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

1982] CASSIINAE—SENNA 69
tionary Unes, not closely allied to one another: sect. Senna; some series of sect.
Chamaeflstula and several of sect. Peiranisia. Alternative hypotheses, that a
similar gland has developed independently in different branches of the genus,
each from glandless predecessors, or that the glanduliferous and glandless species
form mutually exclusive natural groups, are not supported by other data. In some
species of sect. Peiranisia ser. Interglandulosae and of sect. Chamaeflstula ser.
Brachycarpae and Confertae, as in all species of sect. Chamaeflstula ser. Coriaceae
and Laxiflorae, a nectariferous gland simUar to that of the leafstalk occurs
at base of or attached to one edge of the pedicels. All of these clavate or phalloid
nectaries like the cup- or shield-shaped ones of Chamaecrista, are attractive to
ants, which have been supposed to deter predators. The extent to which this
symbiotic relationship is of real benefit to the Senna partner is unknown.
Inflorescence: The basic unit of the Senna inflorescence is a raceme axillary
to a developed leaf, and an indeterminate sequence of lateral several-flowered
racemes is the inflorescence that either prevails or survives in aU major groups
of species. Reduction and subsequent elimination of distal leaves subtending individual
racemes, sometimes accompanied by shortening of the primary axis,
produces a terminal exserted thyrse or panicle which, when the elementary ra­
cemes are reduced to one or two flowers apiece, may simulate a terminal raceme.
The cauliflorous panicle of some Bacillares is structurally identical to that terminating
an annotinous branchlet except that it is wholly leafless. Apparently
simple cauliflorous racemes in the same series, hke those arising from year-old
wood of some drought-deciduous members of sect. Peiranisia, are panicles re­
duced to one elementary racemose axis. These specializations ofthe inflorescence
no doubt correspond to particular biological advantages that may be gained by
compression of flowering time and energy into a showy but transient display or
by appeal to a particular poUinator. However that may be, a particular inflorescence
type has rarely been stabilized within a group of species otherwise defined,
and its organization may even be modified in the course of its own maturescence.
Flower-number, like leaflet-number, approaches stabUity only when very low.
The scapiform one-flowered raceme of S. pumilio is almost invariable, but those
of the prevailingly biflorous Interglandulosae (S. pallida and kindred) are in fact
one- to three- or rarely four-flowered.
Calyx: The sepals of Senna are prevailingly graduated from short to longer in
an upward spiral and are obtuse at apex. Subequal sepals of ser. Brachycarpae
and Tharpia are certainly derived, as are the lanceolate acute one of S. (Bacil­
lares) acutisepala. Venation of the sepals is palmate except in monotypic sect.
Paradictyon, which see for discussion, and varies only in strength, prominence
and number of primary veins. When proportionately small the sepals may sepa­
rate long before true anthesis, exposing the immature petals and androecium.
Conversely, proportionately ample sepals (as in S. latifolia or S. trolliiflora) per­
manently nidulate all the inner floral parts. The extremes both occur in the one
series Bacillares.
Corolla: Senna displays two basically different types of corolla, one vertically
Fig. 11. Androecia of genus Senna (2). Sect. Chamaefistula: S. {Bacillares) rugosa (G. Don) I.
& B.; 5. {Armatae) armata (S. Wats.) I. & B.; S. {Basiglandulosae) neglecta (Vog.) I. & B. Sect.
Senna: S. {Pictae) nicaraguensis (Benth.) I. & B. Sect. Peiranisia: S. {Interglandulosae) pallida
(Vahl) I. & B. var. pallida: S. {Excelsae) spectabilis (DC.) I. & B.