HEPATICOPSIDA

Range of Gametophytic Structure | Range of sporophytic structures

Development of Elaters | Protection of young sporophyte in various liverworts

� Ambarish Mukherjee.

Spore � The spore is the first cell of the gametophytic generation. Bryophytes are characteristically homosporous but functionally heterosporous in certain dioecious forms. In some members like Sphaecarpos donnellii & S. stipitatus spore tetrads are of two types, two large and two small. A larger spore germinate to produce female thallus & and the smaller onses male thalli. Spores are typically unicellular, but apparently multicellular spores occur in Pellia sp., Porella sp., Pallavicinia sp. & Concephalum sp. due to precocious development within the capsule. The outer wall of the spore may be smooth or variously ornamented.

Protonema � Protonema is formed by the germinating spore but it is typically absent in Hepatics except in the leafy liverworts. In these form the protonema is short lived and its function ends with the formation of the apical bud which develop into the gametophytes. The protonema in leafy liverworts may befilamentous (Cephalozia sp.), Disci form (Radula camplanata), globose (Nardia scalaris) or cylindrical (Lejeunea).

Structural organization � Structurally the gametophytic plant body may be thallose or foliose with various grades of intermediate forms. Thallose gametophytes are typically in the member of Sphaerocarpales, Marchatiales & Anacrogynous jungermanniales; foliose thallus is found in the members of acrogynous jungermanniales, Calobryales & Takakiales. However, in the anacrogynous jungermanniales a whole range of gametophytic structure starting from typical thallose type e.g., in Pellia sp. & Metzgeria sp. to almost foliose type e.g., Fossombronia & Petallophyllum can be found. The mature gametophyte show also a considerable variation in size. The smallest one are found in Zoopsis argentea (anacrogynous jungermanniales) or Carrpos sp. which measure a few mm. The largest thallus is found in Monoclea forsteri which attains a length of 20 cm. and a width of 5 cm.

The typical thallose gametophyte is flat, prostrate, dorsiventrally differentiated dichotomously branched structure with or without midrib. A dorsal surface of the thallus may be smooth as in Sphaerocarpos and Monoclea or may be distinctly areolated. A ventral surface is provided with rhyzoids in all hepatics except Takakiales & Calobryales. In Marchatiales rhyzoids are of two types � Simple and Tuberculate. Rhyzoids are always unicellular. The ventral surface may also have multicellular scales. Scales are absent in Sphaerocarpales and anacrogynous jungermaniales. Scales may be arranged in 2-4 rows (Marchantia, Targinia, Reboulia, Riccia) or scales may be scattered irregularly (Corsinia, Ricciocarpos, Athalamia etc.). Scales may be green and as in Asterella, Cyathodium etc. In addition to Rhyzoids and scales, unicellular or multicellular mucilage hair may be present on the ventral surface as in Pallavicinia, Blassia, Metzgeria etc. In Calobryum music mucilage hair develop on the stem. In some Bryophytes green appendages are developed amidst leaves, they are called Paraphyllia. Their function is to increase photosynthetic surface and retain water. Paraphyllia are typically present in mosses but are also found in some of jungermaniales like Trichocolia tomamtolla, T. paraphyllina, Stephaniella paraphyllina etc.

The internal morphology in the simplest hepatics does not show differentiation of cells e.g., Sphaecarpales & anacrogynous jungermaniales. In some members of Marchantiales also, simple undifferentiated thallus is found, e.g., Monoclea & Dumortiera. In contrast majority of the members of Marhatiales exhibit a more complex thallus with tissue differentiation. In such form the thallus is dorsiventrally differentiated into a lower region of compact parenchymatous cells and an upper thinner layer of chloroplast containing green cells. The upper photosynthetic layer is typically divided into air chambers having air pores. The air chambers may be arranged in a single row (Marchatia lunularia, Targionia, Concephalum etc.) or in several rows (Plagiochasma, Reboulia, Asterella) in many cases air chambers are empty, but in Marchatia, Preissia etc. air chambers contain chlorophyllose filament which may be branched or unbranched. The air pores may be simple or compound. Simple air pores are found in Plagiochasma, Targionia, Asterella, Stephensoniella, Eormotheca, Lumunaria & compound pores consisting of 4-5 super imposed rings of coenocytic cells occur in Marchantia, Preissia, Bucegia etc. In Conocephallum and Reboulia the air pores are simple on thallus and compound on receptacle. In Riccia the pores are ill defined.

Apical growth of thallose gametophytes is maintained by a group of cells in the members of Sphaerocarpales and Marchatiales. In the jungermaniales apical growth is due to activity of a single apical cell.

The foliose liverworts which are differentiated into an axis is a branched structure which is prostrate and dorsiventral in the acrogynous jungermanniales and radial and erect in Calobryales and Takakiale. The branching habit is usually monopodial and not dichotomous. The axis does not exhibit much tissue differentiation in prostrate forms, except that cells of the cortical region and medullary portion may be vary in size and thickness of the wall. In Takakiales and Calobryales central strand of hydroid like cells are evidenced. The apical growth of the axis is maintained by a single apical cell with three cutting faces producing two lateral segments and one ventral segments. The leaf like structures is simple without a midrib and consists of a single layer of polygonal cell containing many chloroplasts. In Takakia the leaves are needle like hollow structures and circular in cross section. In the majority of foliose hepatics (acrogynous jungermanniales) the leaves are characteristically arranged in two lateral rows and one ventral row. The ventral leaves called amphigastria which may be morphologically similar to dorsal leaves, in Herbarta adunca. In some genera apmhigastria may be totally absence e.g., Radula, Plagiochilla. Due to unequal growth of the dorsal and ventral segments of the axis the lateral leaves attain sucubous arrangement (posterior margin overlapping the anterior margin of the next lower leaf) as in Cephalozia, Lophocolea, Lophozia or an incubous arrangement (posterior margin is overlapped by the anterior margin of the next lower leaf) e.g., Porella, Calypogia, Bazzarmia etc. In some leafy liverworts a dorsal leaf are dorsiventrally bilobed into a larger antical lobed and a smaller postical lobe. The postical lobe may transformed into a water sac e.g., Frullania. In Calobryum the leaves are borne in three rows on the erect stem. In Takakia the leaves are arranged more or less irregularly on the erect stem. In these two genera rhyzoids are absent and under ground rhizome system is present. TOP

Range of sporophytic structures:

The sporophytic generation starts with the formation of the zygote resulting from syngamy. In Bryophyte the zygote does not undergo reduction division as in thallophytes, but produces a sporophyte by mitotic division. The sporophyte is differentiated in most bryophyte into foot, seta & capsule. The basal portion of the mature sporophyte called foot is usually a swollen brown red, bulbous or conical mass. It is embedded in the gametophyte and serves as as organ for absorption and anchorage. The middle portion of this sporophyte forms a stalk like structure called seta which serves to push the spore bearing portion above the gametophyte for more effective dispersal of aerial spores. Seta also functions as a conductive organ for supply of nutrient to the upper part of the sporophyte. The distal part of the sporophyte is called the capsule which contains spores and other accessory structures help in spore dispersal.

�� In Riccia the sporophyte is without foot and seta and it is a completely embedded in the tissues of the gametophyte. In Sphaerocarpus and Corsinia the sporophyte as a small foot and an incipient seta consisting of a few cells. In members of Marchantiales the foot is larger more or less spherical and the seta is erratedly short. In majority of Jungermanniales the foot ismore or less conical and the seta is long.

�� The size of liverwort capsules varies from 0.5 mm in acrogynous jungermanniales to 1.25 - .5 mm in Marchantiales and anacrogynous jungermanniales. The capsule have form is nearly spherical in Sphaerocarpus, Corsinia, Pellia and Fossombronia. In other members of Marchantiales the capsule is ovoid to cylindrical. Markedly elongated capsule are found in Monoclea, Calobryum, Haplonitum. The capsule have jacket is uniform in one celled thick an all members of Sphaerocarpales and Marchantiales and two to several cells thick in members of Jungermanniales. In Calobryales the capsule wall is one layered except at the top where it is multi layered. Columella is absent in all members of hepatics. Elaters are present in most species and elaterophores are present in anacrogynous jungermanniales. There is no definite mode of dehiscence in majority of Hepatics where the spores are liberated by disintegration of the capsular covering. In some member of archegoniophore forming Marchantiales and a definite operculum is present e.g., Plagiochasma, Reboulia �& Grimmalobia. In jungermanniales the capsule opens characteristically in four valves. Numbers of spores per capsule varies widely in different genera. In Sphaerocarpus about 200 spore tetrads are found in one capsule. In Marchantiales numbers of spore varies from 2000 to 8000 per capsule. In jungermanniales the number of spore is much more varying in 24,000 to 40,000 per capsule. In all Hepatics the spore mother cells originate from endothecium and the amphithecium gives rise to the capsular wall.

�� The development of sporophyte differ some part broadly in Marchantiales and Jungermanniales. In the former the zygote generally passes through an octant stage, where in Jungermanniales a linear tetrad is form. The proportion of total diploid cells derived from the zygote which takes part in the formation of spore where more in different Hepatics. The proportion is highest in Riccia and decreases in other genera where more differentiated sporophytes are found. This has been designated �progressive sterilization of potentially sporogenous tissue� (Bower, 1890). TOP

Development of Elaters:

Cells of the upper most portion of the developing sporophyte divide periclinally to form an amphithecium and an endothecium. The endothecial cells divide by longitudinal wall to form a lens shaped mass of about 2,000 cells arranged like a bundle of cigers. These cells then differentiated with great regularity into two types � sterile elater producing cells and fertile spore mother producing cells. These cells are disposed like the square of a chess board in transverse section about � form of these cell distend and undergo repeated transverse division to form rows of spores mother cells. The remaining cells do not undergo any further division and they grow in length long narrow cylindrical cells which are ultimately converted into elaters with 1, 2 or 3 spiral branch of thickenings on the inside wall. TOP

Protection of young sporophyte in various liverworts:

Liverworts have evolved specialized structures for protection of young sporophyte calyptra, the residual part of archegonial center provide the 1^st line of protection.

Invlocre, which is an outgrowth of thallus is another protective organ found in Sphaerocarpus, Corsinia, Targionia and Pellia. In Marchantiaceae another additional structure is the perichaetium. In addition to perygynium it is also present in some members.

�� In some acrogynous jungermanniales is pouch like growth of the gametophyte serves to protect the young sporophyte, is called a marsupium found in some genera like Trichocolea, Isotachis, Geocalyx, Calypogia. The marsupium may be formed by outgrowth of the thallus in the same direction as the main axis (Trichocolea & Isotachis), or in a direction at right angle to the growth of main axis (Geocalyx & Calypogia). In both cases the foot of this sporophyte remains embedded deep in the marsupium tissue. TOP

� Dr. Ambarish Mukherjee, Reader in Botany, The University of Burdwan, Burdwan, W.B., India.