The most characteristic feature is, that the _gynœceum is 1-locular_
and formed of _1 carpel, the ventral suture of which is turned
posteriorly_. The fruit, in most instances, is a _pod_ (legume),
which opens generally along both sutures, the two valves twisting
more or less in opposite directions. ~In other instances it opens
along one suture only, or as a pyxidium (Red Clover), or it is
indehiscent, in which case it is more or less berry-like (_e.g._ the
Tamarind, Carob-bean), or it is a drupe (_e.g._ the Tonquin-bean), or
a 1–few-seeded nut (_e.g._ _Melilotus_), or a lomentum, which divides
transversely into as many joints as there are seeds (_Ornithopus_, see
Fig. 513).~
The inflorescences belong to the _centripetal_ type (_i.e._
indefinite); cymes do not occur. The flowers are _zygomorphic_, with
vertical plane of symmetry, seldom regular; _5-merous_ with but a few
exceptions, ☿, and slightly _perigynous_. The following diagram is the
most general (Fig. 511): 5 sepals, with the _unpaired sepal median
and anterior_, 5 petals, 5 + 5 stamens, all in alternating whorls, 1
carpel. The calyx is most frequently gamosepalous, the gynœceum is
narrowed down at the base to a short stalk and, in the majority, is
more or less bent. The seed is most frequently kidney-shaped, with
a smooth, hard and shining testa, the hilum being very distinct.
_Endosperm is wanting_, or is reduced to a thin layer, which is of
service when the seed swells during germination. The vegetative parts
have these features in common, namely, the _leaves are scattered,
stipulate_, and almost always _compound_. Peculiar _sleep-movements_
and _sensitiveness_ are found in some, chiefly in the Mimosas. Many,
probably all, Leguminosæ have _small tubercles on their roots_ which
are produced by a kind of bacterium, and assist in the assimilation
of free nitrogen. Spontaneous movements are exhibited by _Desmodium
gyrans_ (Telegraph-plant).
This family is closely allied to the Rosifloræ, with which it
agrees in the scattered leaves, the presence of stipules, the
generally 5-merous and most frequently perigynous flowers with
eucyclic stamens, and the absence of endosperm. _Amygdalaceæ_
and _Chrysobalanaceæ_, with solitary carpels, approach on one
side to the Leguminosæ, among which genera with drupes are also
found; _Mimosaceæ_, with their many stamens, form a connecting
link on the other side. In this respect the Mimosa-genus
_Affonsea_, and certain Cæsalpineæ and Swartzieæ, are of special
interest in having more than one carpel (syncarp), a condition
which is sometimes met with abnormally in other Leguminosæ, as
well as in Amygdalaceæ. About 7,000 species of the Leguminosæ
are known.
Order 1. =Cæsalpiniaceæ.= These are _leguminous plants with straight
embryo and a flower which is not papilionaceous and has not the
same æstivation_ (Figs. 508–510); but in reality there is not a
single characteristic which absolutely distinguishes them from the
Papilionaceæ.--The majority are arborescent; the leaves as a rule
are pinnate or bipinnate. The flower is 5-merous, most frequently
perigynous and slightly zygomorphic; the calyx is free or gamosepalous,
the corolla polypetalous with _ascending imbricate æstivation_ (_i.e._
the two lowest petals envelop the lateral ones, and these again the
posterior; Fig. 508); 10 _free stamens_; fruit various.
[Illustration: FIGS. 508–510.--_Cassia floribunda._
FIG. 508.--Floral diagram.
FIG. 509.--Flower.