764 
GENERAL CONDITIONS OF PLANT-LIFE. 
can be contrived so as to split white daylight exactly into two halves, the first 
solution transmitting the less refrangible rays from the red to the green, the 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 placing 
them in glass vessels with parallel sides 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. 474; only that instead of the colourless plate of glass, a double window is used, 
the space between 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 satitmm 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. C. 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 boihng in water or by decomposition, they dissolve according to their properties 
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 with 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 difference in the kind or relative proportion of the fundamental colouring- 
^ Such double-walled bell-jars can ba obtained from Warmbrunn and Quilitz, Berlin. 
2 Wolkoff, Jahrb. für wiss. Bot. 1866, vol. V. p. ir. 
^ [Proceedings of the Royal Society, vol. XV. 1867, p. 433. — Quarterly Journal 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. XXL 1873, p. 442. 
Researches on the spectrum-analysis of the green colouring-malter of plants are given by Chautard 
in Ann, de Chim. ct de Physique, Sept. 1874.] 
