200^ FLORAL ENVELOPES — COKOLLA. 



the Orange, and the separation between the parts occurs at this 

 articulation. 



The receptacle bearing the calyx is sometimes united to the pistil, 

 and enlarges, so as to form a part of the fruit, as in the Apple, Pear, 

 Pomegranate, Gooseberry, etc. In these fruits the withered calyx is 

 seen at the apex. Sometimes a persistent calyx increases much after 

 flowering, and encloses the fruit, without being incorporated with it, 

 becoming accrescent (accresco, I increase), as in various species of 

 Physalis (fig. 304); at other times it remains in a withered or 

 marcescent (marcesco, I decay) form, as in 

 Erica ; sometimes it becomes inflated or vesi- 

 cular, as in sea campion. In Trifolium fra- 

 giferum the union of the inflated calyces 

 produces the strawberry-like appearance of 

 the head of flowers when in fruit. 



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 con- 

 spicuous whorl. The gay colours and fra- 

 grant odours of flowers are resident in it. It 

 is present in the greater number of Dicoty- 

 ledons. It is composed of parts which are 

 Fig. 304. usually disposed in one or more verticillate 



rows, and which are caRedi petals {-Treray^ov, a leaf). The petals some- 

 times form a continuous spiral with the calycine segments, but in 

 general they are disposed in a circle, and alternate with the sepals. 



Petals difier more from leaves than sepals do, and are much 

 more nearly allied to the staminal whorl. In some cases, how- 

 ever, they are transformed into leaves, like the calyx, and occasionally 

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

 occasionally this colour is met with, as in some Cobras, Hoya viridi- 

 flora, Gonolobus viridiflorus, and Pentatropis spiralis. As a rule they 

 are highly coloured, the colouring matter being contained in cells, and 

 differing in its nature from the chlorophyll of the leaves. As regards 

 their structure, petals consist of cellular tissue traversed by true 

 spiral vessels, and thin-waUed tubes. In delicate flowers, as Convol- 

 vulus 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 J inch 

 object glass, shows beautiful hexagons, the boundaries of which are 

 ornamented with several inflected loops in the sides of the cells. 



Fig. 304. Accrescent calyx, c, connected witli tlie fruit of Physalis Alkekengi. 



