GROUP IV. — PHANEROGAMIA : ANGIOSPERMiE. 497 



four or six leaves arranged in two whorls, consisting respectively 

 of two or three leaves. 



For purposes of comparative description, it is convenient to 

 regard each turn of the spiral in an acyclic or a hemicyclic flower 

 as equivalent to a whorl : thus a well-defined series with f arrange- 

 ment would represent two whorls. 



As in the case of the foliage-leaves (see p. 45), so in that of the 

 floral leaves, the order of development is as a rule acropetal : 

 hence each whorl of the flower is developed later than the one 

 external to it, and earlier than the one internal to it. When, 

 however, a series of floral organs is becoming degenerate, its de- 

 velopment is retarded ; for instance, in the CompositaB, Valeri- 

 anaceop, and Umbelliferae, the degenerate calyx is developed after 

 the corolla, or even after the androecium. The members of each 

 whorl may be developed either simultaneously or successively. 



In their arrangement, also, the floral leaves resemble the foliage- 

 leaves. When, in an acyclic or hemicyclic flower, the spiral is 

 continuous wnth the same divergence from one series of floral 

 organs to another, the members of the successive series lie on 

 the same radii drawn from the centre of the flower, that is, they 

 are directly superposed. A good example of this is afforded by 

 the terminal flower of the inflorescence of Berberis (Fig. 311 ; 

 occasionally in Epimedium, and also in Gagea among Monocotyle- 

 dons), where the stamens, petals, and sepals are all directly super- 

 posed : similai'ly in Ternstra3mia and Clusia, where the sepals and 

 petals are in a continuous f spiral, the petals are superposed on 

 the sepals. When, on the other hand, the divergence varies from 

 one series to another, direct superposition does not occur, but some 

 form of alternation, as is generally the case in acyclic flowers : for 

 instance, the calyx of certain (pentamerous) forms of Anemone 

 and other RanunculaceoB is arranged with a | divergence, whereas 

 the divergence of the stamens is -^\ or /y. In hemicyclic flowers 

 with a simple spiral perianth and cyclic stamens (e.g. Canna- 

 binaceoe, Chenopodiaceae, Amarantaceee), the stamens are super- 

 posed on the perianth-leaves. 



When the floral leaves arc in whorls consisting of equal numbers 

 of members, the general rule is that the members of the successive 

 whorls alternate with each other: thus, in a flower with calyx, 

 corolla, androecium, and gyneeceum, each consisting of a single 

 whorl of five members, the petals alternate with the sepals, the 

 stamens with the petals, and the carpels with the stamens ; and 



