The flowers are hermaphrodite and constructed on the ordinary 3-merous,
pentacyclic type with petaloid, _epigynous, strongly zygomorphic_
perianth, and generally _one-stamened_ by the suppression of the
other 5 stamens. The family has derived its name from the fact that
the stamen is united with the style into a “_stylar column_” (except
_Burmanniaceæ_). All are herbs; many grow as epiphytes on other plants.
This family and the Scitamineæ occupy correspondingly high
positions among the Monocotyledons; these two families may
therefore be placed close together, although one cannot be
derived from the other. The first of the two orders is very
small, but the second is very rich in species. The Apostasieæ
are best classed with the Orchidaceæ and have no independent
place.
Order 1. =Burmanniaceæ.= This order forms a transitional link between
the Gynandræ and the epigynous Liliifloræ (_Amaryllidaceæ_), in having
a 6-leaved perianth, and 6–8 stamens; but some have a labiate perianth
(the median perianth-leaf of the _outer_ whorl being very large). The
ovary is most frequently unilocular with three parietal placentæ; but
in some it is 3-locular with axile placentation. Capsule. Seeds ∞,
small, with _endosperm_. The relationship to the Orchidaceæ is shown
especially in the very imperfectly developed embryo and in the ovary.
Small, tropical herbs (59 species); some are saprophytes.
[Illustration: FIG. 318.--_A_ Diagram of an Orchid-flower. _B_,
_Cephalanthera_. Stylar-column: _a_ anther; _s_ stigma; at the
foot are seen scars indicating the position of the parts which
have been removed.]
Order 2. =Orchidaceæ.= The epigynous, petaloid perianth is strongly
zygomorphic in having the _posterior_ leaf of the interior whorl, the
_lip_ (labellum), differing from all the other leaves in form, size,
and colour (except _Apostasieæ_); the position of the labellum is very
frequently reversed, being turned _forwards and downwards_ by the
twisting of the ovary (Fig. 318 _A_). _Only_ 1 of the stamens--the
anterior of the external whorl--is developed and bears an anther (by
the twisting of the ovary it is turned posteriorly and upwards);
the others are entirely wanting (indicated by * in Fig. 318 _A_)
or present as staminodes (Fig. 318 _A_, δ δ) (except _Apostasieæ_,
_Cypripedileæ_); the filaments are united with the style to form a
column (Fig. 318 _B_), the _stylar-column_[30] (_gynostemium_), and
the anther (_a_) is thus placed on its apex and exactly behind or over
the stigma (_s_). The anther is 4-locular; the pollen-grains do not
separate (except _Apostasieæ_, _Cypripedileæ_) but remain united either
in tetrads or in masses, which correspond to a pollen-mother-cell
(Fig. 320 _C_, _D_, _E_); or the pollen-grains, formed in each of
the two anther-halves, remain united and form one or a few wax-like
masses (pollen-masses, pollinia). The 3 carpels form a _unilocular
ovary_ with 3 parietal, deeply bifid placentæ (except _Apostasieæ_,
_Selenipedilum_). Only the two lateral carpels are prolonged and
developed into the stigma (Fig. 318 _B_, _s_), while the one lying in
the median line, which is situated just within the anther (Fig. 318
_A_), becomes either rudimentary or developed into the “_rostellum_”
(“a small beak”), on which the sticky bodies (_glandulæ_) arise; by
aid of these the heavy, connected pollen-masses may be glued to the
insects which visit the flower, and pollination is thus secured (in
_Apostasieæ_ and _Cypripedileæ_ the 3 carpels each contribute to the
formation of the stigma). The fruit is a _capsule_ which most often
dehisces _by 6 valves_, 3 of which are broader and bear the placentæ,
and 3 alternating with them are narrower and barren (except _Vanilla_).
The very numerous and exceedingly small seeds have _no endosperm_,
and have a somewhat _spherical embryo without any trace of external
organs_. The testa is membranous and loose.
The Orchids are _all perennial herbs_ with diverse habits and varying
morphological structure (see the genera); the leaves are scattered,
of the usual Liliaceous form, and the inflorescences in all cases are
_racemes or spikes_ (sometimes branched), with subtending bracts, but
without bracteoles.
The forms which are the least modified are described first.
=1.= APOSTASIEÆ. The perianth-leaves are almost alike and free. The
column is straight, with 3 equally-developed stigmas. _Neuwiedia_ has
3 perfect stamens (1 median of the outer whorl, and 2 lateral of the
inner whorl); _Apostasia_ has only 2 perfect (inner lateral) and one
barren (the median of the outer whorl), which however may be entirely
wanting. The 3 _posterior_ stamens are entirely suppressed. The pollen
is powdery. The ovary is 3-locular with axile placenta. 7 species
(Tropical East India, Australia).
[Illustration: FIG. 319.--_Cypripedilum calceolus_: 1 front view
of the flower; 2 lateral view, after the removal of all the
perianth-leaves with the exception of the labellum, which has
been divided longitudinally; 3 the stylar-column; _ov_ ovary;
_s_-_s_ exterior, _p_ interior perianth; _p’_ the labellum; _a_
the two fertile stamens; _a’_ the staminode; _st_ the stigma; _i_
entrance for the insects; _ex_ exit.]
=2.= CYPRIPEDILEÆ.[31] The flower is strongly zygomorphic with a large
boat-shaped labellum. There are two perfect stamens belonging to the
_inner_ whorl, and the median anterior (later on the posterior) stamen
of the outer whorl is transformed into a large, barren, shield-shaped
body (Fig. 319). _Selenipedilum_ has a 3-locular ovary, but
_Cypripedilum_ (Ladies’-slipper) has a unilocular ovary with 3 parietal
placentæ--the typical structure for the Orchids. The pollen-grains
are _separate_ (not in tetrads) and all the 3 lobes of the stigma are
constructed to receive them. This group is therefore, next to the
Apostasieæ, the least modified among the Orchids; in all the following
groups, one of the lobes of the stigma is differently developed from
the others, and there is only one stamen.--Terrestial Orchids.--~The
pollination of _C. calceolus_ is effected by the forcible entrance of
insects into the boat-shaped labellum (Fig. 319 _p’_) at _i_, and their
escape at _ex_ (in 2) where the anthers are situated; in this way the
stigmas will first be touched and then the anthers. The pollen-grains
are surrounded by a sticky mass in order that they may adhere to the
insects.~
=3.= NEOTTIEÆ. The majority are terrestrial Orchids with creeping,
sympodial rhizomes; the blades of the leaves are not detached from the
stem at joints, and have convolute vernation. The anthers do not drop
off, but persist in the withered condition; their _apex_ is brought
in contact with the rostellum (acrotonous Orchids). The pollen-grains
are united in _tetrads_, which, however, often hang loosely together
in pollinia, attached to a sticky part of the rostellum (“adhesive
disc”), so that they adhere to the insects, and are by them transferred
to the stigmas. _Spiranthes. Listera_; _Neottia_. ~_N. nidus-avis_
(Bird’s-nest) is brown (it has little chlorophyll) in colour, has
no foliage-leaves, and lives mainly as a saprophyte; the rhizome
is studded with unbranched, fleshy roots which may form buds at
their extremities.~--_Vanilla_ climbs by aerial roots. The fruit
is fleshy and hardly opens, or does so irregularly.--_Epipactis_,
_Cephalanthera_.--_Epipogon_ and _Limodorum_ are saprophytes without
chlorophyll.
[Illustration: FIG. 320.--A Flower of _Orchis maculata_ (front
view): a stamen; _b_ the cup; _n_ the stigmas; _x_ staminodes;
_sp_ the spur; _spe_ the entrance to it; _sm_-_sl_-_sl_ exterior
perianth-leaves; _pm_ the labellum, and _pl_-_pl_ the other 2
interior perianth-leaves. _B-E Orchis mascula_: _B_ lateral
view of the column; _C_ a pollinium with massulæ (_p_), caudicle
(_c_) and adhesive disc (_d_); _D_ caudicles with the cup (_r_),
front view; the latter is depressed so that the adhesive disc is
seen lying inside it; _E_ a pollinium, more highly magnified;
some massulæ are removed. _F Ophrys aranifera_: rostellum and
the base of the anther-loculus; an adhesive disc is seen on the
right.]
=4.= OPHRYDEÆ. Anthers 2-locular, not falling off, on a very short
column. The anther is united at _its base_ with the rostellum
(_basitonous_ Orchids, Fig. 320 _A_, _B_), while in all other Orchids
it is connected at the apex (acrotonous Orchids). The pollen-grains
in each loculus are united into small “masses” (massulæ), each of
which corresponds to a pollen-mother-cell in the anther, and which
hang together by elastic threads (Fig. 320 _C_, _E_). Each pollinium
is attached at the base by a stalk (caudicle) to an adhesive disc,
formed by the modified stigma (rostellum), and is easily liberated
from it (Fig. 320 _C_, _D_, _F_). The pollinium, which is formed in
an anther-loculus, together with its caudicle and adhesive disc, is
termed “pollinarium” (Fig. 320 _C_).--All Ophrydeæ are terrestrial with
_tuberous roots_, two of which are present in the flowering period, an
older one (from the preceding year) containing the nourishment for
the flowering-shoot of the year, and a young one which is intended to
contain the reserve material for the following year. Inflorescence
terminal.
_Orchis._ The lip has a spur; each of the club-like pollinia is
attached to its own adhesive disc, the discs being enclosed in a common
pouch formed by the rostellum (Fig. 320 _C_, _D_). ~Tubers ovate,
undivided: _O. morio_, _mascula_; tubers palmate: _O. incarnata_,
_maculata_, _majalis_.~--_Ophrys_; no spur, the two adhesive discs
are each enclosed in a separate pouch (Fig. 320 _F_).--_Anacamptis_
and _Serapias_ have one adhesive disc.--_Habenaria_, _Gymnadenia_,
_Platanthera_, _Herminium_, _Nigritella_, _Cœloglossum_, etc., have
naked adhesive discs (no rostellum).
=5.= EPIDENDREÆ. Acrotonous Orchids with deciduous anthers
(except _Malaxis_); 2-8 wax-like pollinia, with or without
caudicles; generally no adhesive discs. _Malaxis_ (the flower is
twisted through a complete circle, causing the labellum to be
turned upwards), _Sturmia_ and _Corallorhiza_[32] (Coral-root);
the latter has a creeping, coral-like rhizome _without roots,
and is destitute of chlorophyll_ except in the ovary. The
other two somewhat resemble the tropical Orchids in having the
lower internodes of the axis of the inflorescence tuberous.
_Liparis_; _Calypso_. Most of the genera are tropical epiphytes
and many have aerial, green tubers formed from one or more
stem-internodes; _Dendrobium_, _Eria_, _Phaius_, _Bletia_,
_Epidendrum_, _Cattleya_, _Lælia_, _Pleurothallis_, _Restrepia_,
_Masdevallia_, _Bulbophyllum_, etc.
=6.= VANDEÆ. These resemble the preceding but have only 2
wax-like pollinia in each anther, which are attached by a
caudicle to the adhesive disc of the rostellum. Nearly all are
tropical epiphytes. _Stanhopea_, _Catasetum_, _Maxillaria_,
_Oncidium_, _Vanda_, _Polystachya_, etc.
6,000 (10,000?) species. The majority live in the Tropics and
occur, especially, as epiphytes on trees or in the crevices of
rocks, to which they are attached by aerial roots. These _aerial
roots_, like those of Araceæ, are covered by several layers of
spirally-thickened cells (tracheides) which contain air and form
the velamen--an apparatus to absorb moisture from the air. The
roots have a white appearance when the cells are filled with
air, which changes to a greenish hue when they are filled with
water, the chlorophyll then shining through. They generally
have horizontal rhizomes; the ascending shoots, which bear the
foliage-leaves, may vary, but they very often swell and assume
the form of a tuber, which persists for several years fresh and
green after the leaves have fallen off (Fig. 321). _Vanilla_
is an exception (see above). Our Orchids are all terrestrial
(or marsh-plants); the largest number of species is found in
calcareous soils.
POLLINATION takes place principally by means of insects,
but self-pollination occurs in some. The lip serves as a
landing-stage for the insect visitors, which, on sucking the
honey, cause the adhesive discs, with the pollinia attached to
them, to adhere to their bodies (generally to the probosces)
and so carry them away to other flowers. In some species parts
of the flower are sensitive or irritable, which has some
connection with the pollination. Without doubt there are a great
many biological differences which are closely connected with
the infinite multiplicity of forms; Darwin (1862) has already
shown an enormous variety, never even dreamt of before, in the
European species. The genus _Catasetum_ has ♂-♀-and ☿-plants
with flowers of such different appearances that they have
been classed in various genera (_Myanthus_, _Monacanthus_).
_Platanthera_ is pollinated by hawk-moths; _Ophrys_, by flies;
_Epipactis latifolia_, by wasps; _Orchis_, by bees, especially
humble-bees, etc.
[Illustration: FIG. 321.--_Chysis bractescens._]
The DISTRIBUTION OF SEEDS is effected by the wind, the seeds
being so exceedingly small and light. Many species moreover have
peculiar, elater-like, fine, hygroscopic hairs in the ovary,
which eject the seeds in a manner similar to the elaters of the
Liverworts.
The USES are few, mostly as ornamental plants in conservatories.
The tubers of several _Orchis_-species are OFFICINAL; they
contain starch and mucilage and are used us “salep.” The fruits
of _Vanilla planifolia_ are used as condiments and differ from
other _Orchid_-fruits in being rather fleshy and in dehiscing
irregularly; the seeds are very small, shining and black.