THE ORDER LEGUMINOSÆ: ILLUSTRATED BY THE SWEET-PEA, THE RED CLOVER,
ACACIA ARMATA, THE SENSITIVE PLANT, THE BARBADOES FLOWER-FENCE, THE
CAROB-TREE, THE TAMARIND, THE SENNA, THE GLEDITSCHIA, THE LOGWOOD,
THE JUDAS-TREE, AND THE KENTUCKY COFFEE-TREE.
This order is a very numerous one, containing above three hundred
genera, and including several highly important plants, both for food
and commerce. As examples of the utility of the Leguminosæ for food,
I need only mention the pea and bean, and all their numerous allies;
and as examples of their importance in medicine and the arts, I may
enumerate senna, liquorice, the tamarind, gum-arabic, and logwood.
Among the ornamental plants belonging to this order are, the Laburnum,
the Furze or Gorse, the Robinia or False Acacia, the true Acacias,
the Sensitive Plant, and the Barbadoes Flower-fence. It will be seen
by this enumeration, that the flowers of the Leguminosæ differ from
each other nearly as much as those of Ranunculaceæ; but when in seed,
they are all easily recognised by their seed-vessels, which are always
legumes, that is, bearing more or less resemblance to the pod of
the common pea. To aid the memory in retaining the great number of
genera included in this order, various methods have been devised of
re-dividing it; and of these I shall adopt the newest, which is also
the simplest, by which they are arranged in three tribes, according to
their flowers.
TRIBE I.—PAPILIONACEOUS FLOWERS.
[Illustration: FIG. 13.—FLOWER, POD, AND TENDRIL, OF THE SWEET-PEA.]
[Illustration: FIG. 14.—STAMENS OF THE SWEET-PEA.]
The flowers of this tribe are called Papilionaceous; because Papilio is
the scientific name of a genus of butterflies, which they were supposed
to resemble. The type of this tribe may be considered the flower of
the sweet-pea (_Lathyrus odoratus_), which has a small green calyx,
cut into five deep notches, but not divided into regular sepals. (See
_a_ and _b_ in _fig._ 13.) The corolla is in five petals, the largest
of which (_c_) stands erect, and is called the vexillum or standard;
below this are two smaller petals (_d_), which are called the algæ or
wings; and below these are two petals, joined together so as to form
a kind of boat (_e_), which are called the carina or keel, and which
serve as a cradle for the stamens and pistil. There are ten stamens,
nine of which have the lower half of their filaments growing together,
so as to form a fleshy substance at the base, as shown in _fig._ 14
at _f_, and the other (_g_) is free. The ovary is oblong, terminating
in a filiform style, with a pointed stigma, as shown at _g_ in _fig._
13; and it is one-celled and many-seeded; the seeds being what we
call the peas. When the petals fall, the pod still retains the calyx
(_b_), and the style (_g_); and these remain on till the seeds are
ripe, when the pod divides naturally into two parts, or valves as they
are called, which curl back so as to discharge the seeds. If the pod
be examined before it bursts, it will be found that the valves are
composed of a fleshy substance, lined with a strong membrane or skin,
and that they are united by two seams, called the dorsal and ventral
sutures. Along the ventral suture (_h_) there runs a kind of nerve,
called the placenta, to which the peas are attached, each pea being
furnished with a little separate stalk, called a funicle. A cook would
be surprised, even in these enlightened times, to be told to take a
legume of _Pisum sativum_, and after separating the two valves at the
dorsal suture, to detach the funicles of the seeds from the placenta;
yet these scientific terms would merely describe the operation of
shelling the peas. It will be seen by this description that the pod
of the pea differs very materially from the seed-vessels of all the
other plants I have had occasion to describe; and that it thus forms
a very distinctive character for the order. The other parts vary in
the different genera: the calyx is sometimes tubular, and sometimes
inflated; sometimes it has only four notches, or teeth as they are
called, instead of five, and sometimes it has five distinct sepals
divided to the base. The parts of the corolla vary also in proportion
to each other, the keel in some of the Australian plants is as long as
the standard; as, for example, in _Kennedia Maryattæ_; and in others
the wings are so small as to be scarcely visible. The stamens of many
of the species are also free, that is, divided to the base; while in
others they resemble those of the sweet-pea, in having nine joined
together and one free; and in others the whole are joined together
at the base. The pods also vary very much in size and form; being
sometimes nearly round, and only one or two-seeded; and in others
long, and containing many seeds, as in the common bean or pea. The
seeds themselves are so different that the tribe has been divided, on
account of them, into two sections: the one consisting of those plants
which, like the common bean, have the seed dividing into two fleshy
seed-leaves or cotyledons, when it begins to germinate; and the other,
the seed-leaves of which are thin. The seeds of the papilionaceous
plants which have thin cotyledons are not eatable; but those with
fleshy cotyledons may be safely used as food. The fleshy cotyledons
do not always rise above the ground; but they do so decidedly in the
bean and the lupine; and if either of these seeds be laid in moist soil
with the hilum or scar downwards, the seed, as soon as it begins to
germinate, will divide into two parts (that is, into two cotyledons),
which will rise above the ground, and become green like leaves; though,
from still retaining their roundish form, they are easily distinguished
from the true leaves, which rise in the centre. Though my readers will
have no difficulty in recognising most of the Leguminosæ which have
papilionaceous flowers, there are some genera, respecting which they
may be interested to learn a few particulars. Thus, the Chorozema
is one of the kinds with thin cotyledons, and consequently its seeds
are not eatable. The legumes of this genus are roundish, and swelled
out, so as to bear but little outward resemblance to a pod. Sophora,
Edwardsia, Virgilia, Podolobium, Callistachys, Brachysema, Burtonia,
Dillwynia, Eutaxia, Pultenæa, Daviesia, and Mirbelia, have all thin
cotyledons, and their ten stamens all separate from each other; but
in Hovea, Platylobium, and Bossiæa, though the cotyledons are thin,
the stamens all grow together at the base. I mention these common
greenhouse shrubs, that my readers may have an opportunity of examining
their botanical construction, and thus verifying their names. The
common furze (_Ulex europæus_), the Spanish broom (_Spartium junceum_),
the Petty whin (_Genista Anglica_), the Laburnum (_Cytisus Laburnum_),
and the common broom, all belong to this division, and consequently
their seeds are not eatable; those of the Laburnum are indeed
poisonous. The distinctions between Spartium, Genista, and Cytisus, are
very slight, lying chiefly in the calyx; and as a proof of this the
common broom, which is now called _Cytisus scoparius_, was formerly
supposed to be a Spartium, and afterwards a Genista.
The common red clover (_Trifolium pratense_) has its flowers in such
dense heads that it is difficult at first sight to discover that they
are Papilionaceous. On examination, however, it will be found that each
separate flower has its standard, wings, and keel, though the wings
are so large as to hide the keel, and nearly to obscure the standard.
The calyx is tubular at the base, but divided above into five long,
awl-shaped teeth, that stand widely apart from each other. The legume
has only one or two seeds, and it is so small as generally to be hidden
by the calyx.
TRIBE II.—MIMOSÆ.
[Illustration: FIG. 15.—FLOWERS AND SPRIG OF ACACIA ARMATA.]
The second division of Leguminosæ comprises those plants which have
heads of flowers either in spikes or balls, like those shown in _fig._
15. This figure represents two heads of flowers of _Acacia armata_,
a well-known greenhouse shrub, of their natural size; and _fig._ 16
shows a head of similar flowers magnified. In the latter, _a_ shows
the calyx, which is five-toothed, and _b_ the petals, which are five
in number and quite regular in shape; _c_ are the stamens, which vary
from ten to two hundred in each flower, and which are raised so high
above the petals as to give a light and tuft-like appearance to the
whole flower. The legumes are very large in proportion to the flower;
and consequently, by a wise provision of nature, only a very few of the
flowers produce seed. The valves of the legumes are not fleshy like
those of the pea, but dry and hard, and when they open they do not curl
back.
[Illustration: FIG. 16.—FLOWER OF ACACIA MAGNIFIED.]
The flowers in the different kinds of Acacia, differ in the corolla,
which has sometimes only four petals, which are occasionally united at
the base, and in the calyx, which is sometimes only four-cleft. The
flowers also in many species are in spikes instead of balls.
[Illustration: FIG. 17.—THE BI-PINNATE LEAF OF AN ACACIA.]
The rest of the plant of _Acacia armata_ is very curious; what appear
to be the leaves (see _d_ in _fig._ 15) are, in fact, only the petioles
of the leaves dilated into what are called phyllodia; the true leaves,
which were of the kind called bi-pinnate, having fallen off, or
never unfolded. The true leaves, however, often appear on seedling
plants; and thus, when seeds are sown of several kinds of Acacia, it
is sometimes difficult to recognise them till they have attained a
considerable age. The stipules of the leaves, (which are to ordinary
leaves what bracts are to flowers,) are in _Acacia armata_, converted
into spines, as shown at _e_. In some kinds of Acacia the true leaves,
with the petioles in their natural state, (see _fig._ 17,) are retained
in the adult plants, as in _Acacia dealbàta_; and in others, the
bi-pinnate leaves are occasionally found attached to the phyllodia, as
in _A. melanoxylon_. The bi-pinnate leaves are composed of from six to
twenty pairs of pinnæ, or compound leaflets (see _f_ in _fig._ 17),
each of which consists of from eight to forty pairs of small leaflets
(_g_). The Gum Arabic tree, _Acacia vera_, has leaves with only two
pairs of pinnæ, but each has eight or ten pairs of small leaflets. The
branches and spines are red, and the heads of flowers are yellow.
There are above three hundred known species of Acacia.
The genus Mimosa differs from Acacia in the corolla being
funnel-shaped, and four or five cleft. There are seldom above fifteen
stamens, which are generally on longer filaments than those of the
Acacia; and the legume is compressed and jointed or articulated between
the seeds, so that the part which contains one seed may be broken off,
without tearing the rest. The Sensitive-plant (_Mimosa pudica_) is a
familiar example of this genus.
The cotyledons of the plants belonging to this tribe are generally
leafy; and the seeds are not eatable. The plants themselves are easily
recognised by their ball or tassel-shaped heads or spikes of flowers;
by the small cup-shape and inconspicuous corolla of each; by the great
number and length of the stamens; and by their bi-pinnate leaves,
or phyllodia supplying the place of leaves—though the phyllodia are
sometimes found in Australian plants with papilionaceous flowers, as,
for example, in _Bossiæa ensata_.
TRIBE III.—CÆSALPINEÆ.
[Illustration: FIG. 18.—FLOWER OF THE BARBADOES FLOWER-FENCE.]
The flowers of the plants contained in this tribe have generally five
regular, widely spreading petals, which are never joined together;
and stamens of unequal length, which with few exceptions are also
perfectly free. The petals are generally of the same size and shape;
though sometimes, as in the Barbadoes Flower-fence (_Poinciana_, or
_Cæsalpinia pulcherrima_), four are of the same shape, and one deformed
(see _fig. 18_). The filaments of some of the stamens are very long and
curving over, but the others are much shorter and erect; the style is
long and slender, ending in a pointed stigma. The legume is flat, and
it looks almost many-celled, from the seeds being divided from each
other by a kind of spongy substance, frequently found in the pods of
plants belonging to this division. The leaves are bi-pinnate, and the
stem is spiny.
The Carob-tree, or St. John’s bread (_Ceratonia siliqua_), agrees with
the Barbadoes Flower-fence in the pulpy matter dividing the seeds,
though it differs widely in its flowers, which are without petals, and
do not possess any beauty. The pulp of the pods of the Carob tree is
eatable; but that of Poinciana is said to be injurious. The pod of the
Tamarind (_Tamarindus indica_) differs from the preceding species in
having the pulpy matter of its pods contained between the outer and
inner skin of each valve, like the fleshy substance in the pod of the
pea, instead of serving as a bed for the seeds. The flowers of the
tamarind have five equal petals of a brownish yellow, three of them
being streaked with pink; and the anthers are nearly rose-colour. The
stamens and the style both curve upwards. It is the pods prepared with
sugar that form what we call Tamarinds. In _Cassia lanceolata_, the
leaves of which furnish senna, the flowers have a bright yellow corolla
of five concave petals, three of which are somewhat larger than the
others. The stamens are also unequal in length; and the style curves
upwards. The legume is kidney-shaped, and the cells are divided from
each other by thin membraneous partitions. The Gleditschia or Honey
Locusts, now so frequently planted in our shrubberies on account of
the lightness and elegance of their foliage, belong to this division,
and some of them, particularly the Chinese Thorny Acacia (_Gleditschia
horrida_), are remarkable for their thorns proceeding from the trunk
and large branches, as well as from the axils of the leaves. The
Logwood (_Hæmatoxylon Campechianum_), has inconspicuous yellow flowers,
the petals being very little longer than the calyx; and the legume has
seldom more than two seeds. Though it is considered a tree, the stem
is seldom thicker than the arm of a man, and it is generally crooked;
chips of the wood are used for dyeing purple. The Judas-tree (_Cercis
siliquastrum_) is another species belonging to this division, as,
though the flowers appear of the papilionaceous kind, they are, in
fact, composed of five petals, nearly equal in size, but having the
wings the largest. There are ten stamens, free, and of unequal length.
The legume is oblong and many-seeded; and it opens only on the dorsal
suture, the other side to which the seeds are attached being slightly
winged. The flowers are each on a separate flower-stalk or pedicel, but
they rise from the trunk and branches in tufts or fascicles. The leaves
are simple and cordate; and they do not appear till the flowers have
faded.
[Illustration: FIG. 19.—FLOWERS OF THE KENTUCKY COFFEE-TREE.]
The Kentucky Coffee-tree (_Gymnocladus canadensis_) is the last plant
belonging to this division that I shall attempt to describe. This
tree is called in Canada, Chicot, or the stump-tree, from its having
no visible buds, and thus appearing like a dead stump in winter. The
flowers of this plant are white, and they are produced in racemes, but
they bear no resemblance to the pea flowers, having rather a star-like
appearance, like those of the Jasmine (see _fig._ 19). The calyx (_a_)
is tubular; and the upper part or limb is divided into five parts
(_b_), which alternate with the petals of the corolla (_c_). There
are ten stamens, but they are completely enclosed in the tube of the
calyx. The pod is very large, the valves becoming hard and bony when
dry; and the seeds are like large beans, the pod being deeply indented
between the seeds. The leaves are bi-pinnate, with from four to seven
pairs of pinnæ; the lower having only one small leaflet, but the rest
bearing from six to eight pairs of leaflets each. This tree must not
be confounded with the true Coffee-tree, which belongs to Rubiaceæ,
and from which it is perfectly distinct in every respect; and it only
takes its American name from its beans having been used as a substitute
for coffee. The outer bark of this tree, when it becomes old, splits
off in narrow strips and rolls up; and its timber, like that of the
Robinia or False Acacia, having very little sap wood, is thus very
strong in quite young trees, though it is of little value when the tree
is full-grown.
The species contained in the first and second divisions of this order
will be easily recognised by botanical students; and though those of
the third division are much more difficult to find out, still there
is a kind of family likeness, particularly in the leaves, which will
enable the eye, with a little practice, to recognise them. The student
should visit the hothouses of botanic gardens and nurseries, and should
there endeavour to pick out plants belonging to this order.