CHAr. II. 
COROLLA. 
169 
If the flower has no corolla, it is said to be apetalous. 
Sometimes a petal is lengthened at the base into a hollow 
tube, as in Orchis, &c. : this is called the spur or calcar, and 
by some nectarotheca. 
In Umbelliferse the petal is abruptly acuminate ; and the 
acumen is indexed. The latter is named the lacinula. 
A corolla is said to be regular when its segments form 
equal rays of a circle supposed to be described with the axis 
of the flower for a centre. If they are unequal, the corolla 
is called irregular. Eqzial and unequal are occasionally sub- 
stituted for regular and irregular. 
In anatomical structure, the petal should agree with a leaf, 
of which it is a mere modification ; and, in fiict, it does so in 
all that is important, its differences consisting chiefly in a 
diminished size, an attenuation and colouring of the tissue, 
with a suppression of the pleurenchyma. Like a leaf, petals 
consist of a flat plate of parenchyma, articulated with the 
stem, traversed by veins, and frequently having stomates upon 
its surface. Their veins consist almost entirely of delicate 
spiral vessels, upon which the parenchyma is immediately 
placed. It is therefore by mistake that De Candolle has 
stated [Organogr., p. 454.) that stomates and spiral vessels 
are usually absent. 
The petals are usually deciduous soon after flowering, or 
even at the instant of expansion ; a very rare instance of 
their persistence and change from minute colourless bodies 
into leafy, richly coloured expansions, occurs in Melanorrhma 
usitatissima. 
Their colours are due to the secretion within the bladders 
of their parenchyma of a peculiar substance: even white 
petals are so in consequence of the deposit of an opaque 
white substance, and not because of the absence of colouring 
matter. 
In most corollas the petals, in their natural state, form but 
one whorl within that of the calyx : but instances exist in 
which they naturally are found in several whorls, as in Nym- 
phaea, Nuphar, Magnolia, &c. It sometimes happens that, if 
there is more than one row of petals, all within the first row 
assume a different appearance from the first ; the filamentous 
