Cassiinae pt 1 NY-Botanical_gardens_Vol. 35_1 - Copy.pdf - Antbase

1982] CASSIINAE—SENNA 73
apical sht looking upward and outward. Later the two slits fuse into one round
or U-shaped pore, divided or not by an exiguous septum. The shts of the four
median stamens, whether separate or confluent, are shifted by imperceptibly
small steps from terminal to a position at first obhquely terminal and finally lateral
infraterminal. Perfectly or partly coalescent beaks, especially in the long abaxial
anthers, develop by more or less abrupt strangulation of the thecae, and these
beaks become variously modified into a truncate tubule, an oblique pollen-cup,
or develop a tonguelike appendage produced beyond the true orifice. Anthers
evenly tapering to a bluntly rounded or abruptly truncate, biporose apex represent
the simplest type in Senna and are presumed to represent that from which the
rest arose.
Pistil: The pistil of Senna, whether aligned along the vertical axis of floral sym­
metry or turned alternately left and right in opposition to a modified abaxial petal,
varies httle in form except for length of style and size and orientation of the
stigma. The number of ovules, on the other hand, varies enormously, with con­
sequences to the pod discussed under the next heading. Curiously there is only
partial correlation between ovule number and capacity of the stigmatic orifice.
The massive hollow stigma of S. (Bacillares) quinquangulata, accommodates
enough pollen grains to fertilize more than 200 ovules, but the stigmatic orifice
of 5. (Coluteoideae) pendula, which may have as many as 150 ovules, remains
minute and punctiform hke that of most pauciovulate species. We suppose that
a simple linear style of moderate length, with punctiform stigma looking upward,
preserves the archetypical form. A distally incurved style, dilated or not below
the stigma, promotes self-fertUization of the flower, whereas the whole architec­
ture of the longistylous flower, especially that of sect. Peiranisia when the long
anthers are shielded by one abaxial petal from contact with the laterally displaced
pistU, seems designed for out-crossing. A trend away from entomophily to autog­
amy which is expressed by simultaneous inward curvature, shortening and dila­
tion of the style has reached a climax in the advanced monocarpic S. uniflora.
In this species the enlarged stigma is poised in the bud to receive precociously
released pollen from anthers directly facing it; and most flowers are, in fact, self-
fecundated before the small ephemeral coroUa expands. Changes in spatial rela­
tionships between pistU and androecium, upon which successful poUination is
contingent, probably have occurred many times in the evolutionary history of
Senna, but once made are conservative and consequently become significant in
interpretation of affinities.
Pod: The simplest form of Senna pod is a piano-compressed bean inertly dehis­
cent along both sutures and enclosing some 10-20 seeds which are laid across a
dry cavity and turned broadside to the valves. From such a pod all other types
seem to be derived, either by multiphcation (less often by loss) of ovules, by
elaboration of a pulpy endocarp enveloping the seeds, by pressures brought to
bear on the valves by the swelling seeds, by epidermal excrescences of wings or
crests arising from either valve or suture or, in a few highly specialized groups,
by development of lomentiform constrictions and hues of fracture between seeds.
Seeds of American sennas are highly vulnerable to attack from larvae of Bru-
chidae, the effects of which, in absence of chemical defenses, are mitigated by
Senna: S. {Senna) angustifolia, properly S. alexandrina (P. Mill.) I. & B.; S. (Pictae) nicaraguensis
(Benth.) I. & B