MONOCOTYLEDONOUS, OR ENDOGENOUS PLANTS.
THE structure, growth, and reproduction of the flower-bearing vegetation
offer objects of the highest interest, though very different from those
furnished by the flowerless class. The plants whose seeds have but one
lobe form a transition from the lowest to the highest class of
vegetables, and include those which furnish the principal articles of
food to man and animals. They are all flower-bearing, and consist of
numerous families of land and water plants. The most important are the
palms, which serve for food in tropical countries, the grasses, which
are cosmopolite and social, covering extensive tracts of country with
rich verdure, and including the Cerealia, which have been cultivated
from such remote antiquity, that the grasses from whence they were
derived are unknown. This monocotyledonous class includes, besides the
palms and cereals already mentioned, the sugar-cane, the bamboo and
other canes, together with reeds, rushes, screw pines, most garden
bulbs, the singular and beautiful race of Orchids, and a multitude of
other ornamental and useful plants.
The seeds of this class consist of a thin skin covering a mass of white
or green matter, which, when ripe, constitutes the starch and flour in
the Cerealia. Within that matter lies a fleshy lobe with the infant
plant imbedded in one corner. The same general structure may be traced
throughout the class, though the lobe with its embryo in the surrounding
matter may be covered with a hard coat, as in the cocoa-nut, or inclosed
in a shell covered with a rich fruit, as in the date. The infant plant,
as it lies in its seed lobe, has a stem with a leaf bud or plumule at
one end, and an embryo root at the other; and, as soon as the seed
begins to germinate, it is nourished by the conversion of the starch of
the surrounding matter into sugar. At first the whole young plant is of
cellular tissue, but, as soon as the seed leaf appears above the soil,
it decomposes the carbonic acid of the atmosphere, absorbs the oxygen,
and consolidates the carbon. Then cords of fibro-vascular tissue are
formed within it, which converge at its base, and unite with the radicle
of the young plant to form the solid centre of the underground stem,
from whence the real roots descend, like slender white cords, which
supply the plant with food. The seed lobe having set the current of the
cell sap in motion disappears, and the aërial or above-ground stem,
which now consists of pith, sap, wood, and skin or bark, grows rapidly
in length to its first node, or thickened part, where the first leaf or
leaves appear, for the first leaves generally appear at the top of the
stem in this class, and are annually succeeded by others rising above
them from the top of the elongated stem, as in the palms and grasses.
The stem or axis of a palm is a cylinder with a graceful plume of leaves
at its summit, and a cone of white roots at its base. A section of the
stem perpendicular to its axis exhibits a mass of cellular tissue with a
number of dark-coloured spots irregularly scattered throughout the whole
with the exception of an envelope of dense cellular tissue, which forms
a kind of bark. The dark spots are the sections of bundles consisting of
two or three very wide and large vascular ducts enclosed in extremely
fine woody fibre and spiral vessels closely pressed together. These
bundles after having formed the ribs and footstalks of the leaves, enter
the upper part of the stem, approach towards its centre, bend down for a
short distance, then turn towards the exterior, interlace with the
bundles of the previous year, which have followed a similar course, and
make a circle of rough marks on the surface of the stem. The stem then
increases in length, and a new plume of leaves crowns its top; the same
process is repeated; and, as this goes on indefinitely, the lower part
of the stem is rough, and sometimes even rugged, as that of the Palmyra
palm. In consequence of this manner of growth, a perpendicular section
of a stem shows a series of curves intersecting each other and
originating in points gradually ascending as the palm grows in height.
These curves proceed from every point of the circumference of the stem,
present their convexities to the centre, and bend round, and enter the
leaves, which are long, lanceolate, and often pinnate with veins running
through them longitudinally. In some palms the greater part of the
centre of the trunk is altogether cellular tissue, but each species has
an arrangement of its own. Some palms, as the date palm, are diœcious,
one plant bearing male, the other female flowers. In these cases, the
pollen is carried from one tree to the other by the wind or by insects,
and, as much is lost in the transit, there is always more produced than
is required to fertilize the female flowers.
The Graminaceæ are very numerous, but the general structure is virtually
the same, whether it be a simple herbaceous grass, a sugar cane, or a
bamboo. The stem of the grasses is jointed, and furnished with long
lanceolate leaves, springing alternately to the right and left of each
successive joint, and the parts of the stem between the joints are
embraced and nearly surrounded, as by a sheath, with the expanded bases
of the leaves; for, in this class of plants, one growth always springs
from the interior of that which precedes it. In youth the stems of the
grasses are solid, and the bundles of woody tissue which surround the
pith are parallel to one another and to the axis of the stem; but, at
the joints, they are turned aside and are condensed into a node or
joint, where they enter into a new arrangement, and pass into the
lanceolate leaf, to form its longitudinal ribs. The compression of the
fibro-vascular tissue is so great as to form a septum across the
interior of the stem as the node. This is the case in all the grasses;
for the pith, which is of moist green cellular tissue when they are
young, soon disappears, so that a full grown grass is a hollow stem
articulated at intervals by solid joints, from whence the long narrow
leaves spring alternately, and as the bases of the leaves are continuous
with the stem, they do not fall off when they wither.
In the majority of cases, the inflorescence of the Graminaceæ consists
of a pistil and three anthers; but in the Anthoxanthum odoratum, or
sweet-scented vernal grass, and some others, there are but two anthers,
while some few species have more than three. In general, the flowers are
hermaphrodite; but in certain genera they are monœcious, as in Zea and
Zizania; and in others they are polygamous, as in Andropogon and
Sorghum. The grasses are, for the most part, low and herbaceous; but the
Arundo Donax of Southern Europe, and the sugar cane and bamboo of the
tropics are lofty strong-growing plants, the latter reaching as much as
50 or 60 feet in height. The stems and leaves of the grass family are
strengthened by silex; many of them are so entirely coated with it, that
their leaves are as sharp as the edge of a knife. It gives hardness to
the beards of wheat and barley, and is often found in concrete masses,
called tabasheer, in the joints of bamboos, which in the Indian jungles
have been set on fire by the friction of their silicious coats during a
gale of wind. As no solid matter can enter the roots of a plant, the
silex must be absorbed in a state of solution; and, as it coats the
surface, which is full of pores, the liquid is removed by perspiration,
and the silex is consolidated. The whole of this family abounds in
sugar, and its farinaceous products are too well known to require any
notice.
The grasses seem to have been the means of revealing the earliest dawn
of plant life, for the Hon. Sidney Osborne discovered that the
colourless protoplasm, or organizable liquid extracted from the roots of
young wheat, produced spontaneously and simultaneously double ovate
vesicles, or cells, such as are found in the roots themselves; and in
that liquid, though hermetically sealed in glass tubes, the formation of
these vesicles or cells was as active after six months as in the liquid
freshly taken from the young plant. Mr. Osborne believed these vesicles
to be the earliest organisms of plant life, and that this is the direct
and prevailing mode of production of the embryo. This accords with the
observations of Messrs. Wenham and Devey.
Many other very remarkable plants belonging to the Monocotyledons might
be mentioned, as the Dracæna Draco, or Dragon tree of Teneriffe, one of
the most ancient trees existing; the Pandanus, or screw pine, with its
aërial roots, indigenous in the islands of Oceania; and the Zostera, or
sea wrack, the only flower-bearing plant except one that inhabits the
ocean; their flowers are minute and bisexual, are rarely produced, but
they cover large areas with long grassy leaves. Bulbous plants, and the
Orchidaceæ, the most splendid ornaments of our gardens and hothouses,
are members of this class. Of the former, the snowdrop, crocus,
colchicum, arum, hyacinth, narcissus, tulip, and lily form a group of
singular beauty.
A bulb is merely a subterranean stem remaining permanently in the
condition of a bud. It consists of a disc, or conical plate, which is
the point of growth whence the flower-bearing stem and the leaves
spring. These are surrounded by leafy scales of a fleshy character
overlapping one another. The Liliaceæ and the Amaryllidaceæ contain many
of the most remarkable and beautiful bulbous plants known in gardens. To
the former order belong the hyacinth, tulip, and onion, in which the
growing point is surrounded by a series of fleshy tunics, each of which
encloses its predecessor; as also the lily itself, in the bulb of which
the growing point is enclosed in fleshy scales imbricated in rows one
above the other. One or two circles of roots descend from the
circumference of the disc in the form of slender, soft, white cords or
threads, with a spongy termination of cellular tissue to imbibe water
and other liquids for the nourishment of the plant. These bulbs are
reproduced by buds, or offsets, developed in the axils of the fleshy
scales, which fall off either the first or second year of growth.
Offsets are also produced by the arum, crocus, and meadow saffron or
colchicum, whose bulbs form a solid mass, and are called corms. These
offsets merely reproduce a _facsimile_ of the parent. In these and all
flowering plants fructification must take place before a new form or
variety can be expected.
The Orchids surpass every plant in the vegetable world, for the variety
of means employed by nature to continue the race. The blossoms are, for
the most part, at once both male and female, though some plants are
diœcious, but, except in rare instances, as in that of the Bee Orchis,
not a blossom can be fertilized without the aid of insects. The flowers
of the Orchideæ are constructed upon a fixed plan, which can be traced
through the innumerable variety of beautiful, singular, and often
grotesque forms which they assume. A corresponding variety is exhibited
in the admirable contrivances and adaptations which enable insects to
detach the pollen from one blossom, and carry it to fertilize another
blossom.
[Illustration: Fig. 72. Orchis mascula:—A, side view of flower, with a
portion cut away; B, section through one side of rostellum, with
included disc and caudicle of pollinium; _a_, anther; _r_, rostellum;
_d_, disc.]
The British Orchideæ belong mainly to three natural groups: the Ophreæ,
which comprise the Orchis, Ophrys, and other common Orchids; the
Neotteæ, comprising Epipactis, Neottia, Spiranthes, &c.; and the
Malaxeæ, represented by Malaxis. Orchids have white, fibrous roots, and
many of them a pair of fleshy tubercles. From thence a straight stem
springs up, ending in a spike of blossoms, each of which is attached to
the stem by a twisted stalk, containing the ovarium, as represented in
fig. 72. Ribbed lanceolate leaves rise from the roots, and some are
attached sparingly to alternate sides of the stem. The calyx is formed
of three pointed sepals, one perpendicular, the others horizontal.
Instead of being green they are usually coloured. Sometimes the whole
plant is green. One petal (fig. 72) is much larger than the others,
occasionally assuming the most extraordinary forms. It is called the
labellum, or lower lip; it secretes nectar, a sweet juice, to attract
insects, and is often produced into a long, hollow, spur-like nectary,
as in fig. 72 _n_. On that side of the spur which is opposite to the
labellum, the organs of reproduction are so placed, that an insect
alighting upon the labellum cannot insert its proboscis into the nectary
tube to eat the honey, without touching them.
[Illustration: Fig. 73. Orchis mascula:—Front view of flower, with
sepals and petals removed; _a_, anther; _r_, rostellum; _s_, stigma;
_l_, labellum.]
Mr. Darwin, in his admirable work on the ‘Fertilization of Orchids,’
assumes the Orchis mascula as a type for explaining the mechanism of the
reproductive organs of the Orchidaceæ generally. Fig. 73 represents a
front view of the flower with all the sepals and petals cut off, except
the labellum, or lip; and fig. 72 is a side view of the same, with the
near half of the labellum cut away, as well as the upper portion of the
near side of the nectary, or spur.
In all common Orchids there is only one stamen, which is confluent with
the three pistils, to form what is called the column. The edges of the
labellum are attached to the sides of the column, leaving a space known
as the chamber or mouth of the nectary, and thus the mouth has the
column on one side, and the labellum on the other. Although the three
pistils are united into one, the three stigmas are not; for the stigma
of the upper pistil is transformed into a pouch-shaped viscous mass
(_r_, fig. 73), called the rostellum, bearing no resemblance whatever to
a stigma, while the other two stigmas form a bi-lobed confluent mass or
stigma below it, through which the pollen grains fertilize the ovary,
which latter forms the apparently twisted stalk, by which the blossom is
attached to the stem. In fact, the rostellum (_r_) projects into the
mouth of the nectary, and overhangs the confluent stigmas (_s s_). The
lowest and narrowest part of a hood-shaped anther (fig. 73 _a a_,) is
attached to the back of the rostellum, and consists of two rather widely
separated oblong cells, which open longitudinally in front. Each cell
contains a pollen-mass, or pollinium. Fig. 74 represents one with its
club-shaped mass of pollen grains, its stalk, and viscid disc. These
objects are seen in situ in fig. 73. A pollinium consists of a number of
wedge-shaped packets of pollen grains held together by exceedingly
elastic fine threads, as in fig. 75; the packets unite into the
club-shaped head, and the threads form the stem; the viscid discs are
formed by the rostellum.
[Illustration: Fig. 74. Pollinium of Orchis mascula:—_p_, pollinium;
_c_, caudicle; _d_, disc.]
[Illustration: Fig. 75. Pollen grains of Orchis mascula.]
At an early period of growth the rostellum consists of a mass of
polygonal cells full of brownish matter, which soon resolve themselves
into two balls of an extremely viscid semifluid substance void of
structure. These two discs are flat on the top, and rounded below. They
lie quite free within the rostellum, except at the back, where each
viscid disc firmly adheres to a small portion of the exterior membrane
of the rostellum. The ends of the two stalks of the pollinia, or
granular masses, are strongly attached to these little discs, or balls.
[Illustration: Fig. 76. Pollinia of Orchis mascula, showing front view
of the discs and caudicles within the rostellum:—_r_, rostellum; _c_,
caudicle; _d_, disc.]
At first the membrane forming the exterior surface of the rostellum is
continuous; but as soon as the flower opens, the slightest touch causes
it to rupture in such a manner as to set free from it, and from one
another, the little discs, while at the same time the anther cells
themselves split in a longitudinal direction from top to bottom. In this
state none of the organs are changed; they are merely set free, and
ready for change, yet maintaining their normal positions. The labellum,
which is the largest petal of the flower, lies on one side of the
nectary tube, and the rostellum projects into it on the other; hence, if
an insect alights on the labellum, and pushes its head into the tube, in
order to reach the honey in the nectary with its proboscis, it cannot
fail to touch and depress the rostellum (see fig. 76), so that one or
both of the viscid discs carrying the pollinia will stick to it, and as
the anther cells are open in front, when the insect withdraws its head,
one or both of the pollinia are drawn out of their cells, and stick
upright on it like horns. In that position they never could fertilize
any blossom which the insect might afterwards visit; but Mr. Darwin has
shown that their position is changed by a contrivance that is not
surpassed in beauty in the whole vegetable world. While a pollinium is
upright on the head of the insect, the little viscid disc which supports
it contracts on being exposed to the air, so as to cause the pollinium
to sweep through ninety degrees towards the apex of the insect’s
proboscis, and this is accomplished in about thirty seconds, the time an
insect would take to fly to another flower. The pollinium in this
position would exactly strike the surface of the stigma when the insect
inserts its proboscis into the nectary of a flower.
It was long ago noticed by Robert Brown, that the stigma is very viscid,
but not so viscid as when touched to pull the whole pollinium off the
insect’s head, yet sufficiently viscid to break the elastic threads by
which the packets of pollen grains are tied together, and leave some of
them on the stigma. Hence a pollinium attached to an insect can be
applied to many stigmas, and fertilize them all. Mr. Darwin mentions
having seen the pollinium of Orchis pyramidalis adhering to the
proboscis of a moth with the stalks alone left, all the packets of
pollen having been left glued to the stigma of the flowers successively
visited. It appears that insects, for the most part, only remove one
pollinium at a time, and that the rostellum returns to its normal
position to prevent the viscid matter of the discs of the remaining
pollinia from being exposed to the air.
[Illustration: Fig. 77. Orchis pyramidalis:—Front view of flower with
upper sepal and petals removed; _a_, anther; _s_, stigma; _r_,
rostellum; _l_, labellum.]
The Orchis pyramidalis is considered by Mr. Darwin to be the most highly
organized species of the British Orchids he has examined. It has sharp
leaves, and a close pyramid of white or rose-coloured blossoms. The
upper sepal and the two upper petals form a hood, protecting the anther
and stigmatic surfaces from the weather. In fig. 77, a front view of a
blossom, these are cut off as well as in fig. 78 _a_, which represents a
side view of the same blossom deprived of half the labellum, and the
upper part of the nectary, or spur. The labellum is long, with three
equal entire lobes, and is produced into an awl-shaped spur, or nectary.
It has a small ridge on each side of its narrow base to guide the
proboscides of insects to the mouth of the nectary, which, besides being
small, is partially closed by the pouch-shaped rostellum, which is
placed very low in this species, and is flanked on each side by a
distinct stigmatic rounded surface (fig. 77 _s s_). The rostellum (_r_)
is hollowed out on its under-side in the middle, and is filled with a
fluid. Instead of two little viscid discs or balls, as in the Orchis
mascula, there is but one which is saddle-shaped (figs. 78, 79),
carrying on its nearly flat top or seat, the two stalks of the pollinia,
of which the two truncated ends firmly adhere to its upper-surface.
Before the membrane of the rostellum ruptures, the saddle-shaped disc
forms part of its continuous surface. The upper membrane of the disc is
rather thick; it is lined with a layer of highly adhesive matter formed
within the rostellum.
[Illustration: Fig. 78. Orchis pyramidalis:—Side view of flower, with a
portion cut away; _a_, anther; _r_, rostellum; _l_, labellum; _n_,
nectary.]
[Illustration: Fig. 79. Disc of Orchis pyramidalis seen from above, with
one pollinium, flattened by force.]
[Illustration: Fig. 80. Pollinia of Orchis pyramidalis, attached to
saddle-shaped disc.]
[Illustration: Fig. 81. Pollinia of Orchis pyramidalis with the disc
contracted.]
[Illustration: Fig. 82. Pollinia of Orchis pyramidalis:—_a_, as
withdrawn by the insertion of a needle; _b_, as after the second
contraction.]
When the flower opens, the saddle-shaped disc is set free by the rupture
of the membrane of the rostellum. Then the rostellum, which projects
into the minute round orifice of the nectary, is easily depressed by the
proboscis of a moth, and as the now naked and sticky under-surface of
the saddle-shaped disc is uncovered, it adheres to the proboscis, and is
withdrawn with it into the air, carrying the two pollinia (fig. 80) on
its exterior surface. Almost instantly the saddle is exposed to the air,
a rapid movement takes place. The two flaps curl inwards (fig. 81), and
embrace the proboscis, and the pollinia, at first parallel, become
divergent. A second movement now takes place, which causes the divergent
pollinia (fig. 81), which are at right angles to the proboscis, to sweep
through ninety degrees towards the tip of the proboscis, so as to become
depressed, and lie on each side of it. Hence, when a moth pushes its
proboscis between the guiding ridges of the labellum into the nectary of
another flower, the two thick ends of the pollinia will exactly strike
against the two stigmas. These stigmas are so viscid that they hold and
rupture the elastic threads which bind the packets of pollen grains
together, and some dark green grains are seen even with the naked eye
remaining on the two white stigmatic surfaces. In the Orchids, as in all
other flowering plants, tubes sent out by the pollen grains penetrate
through the stigmas, and fertilize the ovules in the twisted ovary. The
double movement of the pollinia, while on the moth’s proboscis, is owing
to the rapidity with which the viscid matter contracts and dries. Both
butterflies and moths frequent this Orchis. Mr. Darwin enumerates
twenty-three species of these insects which he had seen with the
pollinia of the Orchis pyramidalis attached to their proboscides. Many
had two or three pairs; the proboscis of the Acherontia had seven, and
that of the Caradium had no less than eleven of these saddles attached
at regular distances from top to bottom of its proboscis. Few of the
species of Orchis are visited by bees.
On account of some resemblance in form, the Ophreæ are named after
insects. The Fly Ophrys differs in no material respect from the other
Orchids. The stem or caudicle of the pollinium, instead of being
straight, as in the Orchis mascula, is doubly and almost rectangularly
bent. The upper membrane of the disc to which the stalk of the pollinium
is fixed, being the summit of the rostellum, is exposed to the air, and
becomes dry when the flower opens, consequently the disc, though viscid
enough on its under-side to stick to an insect’s head, is incapable of
shrinking, and causing that depression of the pollinium, characteristic
of all the species of Orchis. The labellum has no spur, but at its base,
just below the stigma, there is a deep depression representing the
nectary; and as the pollinia, which cannot be shaken out of their cells,
or pouches, are certainly, though rarely, extracted, Mr. Darwin
conceives that small insects crawl along the labellum to its base,
strike against one of the pouches, extract a pollinium, and fly with it
sticking on their head to another blossom, and that while bending their
heads into the hollow at the base of the labellum, the pollinium, owing
to its doubly bent stalk, strikes the sticky stigmatic surface, and
leaves pollen grains on it. There can be no doubt that this plant is
visited occasionally by insects, as it cannot be fructified without
them; but it is scentless, and as no nectar has as yet been found in it,
their motive for visiting it is unknown.
[Illustration: Fig. 83. Epipactis palustris:—Side views of flower, with
lower sepals cut away: A, with lip in natural position; B, with lip
depressed as by an insect.]
The fructification of the Ophrys apifera, or Bee Ophrys, is independent
of insects, for the stalks of the pollinia are extremely long, thin, and
flexible; and although their viscid discs still remain in their pouches,
as soon as the flower expands, and the anther cells or pouches open, the
heavy thick ends of the pollinia fall out of their cells, and hang
freely down in the air exactly opposite to the stigmatic surface. A
breath of air is sufficient to make them vibrate, strike the stigma with
their pollen mass, and leave pollen grains on its sticky surface.
[Illustration: Fig. 84. Epipactis palustris:—C, side view of flower,
with sepals and petals and half the labellum removed: D, front view of
column: _a_, anther; _r_, rostellum; _s_, stigma; _l_, labellum.]
The great tribe of British Neotteæ is characterized by a free anther
standing like a hood behind the stigma; the pollen grains are tied
together with threads, and attached to a viscous cap lying on the top of
the rostellum. The Epipactis palustris is a type of this group of
orchids. Its spike is short, and the pink blossoms stand out
horizontally from the stem on long ribbed footstalks, which contain the
ovaries. Fig. 83 A is a side view of the flower in its natural position,
with the lower sepals alone removed. The labellum, or lowest petal, is
interrupted in the middle by a kind of flexible hinge: the basal part is
a cup-shaped trough, at times abounding in nectar; the extreme part is a
wavy leaf (fig. 83 B). The entrance to the nectary cup is nearly closed
by the hood and the large anther; but, on account of the elasticity of
the hinge, the weight of an insect is sufficient to give access to the
nectar; but no sooner is the labellum relieved of the weight, than it
springs up into its natural position, and the insect creeps backwards
and comes out at the top of the flower with the viscous cap clasped
round its proboscis, and the pollen grains attached to it ready to
fertilize another blossom. In fig. 84, C represents a section of the
Epipactis, and D is a front view of the column.
[Illustration: Fig. 85. Listera ovata:—Side view of flower, with sepals
and petals cut away: _a_, anther; _col_, summit of column; _p_, pollen;
_r_, rostellum; _s_, stigma; _l_, labellum; _n_, nectar-secreting
furrow.]
Of all the British Orchids, the Listera ovata, or Twayblade (fig. 85),
has the most curious structure. It grows in woods and pastures, has a
creeping root, oval leaves, a downy stem, and yellowish green flowers.
Fig. 85 represents a lateral view of a blossom, with all the sepals and
petals removed, except the labellum. In this plant the rostellum (_r_)
is large, thin, convex in front, concave behind, and arches over (_s_)
the stigmatic surface. When the flower is full blown, the anther cells
(_a_) are already open, and the naked and friable pollen grains united
by a few threads, which form the pointed tips of the pollinia, rest upon
the concave back of the rostellum. The labellum, which is contracted at
the base, is exceedingly long, hanging down like a narrow ribbon. It is
divided half-way up, and furrowed along the middle, from the bifurcation
close up to the base of the stigmatic surface (_s_). The borders of the
furrow are globular, and secrete much nectar. The rostellum is
internally divided into a series of longitudinal cells, or chambers,
which contain and expel viscid matter with violence on the slightest
touch, and the viscid matter sets hard in two or three seconds, and soon
assumes a purplish brown tint. So exquisitely sensitive is the
rostellum, that a touch from the thinnest human hair suffices to cause
the explosion. As the pointed tops of the loose pollinia lie on the
crest of the rostellum, they are always caught by the exploded drop.
This never fails. So rapid is the explosion, and so viscid the fluid,
that it is difficult to touch the rostellum with a needle quick enough
not to catch the pollinia already attached to the partially hardened
drop, and consequently the slightest touch of any small insect which
enters the flower, suffices to explode the rostellum, and the pollinia
which attach themselves to its proboscis are carried by it to the next
flower to adhere to the viscid stigma and fertilize it. Mr. Darwin has
seen two Hymenopterous insects retreat from one of these plants with
bright yellow pollinia on their heads, and Mr. C. K. Sprengel saw an
insect of that kind leave pollen upon a stigma. The action and structure
of Neottia Nidus-avis is almost identically the same as that of Listera
ovata.
The Malaxis paludosa, or Bog Malaxis, the smallest of British Orchids,
is a rare plant, and differs from all of them in having its labellum
turned upwards instead of downwards. Its lower margin clasps the column,
making the entrance into the flower tubular. In this orchis the upper
sepal and two upper petals are reflexed, to allow insects freely to
visit the flower. In many orchids the labellum is properly directed
upwards, but assumes its usual position as the lower lip by the twisting
of the ovarium, or pedicel, of the flower. In the Malaxis paludosa,
however, the twisting has been carried to such an excess, that the
flower occupies the same position it would have held if the ovarium had
not been twisted at all, and which the stalk ultimately assumes when
ripe by the process of gradual untwisting. This little plant belongs to
a genus distinguished by having a movable and deciduous anther, and is
one of the British orchids that have allied forms among the exotic
genera. All Dr. Lindley’s vast tribes of Epidendreæ, and the still more
numerous and splendid Vandeæ, have not a single British representative.
The structure of the exotic orchids is often very complicated, and they
possess many properties unknown in the British genera. Their labellum,
or lower lip, is so varied, and sometimes so singular, that it baffles
description; besides, it often possesses peculiar motions, sometimes
from structure, at other times from irritation. In the Bolbophyllum
Rhizophoræ, the labellum is attached to the column by a very narrow thin
strap, elastic as india rubber, which oscillates in a singular manner;
while that of the B. barbigerum has a beard of fine hairs in almost
constant agitation. The irritability of the labellum in many of the
allied forms of the orchids is one of their remarkable properties; the
slightest touch sets them into motion. The Australian genus Caleana
possesses it in the highest degree; for when an insect settles on its
labellum, it suddenly shuts up against the column, and encloses it, as
it were, in a box.
Some of the exotic orchids have a pseudo-diœcious character. Thus some
species of Catasetum have two long horns, or antennæ, attached to the
rostellum, which stand over the labellum, and the pedicels of the
anthers are fastened down in a curved position; so, when an insect
alights upon the labellum and touches the antennæ, the excitement is
conveyed by them to the rostellum, the attached edge of the disc of the
anthers is ruptured, and they straighten themselves with such force,
that not only do they drag the balls of pollen and anther cells from
their places of attachment, but the whole pollinium is jerked forward
over and beyond the tips of the horns, to the distance of two or three
feet. The insect, disturbed by so sharp a blow, or after having eaten
its fill, flies with the pollen adhering to it to fertilize the female
plant, which differs from the male in having no antennæ. Thus the agency
of insects is as requisite to fertilize these semi-diœcious as
hermaphrodite orchids.
The Vanilla, which is cultivated for its aromatic pods in Tahiti,
Bourbon, and the East Indies, does not bear fruit without artificial
aid, which shows that the American insect, which fertilizes it in its
own native home, is not indigenous in the places mentioned. It appears
that many exotic orchids require a less elevated temperature than has
hitherto been supposed.
The form and position of the nectary are exceedingly varied. In certain
species, both of the native and tropical orchids, they are always dry;
but Mr. Darwin has discovered that, in these cases, the walls of the
nectaries are thick and formed of two coats, and that a liquid is
contained between them, to which the insect penetrates by piercing the
inner wall. The exotic orchids are, for the most part, larger, and
require larger insects to fertilize them than our small ones, whose
organs are generally microscopic. A curious instance, both of this and
of the extraordinary form of the nectary, is found in the Angræcum
sesquipedale, a Madagascar orchid, with large six-rayed flowers, like
stars formed of snow-white wax. It has a green whip-like nectary,
sometimes as much as a foot long, and, from the structure of the plant,
it appears that the pollinia never could be withdrawn, until a large
moth, with a wonderfully long proboscis, attempts to drain the last drop
of nectar from the bottom of the nectary.
Notwithstanding the vast diversity in the form of the orchids, they are
homologous in their general structure. Mr. Robert Brown was the first to
observe, that an orchid flower consists of fifteen organs, arranged
alternately, three within three, in five whorls. This accords perfectly
with a system of spiral vessels developed at an early age in all
orchids. Mr. Brown and three of the greatest living botanists[78] have
each traced the spirals from six bundles surrounding the ovary in the
footstalk to the different organs of the flower, and have found that
they consist of fifteen bundles corresponding to the fifteen organs of
the flower; namely, three sepals, three petals, six anthers in two
whorls (three of which are rudimentary), and three pistils, with their
stigmas: these are arranged in alternate whorls, and undergo many
modifications. The pistils and anthers are confluent, and form the
column; the uppermost stigma becomes the rostellum; the three inner
anthers are rudimentary, one forming the front of the column, and the
other two forming the membranous sides of the hood which protects the
pollen; and, lastly, the two lower anthers are united to the sides of
the lowest petal, and form the labellum, which accounts for its great
size, frequent tripartite form, and peculiar manner of attachment.
This system has been wonderfully modified to produce the varied groups
of extra-tropical and brilliant forms of exotic orchids, yet it can be
traced in all.