136 PHYLLOTAXY, OR LEAF-ARRANGEMENT. 



parts. 1 It is characteristic of it that some parts (one or more) 

 are wholly exterior or covering in the bud, and others (one at 

 least) interior or covered, at least the margins. Imbricative 

 aestivation, it will be seen, naturally attends alternate or spiral 

 phyllotaxy (248, and see Fig. 242, 243) ; and if it be main- 

 tained that these sets of three, five, &c., in the blossom are not 

 depressed spirals, but whorls or cycles (as may commonly be 

 the case in the corolla, but hardly in the calyx) , it is not less 

 true that the parts are apt to comport themselves in the exact 

 manner of a depressed spiral. The kinds of regular imbrication 

 of alternate leaves, &c., may be specified by the terms or frac- 

 tions expressive of the particular grade of phyllotaxy (, ^, , 

 |, &c.). But some of them have received special names, which 

 may be employed, as subordinate to the general denomination of 

 imbricate. The most important of these are the 



Equitant, where leaves override, the older successively astride 

 the next younger. The typical instance is that of ancipital or 

 two-ranked (^) conduplicate leaves, successively clasping, at least 

 next the base, as in Iris, Fig. 217. In what Linnaeus termed 

 equitant-triquetrous (well seen in Fig. 240, 241), the leaves are 

 three-ranked (being of the order), and each imperfectly 

 conduplicate. 



Quincuncial aestivation (as in the outer part of Fig. 260) is 

 simply the imbricate aestivation of five leaves (), in which 

 necessarily the first and second are external, the fourth and fifth 

 internal, and the third with one margin external, where it over- 

 lies the fifth, and the other internal, where it is covered by 

 the first. 



Alternative aestivation, as already stated (252), comes from 

 verticillate or cyclic phyllotaxy, and the alternation of successive 

 whorls. When two such whorls, say of three leaves each (as in 

 Fig. 258) , are so condensed or combined as to form apparently 

 one set or circle of six members (as in the flower-leaves of most 

 Liliaceae) , three members alternate with and are covered by the 

 other three, and this sort of imbricate aestivation is produced. 

 More properly, the two series are to be considered separately. 

 Where the parts are four (as in Fig. 395) , the normal imbrica- 

 tion is decussate, two exterior and two interior. This is some- 



1 All the examples referred to result from alternate or spiral phyllotaxy, 

 the former of higher series, the latter of the (Fig. 258, 259), and of the $ 

 (Fig. 260) order. Instead of separating (with DeCandolle and others) the 

 \ arrangement as different in kind from the imbricate (under the name of 

 quincuncial aestivation), we should count it as a typical case. Otherwise the 

 i arrangement might equally claim a generic distinction, also the f, &c. 



