192 INTERNATIONAL BOTANICAL CONGRESS. 
Von Martius placed some plants of Amaranthus tricolor for two months 
under glasses of various colours. Under the yellow glass, the varied 
tints of the leaves were all preserved. The red glass rather impeded 
the development of the leaves, and produced, at the base of the limb, 
yellow instead of green; in the middle of the upper surface, yellow 
instead of dido Been, and below, a red spot instead of purplish-red. 
With the blue glasses, which allowed some green and yellow to pass, 
that which was red or yellow in the leaf had spread, so that there only 
remained a green border or edge. Under the nearly pure violet glasses, 
the foliage became almost uniformly green. Thus, by means of coloured 
glasses, provided they are not yellow, horticulturists may hope to obtain 
at least temporary effects, as to the colouring of variegated foliage. 
The action of electricity on vegetation is so doubtful, so difficult to 
experiment upon, that I dare hardly mention it; but it can easily be 
understood how a building constructed as proposed might facilitate 
experiments on this subject. Respecting the action of plants on the 
surrounding air, and the influence of a certain composition of the 
atmosphere upon vegetation, there would be by these means a large 
field open for experiments. Nothing would be easier than to create 
in the experimental hothouse an atmosphere charged with noxious gas, 
and to ascertain the exact degree of its action by day and by night. 
An atmosphere of carbonic acid gas might also be created, such as is 
supposed to have existed in the coal period. Then it might be seen 
to what extent our present vegetation would take an excess of carbon 
from ihe air, and if its general existence were inconvenienced by it. 
Then it might be ascertained what tribes of plants could bear this con- 
dition, and other families could not have existed, supposing the air 
had formerly had a very strong proportion of carbonic acid gas. 
Until hortieulture can supply physiology with such convenient means 
of experiment, it, in the meantime, advances descriptive botany by the 
valuable publications it issues. The greater part of the old works with 
plates, such as * Hortus Eystettensis, * Hortus Elthamensis, etc.; 
also those of Ventenat, Cels, Redouté, etc.; the ‘Salictum’ and 
‘ Pinetum’ of the Duke of Bedford; and more recently the ‘ Rhodo- 
dendrons of the Himalaya,’ by Dr. Joker: the works of Bateman, 
Pescatore, Reichenbach fil, on Orchids ; Hun many others I could 
name, would never have existed, had there not been rich amateurs 
either to edit or buy them. 
