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In this article we will discuss about the Classification of Helobieae. According to Engler and Prantl, Helobieae consists of five families:- 1. Potamogetonaceae 2. Najadaceae 3. Alismaceae 4. Butomaceae 5. Hydrocharitaceae.
Family # 1. Potamogetonaceae:
Potamogetonaceae are the aquatic herbs with perennial rhizome growing in fresh water or in marshes, often marine. Leaves are submerged or floating, alternate or sometimes opposite; petiole sheathing at the base; sheath free or partially united to the petiole; floating leaves leathery, submerged leaves membranous; minute scales are present in the axils.
Flowers are actinomorphic, bisexual or unisexual, in axillary pedunculate spikes; the peduncle is surrounded by a sheath at the base. Bracts are present or absent and the sheath is more like a spa the in zostera where the spike is turned into a spadix.
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In Ruppia the spike is reduced to 2 flowers only and the peduncle, which remain at first enclosed in the broad swollen sheaths of the uppermost foliage leaves elongates to expose the flowers.
In Posidonia the inflorescence is a compound spike and the spike-branches are enclosed at base by the sheaths of leafy bracts. The lower flowers are bisexual and upper ones are male. In zostera and Phyllospadix the flowers are on flattened spadices enclosed in the spathe-like sheaths of the uppermost leaves.
Phyllospadix is dioecious while zostera has bisexual flowers borne on the upper surface of the membranous spadix. There are two rows of alternating stamens and carpels. Each stamen and the carpel opposite represent a single flower.
On the margin of the spadix there is a row of scales which are considered as bracteoles or as prolongations of the connectives. According to some authors flowers of gostera are unisexual. Zannichellia is monoecious. Groups of 4 or fewer flowers form a cymose umbel surrounded by cup-like entire spathe. The males are long-stalked and the females are short-stalked.
Perianth is usually absent:
In Althenia the male flowers have a short 3-toothed perianth. In Potamogeton the sessile anther has a tail-like projected structure at the base of the connective. This structure is considered by some as an extended part of the connective and the flower is perianthless. Others consider that it is nothing but a sepal on which the sessile anther of the stamen is situated.
There being 4 stamens, the perianth is considered to consist of 4 sepals. The anther being extrorse, it is very unlikely that the connective is extended at the base in the manner. This is also proved by anatomical evidence the vascular supply being separate for the stamen and the perianth segment.
A flower of Potamogeton may be described as consisting of a whorl of 4 sepals, 4 stamens and 4 sessile carpels. Sepals are valvate and shortly clawed. Stamens inserted on the claws of the sepals have sessile 2-locular extrorse anthers; pollens globose.
Gynoecium consists of 4 free carpels, each 1-locular, sessile, superior, containing a single, campylotropous ovule, attached to the inner angle of the carpel; stigma sessile or style very short.
Kunth described the flowers of Potamogeton as unisexual, the male consisting of a single stamen subtended by a bract and the female with a single carpel without any bract or perianth. According to this view the so-called bisexual 4-merous flower is a cluster of 4 flowers and a part of a compound inflorescence. This has been supported by Niki (1937) and also by Uhl (1947) but is not accepted by most botanists.
The bisexual flowers of Ruppia consist of 2 stamens and 4 sessile carpels. Stamens have short broad filament and 2-locular reniform anther, the loculi separated by broad connective. Gynoecium consists of usually 4 carpels, sessile at first but with a long, often spirally coiled stalk at fruiting stage; style is short with peltate or umbonate stigma; ovule is solitary and pendulous in each carpel and campylotropous.
A flower of Zostera consists of a stamen and a carpel only. Each stamen is represented by 2 half anthers separated by a connective and lying flat on the axis of the spadix; the pollens are confervoid. Ovary has a single pendulous ovule and a lateral persistent style and a forked deciduous stigma.
In Cymodocea the male flower consists of 2 longitudinally jointed anthers that represent 2 stamens. The female flower has 2 free carpels, each containing a pendulous ovule; the short style terminates in 2 long, narrow stigmas. The male flower of Zannichellia is long pedicelled; stamen one or two.
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The female flower has a single short-stalked carpel with a solitary pendulous ovule; style short with a spathulate or umbonate stigma. Fruits are druplets or achenes, solitary or in clusters. The seed has a thick and tough testa and a strongly curved embryo.
The family contains 11 genera and about 120 species occurring mostly in the tropical countries of the world. None is of any importance economically. Potamogeton is by far the largest genus having 90 species of which 16 are found in India, the common among them being P. indicus Roxb., P. natans L. and P. Crispin L. Ruppia rostellata Koch and zannichellia palustesis Linn, are also Indian.
The family is subdivided into 5 tribes, viz.,
Tribe I Zostereae: Bisexual or unisexual flowers in compressed spike, saline.
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Tribe II Posidonieae: Bisexual flowers in branched spike; saline.
Tribe III Potamogetoneae: Bisexual flower in simple cylindrical spike; in fresh and brackish water.
Tribe IV Cymodoceae: Unisexual flower, solitary or in cymes, pollen thread-like; saline.
Tribe V Zannichellieae: Unisexual flowers, solitary or in cymes, pollen globose; in fresh or brackish or salt water.
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In the old system of classification of Bentham and Hooker Potamogetonaceae was not treated as a family distinct from Najadaceae. Engler established this new family to accommodate all the genera of Najadaceae excepting only the genus Najas.
From the description of Potamogetonaceae as given here it is observed that the genera under this family of Engler are very variable in characters to be satisfactorily included in a single family and it is more a heterogenous group than a natural family.
Hutchinson further reduced Potamogetonaceae into a family of 2 genera only and created 4 new families, viz., Posidoniaceae, Zosteraceae, Ruppiaceae and Zannichelhaceae, i.e. he raised the 5 tribes of Potamogetonaceae to distinct families and these families Ire distributed into several orders splitting the order Helobieae into altogether 7 orders.
Such treatment of Hutchinson has met with the approval of most recent botanists including Takhtajan who kept them as distinct families and also created another family, viz., Cymodoceaceae with the single genus Cymodocea. Takhtajan, however, places all these new families in Najadales wherein are also inlcluded Aponogetonaceae, Scheuchzeriaceae and Juncaginaceae.
Family # 2. Najadaceae:
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Najadaceae are small annual herbs, submerged in fresh or brackish water with slender much-branched stem, rooting at the nodes. Leaves are numerous, sub-opposite or verticillate, narrow, sessile, with a sheathing base; a pair of minute scales occurring in the axil of the sheath; the margin is toothed and the teeth are spine-like outgrowths of a single or of several epidermal cells.
Plants are monoecious or rarely dioecious; the flower is situated at the base of the newly developing branches, enclosed in a cylindrical spathe. A sac-like perianth 2-lipped at the top is present in a male flower which has a single stamen; the anther is sessile or sub-sessile, 1-4-locular, opening by longitudinal slits; pollens spherical or oblong.
The pedicel is very short at the beginning and gradually elongates pushing the flower out through the enveloping spathe. The perianth-lobes separate before the anther bursts. Pollination takes place under water.
The female flower is perianthless or there is a thin cupular perianth adhering to the carpel; ovary is of one carpel, unilocular, with 2-4 sessile or sub-sessile stigmas and with a solitary, anatropous ovule, erect from the base. Fruit is an achene with a thin pericarp. Seed has a straight embryo, large hypocotyl and radicle and no endosperm; the seed-coat is often sculptured.
This is a unigeneric family with about 30 species occurring in almost all parts of the world. It is represented in India by several species, common among which are Najas marina Linn., N. minor All., N. foveolata A. Br., N. graminea Del., etc.
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The flower in this family being very simple, it was considered by Engler to be a rather primitive family. Most other botanists are of opinion that the simplicity of the flower is due to reduction and not due to primitiveness and place the family at the end of the group starting from the Alismataceae through Potamogetonaceae, Zannichelliaceae, etc..
Family # 3. Alismaceae:
Alismaceae are annual or perennial rhizomatous herbs growing in marshes or in fresh water; often stoloniferous; roots fibrous; milky juice is often present. Leaves basal, floating or erect with the petiole sheathing at the base; blade often sagittate at base; main nerves prominent and parallel.
Flowers are whorled or in racemose or paniculate inflorescences, bisexual or polygamous, actinomorphic and hypogynous. The torus is large, flat or globose. Perianth is differentiated into one whorl of 3 persistent sepals and another whorl of 3 deciduous petals; rarely petals are absent. Stamens-are 6 in number, sometimes more, rarely 3; anthers are 2-locular and extrorse.
Carpels are few to many, free or rarely united at the base, often they are in a whorl or are spirally arranged (Ranalisma); style persistent. Each carpel has a solitary ovule, basal or on the inner angle, or there are several ovules in each carpel on the inner angle, i.e. marginal placenta.
Seeds are curved with horse-shoe-shaped embryo and without endosperm. The family with about 60 species are found in the tropical and temperate countries. In India Sagittaria sinensis Linn., Limnnphyton obtusifolium Miq. and several species of Alisma are common.
Alismaceae is a primitive family. This is quite evident from the floral morphology. The flowers are bisexual, hypogynous, with two whorls of free perianth segments, many free stamens and carpels. Engler includes the family in his primitive order Helobieae and Hutchinson treats it as a family of one of the 2 most primitive orders of Calyeiferae.
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Due to the remarkable resemblance in floral structure of Alismaceae and Ranunculaceae many botanists hold the opinion (Hutchinson and others) that Alismaceae had its origin from Ranunculaceae. The genus Ranalisma which has flowers exactly like Ranunculus provides strong evidence in support of this theory. Ranalisma was discovered in Malaya about 50 years ago.
Morphological examination of the new plant revealed that it was a monocot and belonged to the family Alismaceae and the genus was named Ranalisma from the close resemblance of its flower to that of Ranunculus. The species was named Ranalisma roslrata Stapf.
Later it was found out that an African plant known as Sagittaria humilis Kuntze should be transferred to this new genus as its characters were not considered fitting satisfactorily in Sagittaria. The African species after transfer to Ranalisma has the new name-combination as R. kumile (Ktz.) Hutchinson.
The discovery of the 2 species one at Malaya and the other at Africa proves that the genus had a wide range of distribution in the past and although comparatively recently discovered it is not at all of recent origin. It gives further support that the family Alismacea is a primitive family.
The discovery of the genus Ranalisma thus has a great significance as it supports the theory of origin of the family Alismaceae from Ranunculaceae or at least from Ranales and origin of Monocotyledoneae from the primitive Dicots and most probably from the Ranales.
Family # 4. Butomaceae:
Butomaceae are perennial rhizomatous herbs that grow in fresh water or in swamps; milky juice is usually present. Leaves are ensiform or flat and dilated or with a distinct petiole and elliptic or orbicular blade.
Flowers are bisexual solitary or umbellate. Perianth is in 2 whorls of 3 segments each; the outer ‘sepaline or rarely coloured, imbricate and persistent; the inner segments are petal-like, usually thin and deciduous, imbricate; the inner whorl of perianth rarely absent.
Stamens are 5-many on a small torus; when they are numerous the outer ones are staminodes; the filaments are free, flattened; anthers are basifixed, 2-locular, opening by lateral slits.
Carpels are 6 or more, free or cohering only at the base, sometimes crowded to form a head, superior; ovules many scattered all over the inner surface of carpels, anatropous or campylotropous. Fruit is a cluster of follicles opening by the adaxial suture. Seeds are many, with a straight or horse-shoe-shaped embryo and with no endosperm.
Butomaceae was included in Alismaceae from which it has been segregated as a distinct family on account of the laminar placentation. It is very close to Alismaceae as indicated by the floral morphology and also to Hydrocharitaceae having laminar placentation.
Laminar placentation having been considered as a very primitive character, Hutchinson includes Butamaceae and Hydrocharitaceae in his primitive order Butomales while Alismaceae remains in another order, viz. Alismatales, with Scheuchzeriaceae and Petrosaviaceae.
Butomaceae is a small family of 6 genera and 9 species. The genus Butomus stands apart from the other 5 genera in having anatropous ovules and straight embryo; here the leaves are long and narrow, not having a distinct petiole or lamina and the rhizome is without laticiferous ducts.
The plants of the other genera have laticiferous ducts, leaves with distinct petioles and blades, campylotropous ovules and horse-shoe-shaped embryo. Pichon therefore states that these 5 genera, viz. Ostenia, Hydrocleis, Limnocharis, Tenagocharis and Elattosis should be transfeired to Alismaceae although the placentation in the carpels is laminar.
Both Takhtajan and Cronquist put these 5 genera in a separate family as Limnocharitaceae and include the 3 families Butomaceae (unigeneric), Limnocharitaceae and Alismaceae in the order Alismales (Alismatales) which they consider as the most primitive order among the Monocots.
The peculiar laminar placentalion of Butomaceae, ovules scattered over the inner surface of carpels is found also in Cabomba of Nymphaeaceae and provides additional evidence to support the theory of origin of Monocotyledoneae from Ranales or rather from primitive Dicotyledonae. Of this family Tenagocharis lanceolatus (Kunth) Hochst is quite common in eastern and south-western parts of this country.
Family # 5. Hydrocharitaceae:
Hydrocharitaceae are submerged or floating aquatic herbs growing in fresh water or are marine, usually stolioniferous; “turions” or vegetative buds are formed in some cases which detach from the mother plant and remaining dormant in winter grow to new plants in spring time.
Leaves are radical and crowded or dispersed on elongated stems and are alternate, opposite or whorled, with distinct petiole and blade or sessile and linear or strap- shaped.
Umbels are few flowered, axillary, sessile or pedunculate, monoecious or dioecious, enclosed in a pair of bracts or in a spathe-like bifid bract; often flowers (usually the females) are solitary; the female peduncle is usually long and spirally coiled.
Flowers actinomorphic, epigynous, unisexual, rarely bisexual (Ottelia). Perianth is 1-2- seriate; segments are 3 in each series or rarely 2, free to the base; the outer usually green and valvate, the inner petaloid and imbricate.
The stamens are usually numerous or few or rarely 1, all free, in several trimerous whorls; the inner stamens are often sterile; anthers are 2-locular, locules parallel, opening by longitudinal slits; pollens are globose, rarely filiform (Halophila); rudimemtary ovary rarely present in male flower.
Ovary is inferior, of 2-15 carpels, united to form a single locule, with many orthotropous or anatropous ovules on 3-6 or more parietal placentation; or the placentation may be laminar; styles are usually bipartite; nectar bearing scales are often present in the base of female flowers. Fruits are dry and irregularly dehiscent or pulpy. Seeds many with large straight embryo and no endosperm.
Pollination takes place on water surface or under water. In Vallisneria the male umbel with usually 3 flowers is sessile and with the enclosing spathe gets detached when the flowers mature and floats on the surface of water. The solitary female flower comes also to the surface by the elongation of the peduncle.
The male flowers are carried to the female flower by water current and pollination takes place. After pollination the female flower is drawn back under water by the coiling of the peduncle and the fruit matures under water. In Elpdea, also pollination takes place on the surface of water. The flowers of Ottelia, Hydrocharis, etc. come above water surface and are pollinated by insects.
In the order Helobieae this family is very distinct. Here more than one carpel unite to form a unilocular ovary which is inferior, unlike all other families. However, the seeds in this family are devoid of endosperm and the perianth segments and stamens are free.
Further the habit of the plants are similar to the other families in this order. These characters are in favour of its inclusion in Helobieae and is considered to have a closer relation to Butomaceae due to the presence of laminar placentation.
The family contains 16 genera and about 90 species occurring mostly in the tropical regions of both hemisphere. It is well-represented in India by Hydrilla verticillata Casp. Ottelia alismoides Pers, Vallisneria spiralis Linn., several species of Blyxa and by others. The plants are of no economic importance except that a few are suitable for planting in aquaria.
Floral formula:
