This family (with the exception of Aristolochiaceæ) includes only
parasitic plants. Partly on this ground, and partly because they
all have _epigynous_ flowers, they are considered to belong to the
youngest type (which is expressed in the name ὕστερος, the one that
comes after). It is not certain to which of the preceding families they
are most nearly allied. ~Again, it is a matter of doubt whether the
Aristolochiaceæ are related to the others; they are by Engler united
with Rafflesiaceæ into one family, _Aristolochiales_.~
[Illustration: FIG. 540.--Flower of _Aristolochia clematitis_
(long. sect.). _A_ Before pollination, and _B_ after: _n_ stigma;
_a_ anthers; _t_ an insect; _kf_ ovary.]
Order 1. _Aristolochiaceæ._ The majority are perennial herbs or
twining shrubs, whose stalked, simple, and generally more or less
cordate or reniform leaves are borne in 2 rows and are exstipulate.
The flowers are _hermaphrodite_, _epigynous_, regular or zygomorphic;
perianth-leaves united, _simple_ but most frequently _petaloid_ and
3-merous; 6 or 12 (in _Thottea_ as many as 36) stamens with _extrorse_
anthers. The ovary is more or less completely 4–6-locular with ovules
attached in the inner angles of the loculi (Fig. 540 _kf_). The style
is short, and has a large, radiating stigma (Fig. 540 _n_). Fruit a
capsule. Seeds rich in endosperm.
_Asarum europæum._ Each shoot has 2 reniform foliage-leaves, between
which the terminal flower is borne (the rhizome becomes a sympodium
by development of the bud in the axil of the upper foliage-leaf).
The flower is _regular_ and has a bell-shaped perianth with 3 outer
valvate, and 3 inner small segments (which may be wanting). =12= (2
× 6) free, extrorse stamens, 6 carpels.--_Aristolochia clematitis_
(Birth-wort) has an erect, unbranched stem, bearing many flowers in the
leaf-axils, in a zig-zag row (accessory buds in a unipared scorpioid
cyme). The flowers are zygomorphic (Fig. 540), formed by 3 alternating,
6-merous whorls. The perianth has a lower, much-distended part (_k_),
succeeded by a narrow, bent tube (_r_), which passes over into an
oblique, almost tongue-like projection (6 vascular bundles indicate
that the number 6 is prevalent here, as in _Asarum_); =6= stamens (Fig.
540 _a_), with the dorsal portion turned upwards, are united with the
short style to form a _stylar column_; they are placed quite beneath
the 6 commissural stigmatic rays, which arch over them as short, thick
lobes. ~Protogynous; POLLINATION is effected in _Arist. clematitis_
by small flies; these enter the erect unfertilised flower through
the tube (Fig. 540 _A_, _l_) without being prevented by the stiff,
downwardly-turned hairs which line the tube and prevent their escape;
they find the stigma (_n_) fully developed, and may pollinate it with
the pollen they have brought with them. The stigmas then straighten
and wither (_B_, _n_), the anthers open, and the flies may again be
covered with pollen; but the hairs which blocked up the tube do not
wither until the anthers have shed their pollen, and only then allow
the imprisoned flies to escape and effect cross-pollination. Prior to
pollination, the flowers stand erect, but after this has taken place
they become pendulous, and the perianth soon withers.--_A. sipho_
(Pipe-flower), another species, is a climber, and often grown in
gardens; it has only one row of accessory buds in the leaf-axils.--200
species; chiefly in S. Am. OFFICINAL: the rhizome of _Aristolochia
serpentaria_ (N. Am.).~
[Illustration: FIG. 541.--A fruit of _Myzodendron brachystachyum_
(slightly mag.) germinating on a branch.]
Order 2. =Santalaceæ.= Parasites containing chlorophyll, which, by
the help of peculiar organs of suction (haustoria) on their roots,
live principally on the roots of other plants. Some are herbs,
others under-shrubs. The regular, most frequently ☿-flowers have
a simple perianth, which is gamophyllous, 3- or 5 partite with the
segments valvate in the bud, and a corresponding number of stamens
opposite the perianth-leaves. In the inferior ovary there is a
_free, centrally placed_, often long and curved _placenta_ with
three ovules (one opposite each carpel); these are naked, or in any
case have an extremely insignificant integument. Fruit a nut or
drupe. Seed without testa. Endosperm fleshy. 225 species; chiefly
in the Tropics.--_Thesium_, a native, is a herb with scattered,
linear leaves and small 5-merous flowers (P5, A5, G3) in erect
racemes; the subtending bracts are displaced on the flower-stalks.
Fruit a nut.--_Osyris_ (diœcious shrub; 3-merous flowers) is
another European genus.--_Santalum album_, which grows in E. Ind.,
yields the valuable, scented Sandalwood, the oil of which is used
medicinally.--_Quinchamalium._
_Myzodendron_ is a reduced form of the Santalaceæ; the ♂-flowers
are without perianth; the perianth of the ♀-flower is 3-merous.
About 7 species; S. Am.; parasitic on a Beech (_Nothofagus_).
The fruit has 3 feathery brushes, alternating with the lobes
of the stigma, which serve as flying organs and to attach the
fruits to a branch (Fig. 541), the brushes twining round as
soon as they come in contact with it. There is only 1 seed in
the fruit, which germinates by a long, negatively heliotropic
hypocotyl, and is attached by a radicle modified into an
haustorium.
Order 3. =Loranthaceæ= (=Mistletoes=). Plants containing chlorophyll
which are parasites on trees, and most frequently have opposite,
simple, entire leaves and regular, epigynous, often unisexual, 2- or
3-merous flowers, with single or double perianth. Stamens equal in
number and opposite to the perianth-leaves, free, or in varying degrees
united to one another. The inferior ovary is constructed as in the
Santalaceæ, the ovules being situated on a low, free, centrally-placed
placenta, but the placenta and ovules unite with the wall of the ovary
into _one connected, parenchymatous mass_, in which _the embryo-sacs
are imbedded_. Only 1 (less frequently 2–3) of the 1–6 embryo-sacs
is fertile. The number of the carpels however varies. The fruit is a
_1-seeded berry_, whose inner layer is changed into a _tough slimy
mass_ (bird-lime), which serves to attach the fruits to other plants.
The two groups, _Loranthoideæ_ and _Viscoideæ_, are
distinguished by the fact that the former has a distinct
“calyculus,” _i.e._ an entire or lobed, or dentate swelling
on the receptacle below the perianth. The majority of the
Loranthoideæ have a petaloid perianth; in all the Viscoideæ, on
the other hand, it is sepaloid.
[Illustration: FIG. 542.--_Viscum album_: _A_ branch with leaves
and berries: _a_ scale-leaves; _b_ foliage-leaves; _n_ _m_ _n_
flowers; _B_ seedling, the bark of the branch being removed; _C_
an older embryo which still retains the cotyledons.]
[Illustration: FIG. 543.--To the left the Rafflesiaceous _Cytinus
hypocistus_, parasitic on the roots of _Cistus_. To the right the
Balanophoraceous _Cynomorium coccineum_, parasitic on the roots
of _Salicornia_.]
The Mistletoe (_Viscum album_, Fig. 542) is a native, evergreen plant
which may be found growing on almost any of our trees (sometimes on
the Oak), and, like other Loranthaceæ, it produces swellings of the
affected branches. ~Its spherical white berries (Fig. 542 _A_) enclose
(1–) 2–3 green embryos; they are eaten by birds (especially Thrushes),
and are partly sown with their excrement, partly struck or brushed
off the branches of the trees, the seed being enclosed, at maturity,
by viscin, _i.e._ “bird-lime.” The seeds may also germinate on the
branches, without having first passed through the alimentary canal
of the birds. On germination, the hypocotyl-axis first appears, as
in Fig. 541, and bends towards the branch; the apex of the root then
broadens, and forms at the end a disc-like haustorium, from the centre
of which a root-like body grows through the bark into the wood, and
ramifies between the bark and wood. Suckers are developed on the root
like strands which are formed in this manner, without, however, having
a rootcap; they are green, and penetrate the wood by the medullary
rays (Fig. 542 _C_). Adventitious buds may also be developed from the
root-like strands which break through the bark and emerge as young
plants. The young stem quickly ceases its longitudinal growth, and
lateral shoots are developed from the axils of its foliage-leaves.
These and all following shoots have a similar structure; each of
them bears a pair of scale-leaves (Fig. 542 _A_, _a_) and a pair
of foliage-leaves (Fig. 542 _A_, _b_), and then terminates its
growth, if it does not produce an inflorescence; new lateral shoots
proceed from the axils of the foliage-leaves, and the branching, in
consequence, is extremely regular and falsely dichotomous. Only one
internode (shoot-generation) is formed each year, so that each fork
indicates one year. The foliage-leaves fall off in the second year.
The inflorescence is a 3(-5)-flowered dichasium (Fig. 542 _A_, _m_ is
the central flower, _n_ the lateral). The plants are _diœcious_; the
♂-flower as a rule is 2-merous: perianth 2 + 2, each leaf of which
bears on its inner side 6–20 pollen-sacs, each of which opens by a
pore; this relationship may be considered to have arisen from the union
of the perianth-leaves with the multilocular stamens (2 + 2) placed
opposite them. The ♀-flowers always have Pr 2 + 2, G2.--_Loranthus_
is also found in Europe (it has a 3-merous flower), especially in
the central and south-eastern districts, on _Quercus cerris_ and _Q.
pubescens_; but the great majority of the 520 species grow in the
Tropics on trees which they ornament with their often brightly-coloured
flowers, and ultimately kill when present in too great numbers. The
pollination in the numerous Loranthaceæ with unisexual flowers, is
effected by the wind. In _Viscum album_ this takes place in autumn,
the actual fertilisation in the following spring, and the maturity in
November or December; in the succeeding month of May the berry is ready
to germinate, and falls off.~
USES. Birdlime from _Viscum album_.
Order 4. =Rafflesiaceæ= and Order 5. =Balanophoraceæ=.
These orders comprise _root-parasites_, almost entirely
devoid of chlorophyll; they are reddish or yellow, without
foliage-leaves (Fig. 543). As far as our knowledge of these
rare tropical plants extends, they have thalloid organs of
vegetation resembling the root-like strands of _Viscum_, or
they are filamentous and branched like Fungus-hyphæ; they live
in and on the tissues of the host-plant, from which their
flowering-shoots, often of mushroom-like form, are subsequently
developed (Fig. 543). In order to unfold they must often break
through the tissues of the host-plant.
Of the RAFFLESIACEÆ, _Cytinus hypocistus_ is found in S. Europe
living on roots of _Cistus_-plants and to some extent resembling
_Monotropa_ (Fig. 543). _Rafflesia_ is the best known; it lives
on roots of _Cissus_-species (belonging to the Ampelidaceæ) in
Java; its yellowish-red, stinking flowers attain a gigantic
size (one metre or more in diameter), and are borne almost
directly on the roots of the host-plant. Besides these there
are other genera: _Brugmansia_, _Pilostyles_, _Hydnora_.--To
BALANOPHORACEÆ (Fig. 543) belong: _Balanophora_, _Langsdorffia_,
_Scybalium_, _Sarcophyte_, _Helosis_, etc., and in S. Europe,
_Cynomorium coccineum_.