282 THE FLOWER. 



cially in irregular flowers, where the overlapping of parts does not 

 altogether accord with what must needs be their order of succes- 

 sion on the axis. In the 5-merous calyx and corolla of all truly 

 papilionaceous flowers, for example, one edge of the sepal or the 

 petal No. 2 is placed under instead of over the adjacent edge of 

 No. 4, in consequence of which three, instead of only one, of the 

 leaves have one edge covered and the other external ; as is shown 

 in Fig. 282. Since, in the corolla of this kind of blossom, the ex- 

 terior petal, here the vexillum (468), is the larger, and at first em- 

 braces all the rest (as is seen in the separate diagram of the 

 corolla, Fig. 359), this modification of imbricative sestivation has 

 received the name of vexillary. As nearly the same thing occurs 

 in the Violet, it is probably caused by some slight dislocation that 

 takes place during the early growth of organs in the irregular 

 blossom, which the study of their development should explain. It 

 is not restricted to irregular flowers, however, but occurs as a cas- 

 ual variation, or perhaps more frequently than the quincuncial, in 

 the regular corolla of the Linden (as is shown in Fig. 306). A 

 slight obliquity in the position of the petal No. 2, assumed at an 

 early period, would account for the whole anomaly. That this 

 suggests the true explanation is almost demonstrated by the vary- 

 ing aestivation of the corolla of the Linden ; in which the same 

 bunch of blossoms often furnishes instances of regular quincuncial 

 imbrication, of the modification here referred to, and of the similar 

 disposition of the fifth petal, throwing one of its edges outwards 

 also. If the first petal were also to partake of this slight obliquity, 

 the imbricative would be completely converted into what is vari- 

 ously named 



498. The contorted, twisted, or convolutive sestivation (Fig. 360, 

 the corolla, and 361). In this mode, the leaves of the circle are all, 

 at least apparently, inserted at the same height, and all occupy the 

 same relative position : one edge of each, being directed obliquely 

 inwards, is covered by the adjacent leaf on that side, while the 

 other covers the corresponding margin of the contiguous leaf on 

 the other side. This is owing to a more or less evident torsion 

 or twisting of each member on its axis early in its develop- 

 ment ; so that the leaves of the floral verticil, instead of forming 

 arcs of a circle, or sides of a polygon having for its centre that of 

 the blossom, severally assume an oblique direction, by which one 

 edge is carried partly inward and the other outward. This con- 



