AESTIVATION OR PRAKFLOKATION. 271 



imbricated. We have here the advantage of being able to number 

 the successive sepals, or petals, since the third leaf is not only recog- 

 nizable by its intermediate position, but also indicates the direction 

 in which the spiral turns (Fig. 20G and Fig. 439). 



495. It must be recollected, in the comparison, that the parts of 

 successive cycles are superposed in the foliage, while those of the 

 floral circles alternate. Regular imbrication in the 4-merous flower 

 gives two outer and two inner members in aestivation (as in the 

 calyx of Cruciferous blossoms, Fig. 3G7), on the principle of two 

 decussating pairs of leaves (441) ; or it may sometimes be refer- 

 able to a modification of some alternate spiral arrangement. 



496. The degree of overlapping depends upon the breadth of the 

 parts and the slate of the bud ; it naturally grows less and loss as 

 the bud expands and is ready to open. It, is from the full-grown 

 flower-bud, just before anthesis (or the opening of the blossom), that 

 our diagrams are usually taken ; in which the parts are represented 

 as moderately or slightly overlapping. The eame overlapping car- 

 ried to a greater extent will cause the outer leaf to envelope all the 

 rest, and each succeeding one to envelop those within ; as shown in 

 Fig. 438 from one circle of petals of a Magnolia taken in an early 

 state of the hud. To this, however, has not improperly been applied 

 the name of convolute, from its similarity to the convolute vernation 

 of the leaves of the branch (257), similarly rolled up one within the 

 other. But it is practically inconvenient, and wrong in principle, to 

 designate different degrees of the very same mode by distinct names ; 

 fjrthcrmore, it is to the next general mode of aestivation that the 

 name of convolute is more commonly applied, at least in recent sys- 

 tematic botanical writings. 



497. There are numerous cases cf imbricative aestivation, espe- 

 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, foi* 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. 358. Since, in the corolla of this kind of blossom, the ex- 

 terior petal, here the vexillum (472), is the larger, and at first em- 

 braces all the rest, this modification of imbricative; aestivation has 

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



