142 
ORGANOGRAPHY. 
BOOK I. 
When the corolla is very small, or when it forms a part of 
an anthodium, it is called corollula : that of a floret is so called. 
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 nectar otlieca. 
In Umbelliferse the petal is abruptly acuminate ; and the 
acumen is inflexed. 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. Equal and unequal are occasionally 
substituted for regular and irregular. 
In anatomical structure, the petal should agree with a leaf, 
of which it is a mere modification ; and, in fact, it does so 
in all that is important, its differences consisting chiefly in 
an attenuation and colouring of the tissue, with a suppression of 
woody fibre. Like a leaf, petals consist of a flat plate of paren- 
chyma, articulated with the stem, traversed by veins, and fre- 
quently 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 mis- 
take that De Candolle has stated (Organogr.^ p. 454.) that 
stomates and spiral vessels are usually absent. The latter may 
be very readily seen in the corolla of Anagallis, where they 
form a beautiful microscopical object, as I first learned from 
Mr. Solly. 
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 Melanorrhaea 
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 whoi ls, as in Nymph- 
