ALTERNATION OF THE FLOKAL ORGANS. 235 



posite leaves, combined two by two, or with quaternaiy verticillate 

 leaves (251) ; either of which would give sets of parts in fours. 



440. The Alternation of the Floral Organs. ^Ve learn from obser- 

 vation that, as a general rule, the parts of the successive circles of 

 the flower alternate with each other. The five 



petals of the flower represented in Fig. 334, for ex- 

 ample, are not opposed to the five sepals (that is, 

 situated directly above or before them), but alter- 

 nate with them, that is, or stand over the intervals 

 between them ; the five stamens in like manner al- 

 ternate with the petals, and the five pistils with the 

 stamens, as is shown in the diagram. Fig. 335. The same is the 

 case in Fig. 353, the several organs of a flower with its parts in 

 threes ; and in fact this is the rule, the few exceptions to which 

 have to be separately accounted for. 



441. This comports with the more usual phyllotaxis in opposite 

 and verticillate leaves, where the successive pairs decussate, or cross 

 each other at right angles (251), or the leaves of one verticil several- 

 ly correspond to the intervals of that underneath, making twice as 

 many vertical ranks as there are parts in the whorl. The alternation 

 of the floral organs is therefore most readily explained on the assump- 

 tion that the several circles are true decussating verticils. But the 

 inspection of a flower-bud with the parts imbricated in aestivation 

 (494) shows that the several members of the same set do not origi- 

 nate exactly in the same plane. The five petals, for example, in 

 the cross-section of the pentamerous blossom shown in Fig. 335 (and 

 the same arrangement is still more frequently seen in the calyx), 

 are so situated, that two are exterior in the bud, and therefore in- 

 serted loAver on the axis than the rest, the third is intermediate, and 

 two others are entirely interior, or inserted higher than the rest. In 

 fact, they exactly correspond with a cycle of alternate leaves of the 

 quincuncial or five-ranked arrangement, on an extremely abbreviated 

 axis, or on a horizontal plane, as is at once seen by comparing the 

 ground plan. Fig. 335, with Fig. 206. Compare also Fig. 355 with 

 Fig. 203. Also, when the parts are in fours, two are almost always 

 exterior in the bud, and two interior. Moreover, whenever the 

 floral envelopes, or the stamens or pistils, are more numei'ous, so as 

 to occupy several rows, the spiral disposition is the more manifest. 

 It is most natural, accordingly, to assume that the calyx, corolla, 



FIG. 355. Cross-section of the flower-bud of Fig. 353, to sho^y the alternatioa of parts. 



