336 MORPHOLOGY OF PETALS. [BOOK i. 



of a capitulum, it is called corollula : that of a floret is so 

 named. 



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 Umbelliferous plants 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 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 fact, 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 pleurenchym. Like a leaf, petals 

 consist of a flat plate of parenchym, 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 parenchym is immediately placed. 

 It is therefore by mistake that De Candolle has stated 

 (Organogr., p. 454.) that stomates and spiral vessels are 

 usually absent. In some plants the petals remain after the 

 flowering is over, and then grow into true leaves. 



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 Melanorrhsea 

 usitatissima. 



Their colours are due to the secretion within the cells of 

 their parenchym 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 



