1 76 FLORAL ENVELOPES. COROLLA. 



at other times it remains in a withered or marcescent (marcesco I 

 decay) form ; sometimes it becomes inflated or vesicular. In Trifolium 

 fragiferum, the union of the inflated calyces causes the strawberry-like 

 appearance of the head of flowers when in fruit. 



368. Corolla. The corolla is the more or less coloured inner floral 

 envelope, forming the whorl of leaves between the calyx and the 

 stamens. It is generally the most conspicuous whorl. The gay colours 

 and fragrant odours of flowers are resident in it. It is present in the 

 greater number of Exogens. It is composed of parts which are usually 

 disposed in one or more verticillate rows, and which are called petals 

 (viTathoit, a leaf). The petals sometimes form a continuous spiral with 

 the calycine segments, but in general they are disposed in a circle, and 

 alternate with the sepals. 



369. Petals differ more from leaves than sepals do, and are much 

 more nearly allied to the next whorl or the stamens. In some cases, 

 however, they are transformed into leaves like the calyx, and occa- 

 sionally leaf-buds are developed in their axil. They are seldom green, 

 although occasionally this colour is met with, as in some Coba?as, 

 Hoya viridiflora, Gonolobus viridiflorus, and Pentatropis spiralis. 

 They are generally white, red, blue, or yellow, or exhibit some colour 

 produced by an intermixture of these. The colouring matter is con- 

 tained in cells, and differs in its nature from the chlorophylle of the 

 leaves. As regards their structure, petals consist of cellular tissue 

 traversed by true spiral vessels, and thin-walled tubes. In delicate 

 flowers, as Convolvulus and Anagallis, these vessels are easily seen 

 under the microscope. Petals do not usually present numerous layers 

 of cells like the leaves, neither is the epidermis always distinct although 

 in some instances it may be detached, especially from the surface next 

 the calyx. The cuticle of the petal of a Pelargonium, when viewed 

 with a \ or inch object glass, shows beautiful hexagons, the boun- 

 daries of which are ornamented with several inflected loops in the 

 sides of the cells. 



370. On the outer surface of petals, corresponding to the lower side 

 of leaves, stomata are sometimes found. Petals are generally glabrous 

 or smooth ; but, in some instances, hairs are produced on then: sur- 

 face. Petaline hairs, though sparse and scattered, present occasionally 

 the same arrangement as those which occur on the leaves : thus, in 

 Bombaceaj they are stellate. Coloured hairs are seen on the petals 

 of Menyanthes, and on the segments of the perianth of the Iris. 

 Although petals are usually very thin and delicate in their texture, 

 they occasionally become thick and fleshy, as in Stapelia and Eaf- 

 flesia ; or dry, as in Heaths ; or hard and stiff, as in Xylopia. A petal 

 often consists of two portions the lower narrow, resembling the 

 petiole of a leaf, and called the unguis or clatv ; the upper broader, like 

 the blade of a leaf, and called the lamina or limb. These parts are 



