To this family belong _only water- or marsh-plants_; _the endosperm is
wanting_, and they possess an embryo with a very _large hypocotyl_
prolonged downwards and often club-like. The perianth is often
differentiated into calyx and corolla; the flower is regular, and in
the first orders to be considered, may be reduced to the ordinary
Monocotyledonous type; there are, however, _usually found two_
3-_merous whorls of carpels_ (Fig. 282), and thus in all 6 whorls, or
again, the _number of carpels may be indefinite_; the number of stamens
also may be increased, either by the division of the members of a
whorl, or by the development of additional whorls. _Syncarps_,[26] with
nut or follicular fruitlets, are _very common_, for example, in the
first orders; in the last (Hydrocharitaceæ) the carpels are not only
united, but the ovary is even inferior.
The primitive type appears to be a hypogynous flower, similar
to that of the Juncaginaceæ or Alismaceæ, with several 3-merous
whorls, and free carpels, each with many ovules; the green
perianth in this instance being no doubt older than the coloured
ones. If we take a flower with this structure as the starting
point, then the family developes partly into epigynous forms,
partly into others which are so strongly reduced and exceptional
that it is scarcely possible to refer them to the ordinary
type. The family, through the peculiar _Zostereæ_, appears to
approach the Araceæ, in which _Potamogetonaceæ_ and _Najadaceæ_
are included by some authorities. However, the inclusion of
_Potamogeton_, and with it _Ruppia_ and _Zannichellia_, in the
Juncaginaceæ appears quite correct. It would scarcely be right
to separate _Zostereæ_ from these. Great stress has often been
laid upon the similarity with the Ranunculaceæ which is found
in the Alismaceæ, but it is scarcely more than an analogous
resemblance.
Order 1. =Juncaginaceæ.= The ☿, regular, _hypogynous_ flowers have
the _perianth_ 3 + 3, _sepaloid_, stamens 3 + 3 (with extrorse
anthers), and carpels 3 + 3 (free or united), of which last, however,
one whorl may be suppressed (in _Triglochin maritima_ all 6 carpels
are developed, in _T. palustris_ the inner whorl is unfertile).
Inflorescence long spikes. Embryo _straight_.--Marsh-plants with
radical, rush-like leaves, arranged in two rows, and often sheathing
and ligulate (“squamulæ intravaginales”); the inflorescence is a spike
or raceme.--_Scheuchzeria._ Carpels almost free; in each at least two
ovules. Follicles.--_Triglochin_ has long, fine racemes without bracts
or bracteoles; one ovule in each carpel. The carpels in the two native
species are united, but separate when ripe as a schizocarp, loosening
from below; they open along the ventral suture or remain closed; a
linear central column remains. ~The most reduced is Lilæa (1–2 sp.
Am.)--Protogynous. About 10 species. Temp. Fossils in Tertiary.~
Order 2. =Potamogetonaceæ.= The aquatic plants belonging to this order
are perennial, living entirely submerged, or with floating leaves, and
preferring still water. The leaves are alternate, in some linear and
grass-like, in others there is an elliptical floating blade, supported
by a linear submerged petiole. Axillary scales. The fruit is generally
a syncarp with _nuts_ or _drupes_; the _embryo is curved_, of very
various forms.
_Potamogeton_ (Pond-weed). The rhizome is creeping, sympodial (with two
internodes in each shoot-generation); the inflorescence is a terminal,
many-flowered spike, without floral-leaves; below it are found 2
foliage-leaves placed nearly at the same height, from whose axils
the branching is continued cymosely. The flowers are ☿, 4-_merous_,
naked, and consist only of 4 _stamens_, with the _connectives, broadly
developed_ at the back of the anthers, _resembling a perianth_,
and of 4 _free, sessile carpels_. They are common plants in fresh
water. ~The spike, during the flowering, is raised above the water.
Wind-pollinated and protogynous.--Closely allied is _Ruppia_ (Tassel
Pond-weed), in salt or brackish water. The spike has only two naked
flowers, each consisting of 2 stamens and 4 carpels. The stalks of the
individual carpels are considerably prolonged.--_Zannichellia_ (Horned
Pond-weed) is monœcious; the ♀-flower consists of 4 (2–9) carpels, with
membranous, bell-shaped perianth; long styles; the ♂-flower has 1 (-2)
stamens. _Althenia._~
_Zostera_ (Grass-wrack) is an entirely submerged, marine plant
with creeping rhizome (with displacement of buds) and strap-shaped
leaves. The flowering shoots are sympodia with displacement of the
axes (Fig. 281). The inflorescence is a peculiar, flatly-compressed
spike, on _one_ side of which the flowers are borne (Fig. 281). ~This
inflorescence may be considered, no doubt correctly, to be derived
from the symmetrical spike of _Potamogeton_ by strongly dorsiventral
development, and by a strong suppression of the floral parts taking
place simultaneously. Two rows of flowers are developed, but of these
one is so pressed into the other that apparently only one is present.~
Each flower consists of only 1 stamen and 1 carpel situated at the same
height (Fig. 281); the unilocular ovary encloses 1 pendulous ovule
and bears a bifid style. As regards the perianth (?) one leaf may be
present (_Z. nana_, Fig. 281 _D_). The pollen-grains are filamentous.
Pollination takes place under water. ~_Posidonia_ and _Cymodocea_ are
allied to these. About 70 species.~
[Illustration: FIG. 281.--_Zostera._ A Diagram of the
branching of the floral shoots: _I_, _II_ ... are the successive
shoot-generations, every other one being shaded; _g_{1}_
_g_{2}_ ... fore-leaves; _sp_{1} sp_{2}_ ... spathes for the
successive spikes. Each shoot is united for some distance with
the parent axis (indicated by the half-shaded internodes). Each
shoot commences with a fore-leaf turning towards the parent
axis, _g_; succeeding this is the spathe, _sp_; and then the
inflorescence. The fore-leaf supports a new lateral shoot.
_B_ Diagram of a shoot, _II_, which is borne laterally in the
axil of the fore-leaf _g_{1}_, on the shoot _I_, _g_{2}_ its
fore-leaf; _sp_{2}_ its spathe; _sti_ squamulæ intravaginales.
_II_ Is the spadix with stamens and carpels; _b_ a perianth-leaf
(or connective expansion, similar to those which occur in
_Potamogeton_). _C_ The upper portion of a young spadix with
development of flowers. _D_ Part of a spadix with 2 flowers; the
parts which theoretically belong to one another are connected by
a dotted line.]
Order 3. =Aponogetonaceæ.= Aquatic plants with tuberous stem. They
have a single, petaloid perianth (3–2–1–leaved), most frequently 6
stamens and 3(-6) carpels. Straight embryo.--About 15 species (Africa,
Madagascar, Tropical Asia and Australia).--_Aponogeton distachyos_ and
_A._ (_Ouvirandra_) _fenestralis_ are grown in conservatories; the
latter has lattice-like, perforated leaves.
Order 4. =Najadaceæ.= Only one genus _Najas_ (about 10 species);
annual fresh water plants with leaves in pairs and solitary, unisexual
flowers. The ♂ flower is remarkable in having a terminal stamen, which
has either 4 longitudinal loculi or 1 central one; on this account the
stamen of _Najas_ is considered by some authorities to be a stem and
not a leaf-structure. The unilocular gynœceum and the single, erect,
anatropous ovule are also terminal. Pollination takes place under the
water.
Order 5. =Alismaceæ.= The regular, _hypogynous_ flowers are in some
species unisexual by the suppression of either andrœcium or gynœceum;
they have a 6-merous perianth, _generally_ differentiated into 3
sepals and 3 petals; generally 6 _stamens in the outer whorl_ (by the
division of the 3; Fig. 282) and often several 3-merous whorls inside
these, and 6–∞ _free_ carpels arranged cyclically or spirally. Fruit a
syncarp.--Marsh- or water-plants with radical leaves and long-stalked
inflorescences.
=A.= _Butomeæ. Follicles with many seeds, which are borne on
nearly the whole of the inner surface of the cyclic carpels_ (as in
Nymphæaceæ). Embryo _straight_.--_Butomus_ (Flowering Rush, Fig.
282), has an umbel (generally composed of 3 helicoid cymes). _S_
3, _P_ 3, stamens 9 (6 + 3, _i.e._ the outer whorl doubled), _G_ 3
+ 3. ~_B. umbellatus_; creeping rhizome with triangular Iris-like
leaves.--_Hydrocleis. Limnocharis._~
[Illustration: FIG. 282.--Diagram of _Butomus_: _f_ bracteole.]
=B.= _Alismeæ._ Fruit achenes. Latex common (in the intercellular
spaces). The flowers are arranged most frequently in single or
compound whorls. Embryo _curved_, horse-shoe shaped.--_Alisma_ has
_S_ 3, _P_ 3, _A_ 6 (in 1 whorl, grouped in pairs, _i.e._ doubled in
front of the sepals), and 1 _whorl_ of 1-seeded achenes on a flat
receptacle. The leaves are most frequently radicle, long-stalked;
the lamina have curved longitudinal veins, and a richly branched
venation. _A. plantago._--_Elisma_ (_E. natans_) has epitropous
(turned inwards) ovules, whilst the ovules of _Alisma_, _Sagittaria_
and others are apotropous (turned outwards).--_Echinodorus_ (_E.
ranunculoides_) has a convex receptacle, carpels many, united and
capitate. _Damasonium_.--_Sagittaria_ (Arrow-head) has _monœcious_
flowers, several whorls of stamens and _spirally-arranged achenes_ on
a very convex receptacle. ~_S. sagittifolia_ reproduces by tuberous
buds formed at the end of long, submerged branches. The leaves, in deep
and rapidly running water, are long and strap-shaped, but in the air
arrow-shaped.~
Honey is secreted in the flower and pollination effected
by insects. _Alisma plantago_ has 12 nectaries. The
submerged flowers of _Elisma natans_ remain closed and are
self-pollinated. _Butomus_ has protandrous flowers. There are
about 50 species, which mostly grow outside the Tropics.--Uses
insignificant. The rhizome of some is farinaceous.
Order 6. =Hydrocharitaceæ.= This order differs chiefly from the
preceding in its _epigynous_ flowers. These are in general unisexual
(_diœcious_), and surrounded by a 2-leaved or bipartite _spathe_; they
are 3-merous in all whorls, but the number of whorls is generally
greater than 5, sometimes even indefinite. The perianth is divided
into _calyx_ and _corolla_. The ovary is _unilocular_ with parietal
placentation, or more or less incompletely plurilocular. The fruit
is berry-like, but usually ruptures irregularly when ripe. Embryo
straight.--Most often submerged water-plants, leaves seldom floating on
the surface. Axillary scales (_squamulæ intravaginales_).
_Hydrocharis._ Floating water-plants with round cordate leaves; S3, P3
(folded in the bud); ♂-flowers: 3 (-more) flowers inside each spathe;
stamens 9–15, the most internal sterile. ♀-flowers solitary; three
staminodes; ovary 6-locular, with many ovules attached to the septa;
styles 6, short, bifid. [The petals of the ♀-flowers bear nectaries at
the base. In this and the following genus the pollination is without
doubt effected by insects.] ~_H. morsus ranæ_ (Frog-bit) has runners;
it hibernates by means of special winter-buds.~--_Stratiotes_; floating
plants with a rosette of linear, thick, stiff leaves with spiny margin,
springing from a short stem, from which numerous roots descend into
the mud. Inflorescence, perianth, and ovary nearly the same as in
_Hydrocharis_, but the ♂-flower has 12 stamens in 3 whorls, of which
the outer 6 are in 1 whorl (dédoublement), and inside the perianth in
both flowers there are numerous (15–30) nectaries (staminodes?). _S.
aloides_ (Water-soldier); in N. Eur. only ♀-plants.--~_Vallisneria
spiralis_ is a tropical or sub-tropical plant, growing gregariously
on the mud in fresh water. The leaves are grass-like, and the plants
diœcious; the ♂-flowers are detached from the plant, and rise to
the surface of the water, where they pollinate the ♀-flowers. These
are borne on long, spirally-twisted peduncles which contract after
pollination, so that the ♀-flower is again drawn under the water,
and the fruits ripen deeply submerged.--_Elodea canadensis_ is also
an entirely submerged plant. The leaves are arranged in whorls on a
well-developed stem. Only ♀-plants in Europe (introduced about 1836
from N. Am). This plant spreads with great rapidity throughout the
country, the reproduction being entirely vegetative. _Hydrilla_,
_Halophila_, _Thalassia_, _Enhalus_.--In many of these genera the
number of whorls in the flower is remarkably reduced; for example, in
_Vallisneria_, in the ♂-flowers to 2: Pr 3, A (1-) 3, in the ♀ to 3: Pr
3, Staminodes 3, G 3.--About 40 species; Temp. and Trop.~