OBGAKS OF BKPKODUCTION. 211 



contact by their margins throughout their whole length, with- 

 out any overlapping. This form of aestivation may be generally 

 distinguished, even when the flowers are expanded, by the mar- 

 gins of its component parts being slightly thickened, or at all 

 events not thinner than the rest of the organ ; whereas in all 

 forms of imbricate or spiral aestivation, the overlapping margins 

 are usually thinner, as may be well seen in the sepals of the 

 Geranium. When the component sepals, or petals, instead of 

 being flattened, are folded inwards at the points where they 

 come in contact (^_^. 429), the aestivation is induplicate, sls in 



Fiff. 428. Fiff. 429. Fiff. 430. 



( ) 



Fig. 428. Diagram to illustrate valvate asstivation. Fig. 429. Diagram 



to illustrate iuduplicate estivation. Fig. 430. Diagram to illustrate 



reduplicate aestivation. 



the petals of Guaziima ulmifolia, and in the calyx of some 

 species of Clematk. When the margins are turned outwards 

 under the same circumstances (Jig. 430), the aestivation is re- 

 duplicate, as in the calyx of the Hollyhock {Althcea rosea), and 

 some other Malvaceous Plants, and in the corolla of the Potato. 



When the parts of a whorl are at the same height, or ap- 

 parently so, as in the ordinary forms of circular aestivation, and 

 one margin of each part is directed obliquely inwards, and is over- 

 lapped by the part adjacent on that side, while the other margin 

 covers the corresponding margin of the adjoining part on the 

 other side, so that the whole presents a more or less twisted 

 appearance (Jig. 431), the aestivation is contorted or twisted. 

 This form may be considered as intermediate between the 

 Circular and Imbricated forms of aestivation. It occurs very 

 frequently in the corolla, but is very rare in the calyx. Ex- 

 amples may be seen in the corolla of the HoUyhock {Althcsa 

 rosea) and other Malvaceous Plants; in that of the common 

 Flax (Linum), and generally in the order Linaceae; in the St. 

 John's Wort {Hypericum); in the Periwinkle (Vinca), and in 

 many other plants of the same order to which this plant 

 belongs. 



2. Imbricated, Imhricative, or Spiral Mstivation. — We shall 

 describe five forms of this kind of eestivation, i.e., the imbricate, 

 convolute or enveloping, q^uincuncial, cochlear, and vexillary. The 

 p2 



