal Society, vol. XXI, 1875, p .142.] 



