ALTERNATION OF THE FLORAL ORGANS. 241 



each set would answer to a pair of leaves. So, likewise, the or- 

 gans of a trimerous flower (viz. one with its parts in threes, as in 

 Fig. 277) may be taken, each set as a cycle of alternate leaves of 

 the tristichous mode (171), with the axis depressed, which would 

 throw the parts into successive whorls of threes, or as a proper ver- 

 ticil of three leaves ; while those of a pentamerous or quinary flower 

 (with the parts in fives, as in Fig. 278) would answer to the cycles 

 of the f arrangement (239) of alternate leaves, or to proper five- 

 leaved verticils. So the whorls of a tetramerous flower are to be 

 compared with the case of decussating opposite leaves (250) com- 

 bined two by two, or directly with quaternary verticillate leaves ; 

 either of which would give sets of parts in fours. 



440. The Alternation of the Floral Organs, We learn from obser- 

 vation that the parts of the successive circles of the flower almost 

 universally alternate with each other. The five petals of the flower 

 represented in Fig. 256, for example, are not opposed to the five 

 sepals, that is situated directly above or before them, but alternate 

 with, or stand over the intervals between them ; the five stamens 

 in like manner alternate with the petals, and the five pistils with 

 the stamens, as is shown in the diagram, Fig. 257. The same is 

 the case in the trimerous flower, Fig. 277 ; and in fact this is the 

 regular rule, the few exceptions to which have to be separately 

 accounted for. 



441. This alternation comports with the more usual phyllotaxis 

 in opposite and verticillate leaves, where the successive pairs de- 

 cussate, or cross each other at right angles (250), or the leaves of 

 one verticil severally correspond to the intervals of that underneath, 

 making twice as many vertical ranks as there are parts in the 

 whorl (251). The alternation of the floral organs is therefore 

 most readily explained on the assumption that the several circles 

 are true decussating verticils ; when it only remains to discover 

 the real connection between the opposite-leaved or verticillate and 

 the spiral phyllotaxis, and to obtain some expression which will 

 harmonize the two modes ; both of which are often met with on 

 the same axis. But the inspection of a flower-bud with the 

 parts imbricated in estivation (492) shows that the several mem- 

 bers of the same set do not originate exactly in the same plane. 

 The five petals, for example, in the cross-section of the pentame- 

 rous blossom shown in Fig. 257 (and the same arrangement is still 

 more frequently seen in the calyx), are so situated, that two are 



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