ACTION OF LIGHT ON VEGETATION. 
763 
their cells cannot divide without the assistance of light;' while the exact converse is 
the fact, — they do not divide because they do not grow. This error prevails throughout 
the whole treatise, which in other respects contains a number of instructive observa- 
tions. Moreover, Prantl's measurements show that even in small etiolated leaves 
{Phaseolus) numerous cell-divisions take placed 
It must be observed in addition that the very small growth of leaves in the dark 
is not a universal phenomenon even amongst Dicotyledons. The leaves produced 
from the tuberous roots of the Dahlia and Beet grown in the dark, and even those 
of Phaseotus, attain very considerable dimensions, and sometimes, especially when the 
temperature is high, almost the size of those developed in the light'^. 
Contri'vances for obserinng plants in light of different colours (or of different refrangi- 
bility). In order to allow light of different degrees of refrangibility to act upon plants, 
three methods may be adopted:— (i) The use of the spectrum; (2) The removal of 
particular rays by absorbent media (glass or fluids) ; and (3) Coloured flames. 
(1) If a ray of light is decomposed by passing it through a prism, it is possible 
to expose small plants or parts of plants to the action of narrow zones of the spec- 
trum ; and hence to allow light of approximately equal refrangibility to act upon 
them. Draper, Gardner^, Guillemin, and Pfeffer have worked in this manner. In 
using the spectrum it must however be observed that the intensity of the light in 
its different parts is less than that of the light that passes through the slit in pro- 
portion to the breadth of each part. If the spectrum at the distance from the 
prism where the observation is made is, for instance, 200 mm. long, but the slit only 
I mm. broad, the mean intensity of light of the whole spectrum is only ^200 of that 
which passes through the slit, even if no light is otherwise lost, which is seldom the 
case. Only a small luminous intensity must therefore be expected in the spectrum. 
In order to obviate this difficulty, it is necessary that very intense light pass through 
the slit, which may be effected by the use of condensing lenses. If, as is usually the 
case, sunlight is employed, the ray to be decomposed must be kept in a fixed position 
by a heliostat, or at least by a moveable mirror. 
(2) Absorbent media. The defects which have been mentioned in observations 
with the spectrum, as well as the considerable cost of a heliostat, are avoided when 
coloured light is obtained by means of absorbent media. For this purpose discs of 
coloured glass or strata of fluids enclosed between colourless glass plates may be 
used. These last possess the advantage that almost any required amount of space 
may be illuminated by the light in question, and that the transmitted light only 
loses so much in intensity as is due to the small amount of absorption of the 
transmitted rays by the coloured medium. It is a mistake, though a very common 
one, to think that observations made with coloured screens are less exact than those 
made with the spectrum ; in general it is just the reverse ; and which method should 
have the preference must be decided in each particular case. 
The use of absorbent media is always subject to the disadvantage that they do 
not generally transmit light of a single colour, but several different kinds of rays. 
This disadvantage is especially the case with coloured glass plates; and, with the 
exception of the deep red ruby and the very dark blue cobalt glass, there are scarcely 
any kinds which answer our purpose. It is more practicable to obtain coloured fluids 
of the desired quality, although here also the number that can be used is small. 
The two which have been already mentioned are particularly useful, wz. a saturated 
solution of potassium bichromate, and a dark solution of ammoniacal copper oxide ; by 
means of these, with the right concentration and thickness of the stratum, experiments 
1 Arb. des bot. Instit. Würzburg, 1873, Heft III. p. 384. 
^ See infra, Sect. 20. 
^ Gardner, Frorisp'o Nolizen, 1844, vol. 30. No. 11. — Guillemin, Ann. des Sei. Nat. 18^7, vol. 
VII. p. 160. 
