828 DEVELOPMENT OF THE FLORAL ENVELOPES. 
nalis, those of the Victoria regia, of several Cacti, and of many 
Aroidacez, present us with the most marked illustrations of this 
evolution of heat. (See also Development, of Heat by Plants, 
page 855.) 
That the heat thus evolved is dependent upon the combina- 
tion of the oxygen of the air with the carbon of the flower was 
conclusively proved by the experiments of Vrolik and De Vries ; 
for they showed that the evolution of heat by the spadix of an 
Arum was much greater when it was placed in oxygen gas than 
in ordinary atmospheric air, and that when introduced into 
carbon dioxide or nitrogen gases it ceased altogether. 
Colour of Flowers.—All the colours of flowers otherwise than 
green depend on colouring matter dissolved in the watery cell- 
sap, and chromo-plastids, the nature of which was very imper- 
fectly known until the recent observations of Schimper, Meyer, 
and Schmidt (see page 809), though spectroscopic analysis had 
done something towards grouping them into series. The changes 
in colour which many corollas undergo are supposed to depend 
on the oxidation of these bodies. Most of the Boraginacez pass 
from pink to blue, from their first expansion, till they are fully 
open; the garden Convolvulus changes from pink to a fine 
purple in the same period. Cultivation will effect great changes 
in this respect, but there is a limit to its influence. The Dahha 
and Tulip are naturally yellow, and under cultivation may be 
made to assume all shades of red, orange, and white, but no 
tint of blue; Pelargoniums and the Hydrangea will take on 
various shades of blue, purple, red, and white, but never a 
yellow. These facts led De Candolle to divide flowers in this 
aspect into two series—a xanthic which has yellow for its base, 
and a cyanic which has blue—either of which can be made red 
or white, but will not assume the basic colour of the other. 
There seem to be a few exceptions to this rule; e.g. Myosotis 
versicolor changes from yellow in the bud to blue in the open 
corolla, and the Hyacinth is not unfrequently a pale yellow. 
Development of the Floral Envelopes.—The manner in which 
the floral envelopes are developed may be shortly summed up as 
follows :— 
They are subject to the same laws of development as the usual 
foliage leaves, and make their first appearance as little cellular 
processes, which grow by additions to their bases or points of 
attachment to the axis. 
The calyx is commonly developed before the corolla. 
When a calyx is polysepalous, or a corolla polypetalous, the 
component sepals or petals make their first appearance in the 
form of little distinct papillze or tumours, the number of which 
corresponds to the separate parts of the future calyx or corolla. 
When a calyx is gamosepalous, or a corolla gamopetalous, the 
first appearance of these organs is in the form of a little ring, 
which ultimately becomes the tube of the calyx or corolla, as 
thé case may be. When these present lobes or teeth, as they 
