184 THE REV. б. HENSLOW ON THE ORIGIN OF FLORAL Ж8ТІУАТІОХ8. 
diagram of Snap-dragon, p. 583, the 1st sepal is no longer the posterior one but the left 
anterior; the spiral is left-handed; and the posterior вера! is the 5th, and completely 
within the others. The corollas of all three genera have the same estivation, and are 
constructed on this plan, which I call half-imbricate, having, at least most commonly, 
the 5th petal as the anterior one. 
It is rare to find any other part than the 5th in the anterior position of the zygomor- 
phic whorl, when its estivation is half-imbricate. There is, however, one case figured by 
Le Maout and Decaisne, viz. the diagram of Streptocarpus (р. 598), in which the 4th petal 
is the anterior one. It is also thus in Azalea and Rhododendron, in which the 5th petal 
(always the single petal, which is entirely within the others) is the posterior one. Lastly, 
in Pelargonium alone all these arrangements may be found. 
The above shows that no necessity can be seen for No. 1 of a whorl being always the 
same in position and therefore always homologous. 
I have already remarked that this arrangement furnishes conditions varying from 
perfect regularity in the whorls to extremely irregular ones. A few references to the 
pages of Le Maout and Decaisne will illustrate this. 
1. Corollas regular or very slightly irregular :— 
Pittosporum, р. 248; Rue, р. 317; Parnassia, p. 408; Elder, 481. 
2. Corollas partially or decidedly irregular :— 
Moringa, р. 235; Dictamnus, р. 316; Horsechestnut, р. 357; Cassia, р. 367; Rho- 
dodendron, p. 516; Henbane, p. 578; Vitex, p. 615. 
3. Very irregular corollas :— 
Pansy, p. 241; Lathyrus, р. 369; Snapdragon, р. 87; Lamium, p. 620. 
Irregular and zygomorphic flowers are sometimes produced without any departure 
from the quincunx ; for it should be observed that both the quincuncial and the half- | 
imbricate sestivations can be divided symmetrically into two similar halves (as repre- 
sented in Tab. XXV. figs. 4and 5) by median lines. Thus is it with Polygala (Tab. XXV. 
fig. 11), which somewhat resembles the papilionaceous; but the estivation is simply 
quincuncial and without any modification. Similarly amongst gamopetalous corollas 
the diagrams given of Alloplectus, p. 597, as well as that of Achimenes, p. 596, and 
Adhatoda, p. 606, though possessing irregular bilateral flowers, have yet their petals 
arranged on the quincuncial plan. 
Irregular flowers with a quineuncial æstivation would seem to be comparatively rare ; 
and when it does occur the various parts are not always homologous. Thus in Ramondia, 
p. 600, Alloplectus, p. 597, and Achimenes, p. 596, it is in the 4th part, while in Stilbe, 
p- 613, it is in the 3rd part, and in Муоротит, р. 610, it is in the 1st part, which 
respectively occupy the anterior position. 
5. Imbricate proper.—In this sestivation the parts of the whorl may be three in 
number, as in the tristichous; four, as in Holly, p. 339, and .Euonymus, р. 842; or more 
commonly five or more. The parts overlap one another in succession, so that the first 
is wholly exterior, and the ‘last’ (which is really the third of the original quincunx 
from which it is derived) is entirely within the spiral. Le Maout and Decaisne number 
the parts in succession, fig. 463, р. 86; but this is not, nor can it possibly be correct, at 
least in accordance with phyllotactical principles; for if it were, we should have five parts 
