14-2 ORGANOGRAPHY. BOOK I. 



WTien xhe corolla is very small, or when it forms a part of 

 an antliodimn, 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 calcai\ and 

 by some nectar otiieca. 



In Umbelliferae 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 dilfei-ences 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 colovn*ed expansions, occurs in Melanorrhaea 

 usitatissima. 



Their colours are due to the secretion wdthin 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 natiu'al state, form but 

 one whorl within that of the calyx : but instances exist in 

 which they naturally are found in several whorls, as in Nymph- 



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