OF THE GENUS MILLETTIA. 191 
varies in length from twice the calyx down to very small dimensions in the 
subsessile flowers of some Albifloræ and allied sections. 
The clothing of the petals is usually constant through sections. Many 
sections have petals with beautifully coloured, adpressed, silky pubescence, 
while in others, such as the 7runcaticalyces, they are quite glabrous. But of 
all the parts of the flower it is the base of the standard which gives the best 
superficial indication of the sectional grouping of the genus. The special 
modifications of this organ are probably concerned in the opening and closing 
of the nectar-producing region round the base of the ovary when the flower 
is visited and pollinated by insects. In all the Millettias this cavity is 
enclosed by the base of the staminal sheath. It is, however, always accessible 
on the upper side in consequence of the upper or vexillary stamen being 
quite free or, more usually, free at the base only from the others, which, 
moreover, gape open a little on both sides of it. Where this stamen does not 
itself close the aperture while the flower is undisturbed, the bases of the wing 
and keel petals perform the same protective function, and it is, I conjecture, 
for the proper opening and closing of the nectary space, and for the leading 
of the visiting insect into the best position for pollination during its search. 
for honey, that the numerous remarkable modifications of the staminal sheath. 
and the petal bases occur. 
It is, of course, as in other Papilionacez, necessary for cross-fertilization 
that an insect alighting on the flower for the purpose of obtaining the nectar 
should press down by its weight the elastic wing and keel petals, until its 
body comes into contact with the comparatively stiff stamens and the pistil 
which lies concealed among them. When the nectary space is closed by the 
upper stamen, the latter is sometimes grasped by two callous projections at 
the base of the standard or enfolded by the whole length of its claw, so that 
the least depression of the staminal sheath by the weight, for instance, of an 
insect resting on it, draws it away from the upper stamen so held and opens 
a way to the nectar. In cases where the openings are normally closed by 
the wings and keel or by projections from their bases (as when they are 
hooked), there are various devices for holding these petals together in such a 
way that an insect visitor will open them. Sometimes their upper edges are 
held together in a noteh of the standard so shallow that the least pressure 
disengages them from it and causes them to slide apart over the smooth 
staminal cylinder until the nectarian openings are disclosed. This notch is 
usually formed by callous thickenings of the standard base or by the infolding 
of its margin into stiff auricles. Where both wings and keel cover the 
openings and it is necessary that they should both be pressed down together, 
their coherence is secured by a fold of the keel petals fitting into a pocket in 
the wings usually halfway down their length and near their upper edge. 
Another means of holding the interior petals together is secured by the 
curious device, which obtains throughout the Jfulgentes, in which delicate 
