138 PHYLLOTAXY, OR LEAF-ARRANGEMENT. 



form, we have only to reverse a single overlapping on the left- 

 hand side of the figure. To restore Fig. 262 to the convolute, 

 we have only to reverse a single overlapping at the lower right- 

 hand side. Changes like these, or the reverse, are not rare in 

 several species, particularly in the corolla. The normal form 

 and the deviation often occur in different flowers on the same 

 individual, thus indicating an easy passage between the imbricate 

 aestivation in the proper sense and the 



258. Convolute, otherwise called Obvolute or Contorted, or 

 Twisted, Fig. 263, and inner circle of Fig. 260. Here each leaf 

 successively overlaps a preceding and is overlapped by a following 

 one, all having a slight and equal obliquity of position, so that 

 all alike have an exterior and an interior (or a covering and a 

 covered) margin, and all appear to be as it were rolled up to- 

 gether. This is strikingly so when the parts are 

 broad and much overlapped, as in Fig. 264. Brown 

 included this among the forms of imbricate aestiva- 

 tion, and so does Eichler, particularly distinguishing 

 it, however, under the name here preferred. The 

 occasional transitions would justify such classifica- 

 tion. But in most cases it is so uniform, and in the corolla so 

 completely characteristic of whole families (such as Malvaceae. 

 Onagraceae, Apocynaceae, Gentianaceae, Polemoniaceae, &c.), and 

 is so distinct in its nature, that it may well take rank among 

 the primary kinds of aestivation. As to its nature, it is evident 

 that while the imbricate mode (at least the ternary, quinary. 

 &c.) indicates or imitates spiral phj-llotaxy (some members be- 

 ing within or with higher insertion than others) , the convolute 

 and the valvate (having all the members of the series on the 

 same plane) answer to verticillate phyllotaxy, or to whorls 

 instead of depressed spirals. 1 The name which this mode of 



Henslow, in Trans. Linn. Soc. ser. 2, i. 178, proposes to call the former half- 

 imbricate; the latter (following the faulty French example) is his imbricate 

 proper. 



The subimbricate mode has two varieties, distinguished by Eichler (in 

 Bliithendiagramme) as ascensive, when the lower or anterior (f. e. the pieces 

 next the subtending bract or leaf) are successively exterior (as in Fig. 261), 

 and descensive, when the covering is from the upper side, i. e. from the side 

 next the axis. 



1 Still, as those members of a quincunx which normally should be wholly 

 external do sometimes become internal during their development in the bud, 

 similar changes may be conceived to change a quincuncial into a convolute 

 disposition ; but, to effect this, three out of the five overlappings would have 

 to be reversed. 



FIG. 264. Convolute (also called contorted) aestivation of a corolla. 



