106 MORPHOLOGY, OR COMPARATIVE ANATOMY. 



Open. This is also called " straight." The parts of the whorl, 

 usually the calyx, are so rudimentary or arrested in growth, that 

 they do not even meet, as in the Umbelliferoe, Rubiaccce, &c. Hence 

 this aestivation may be said to be indeterminate. Gr. H.] 



The calyx and corolla may both have the same aestivation, or they may 

 be different ; and their characters may hold good for all the species of a 

 genus, as in Hypencwn. in which the calyx is quincuncial and the corolla 

 contorted, or even for all the genera of an order, as of Malvaceae, in which 

 the calyx is valvate and the corolla convolute or contorted ; but it is 

 very common for a species to have several varieties in different individual 

 flowers, even on the same plant. 



The direction o the spiral in imbricated aestivations is variable, 

 often in the same plant : occasionally the direction changes in 

 passing from the calyx to the corolla ; at other times it remains the 

 same ; and this character is sometimes constant, in other cases very 

 inconstant. In determining the direction of spirals, right-hand or 

 left-hand, it is usual to suppose one's self standing in the axis of 

 the organ ; but many authors suppose themselves standing in front 

 of it for instance, in the place of the bract of a flower, which gives 

 the exact opposite of the former ; hence great confusion in the ap- 

 plication of the terms dextrorse and sinistrorse. 



Calyx. The calyx is the outermost circle of the floral envelopes. 

 It is composed of phylJomes or modifications of leaves, called sepals; 

 according as the sepals are distinct or coherent, the calyx is termed 

 polysepalous (or dicdysepalous), or monosepalous (or gamosepalous). 



The exceptions to the absolutely external position of the calyx have 

 been pointed out. 



The Sepals generally bear more or less resemblance to bracts, 

 being attached by a broad base, seldom articulated, without any 

 stalk, and of a green foliaceous texture ; not tmfrequently, how- 

 ever, their texture is of the coloured and delicate nature described 

 as petaloid. They are usually entire, but the margins are some- 

 times cut, as in the Rose (fig. 158), and they are occasionally re- 

 duced to scale-like, or even feathery or hair-like processes. They 

 are likewise subject to the production of pouches, spurs, &c., es- 

 pecially at the lower part, both when distinct and when coherent ; 

 and the apex is often more or less prolonged into a point or spine. 

 Their mode of venation is usually like that of the sheath of the leaf. 



Some confusion is liable to arise in the condition called a superior calyx, 

 where the segments are totally free : if we suppose an adherent tube to 

 exist below, such a calyx would be monosepalous ; but the so-called 

 calyx-tube is usually a cup-like receptacle, and the sepals originate or 





