Marselia- Water fern

Dr. Pallavi

J.N.L. College Khagaul

Distribution and Classification• Marsilea is commonly known as “water clover” or “water fern” (although it

is a fern but hardly resembles a true fern).

• Either the species are hydrophytic or amphibious i.e., they grow rooted in mud or marshes and shallow pools or are completely submerged

• CLASSIFICATION

Division- Pteridophyta

Class- Filicopsida

Order- Hydropteridales

Family- Marsileaceae

Genus- Marsilea

Morphology:

• Plant body is sporophytic andherbaceous plant. The plant bodyis differentiated into rhizome,leaves and roots.

Rhizome:

• All the species possess a rhizomewhich creeps on or just beneaththe soil surface. It is slender,dichotomously branched withdistinct nodes and internodes andis capable of indefinite growth inall directions.

Leaves:

• They are borne alternately on upper side of rhizome at nodes, in tworows. They show circinate vernation. In submerged plants the petioleis a long and the lamina floats over the surface of water but in muddyor marshy plants the petiole of the leaf is short and rigid with shortlamina spreading in the air.

• The lamina consists of 4 leaflets/pinnae which are present at the apexof petiole. Near the base of petiole the stalked bean-shapedsporocarps are borne.

Roots:

The roots are adventitious, arising from the underside of the node ofrhizome, either singly or in groups.

AnatomyT. S. Rhizome (stem):Young rhizome is protostelic in nature i.e., pith is absent and xylem iscompletely surrounded by phloem ,however, in older stem pith is developed inthe centre and the stele becomes amphiphloic siphonostelic type. Thus,internally rhizome consist of epidermis, cortex, followed by stele which is madeup of outer endodermis, outer pericycle, outer phloem, xylem, inner phloem,inner pericycle and inner endodermis.T.S. leaflet:T. S. of the leaflet shows upper and lower epidermis, in between mesophylltissue is present (differentiated into palisade and spongy parenchyma) andvascular bundles. In submerged species mesophyll region is not differentiatedinto palisade and spongy parenchyma.T. S. Root:T. S. of root is circular in outline and can be differentiated into epidermis orpiliferous layer, cortex and stele (Protostelic).

ReproductionMarsilea reproduces by two methods:

(i) Vegetative/Asexual reproduction

• It takes place by means of tubers which are produced in dryconditions from the rhizome. First a branch is given out from therhizome, which later on swells up due to the accumulation of foodmaterial. The structure is termed as tuber and is capable of tidingover the unfavourable conditions. On the return of favourableconditions it germinates to produce a new sporophytic plant, e.g. ,M.hirsuta, M. quadrifolia.

(ii) Sexual Reproduction:

Sporophytic Phase:

Spore producing organs:

• Marsilea is heterosporous i. e., it produce two types of spores—microspores and megaspores. These spores are produced inmicrosporangia and megasporangia, respectively. These sporangia areborne in special type of spore producing organ called sporocarp. Thesporocarp are born laterally on the short and lateral branches (peduncle) ofthe petiole of leaf either near the base or a little higher up.

• They arise solitary or in clusters. Number of sporocarp differs in differentspecies and varies from 1 to 20 or more. Each sporocarp is an oval or beanshaped biconvex, flattened structure. It is green and soft when it is youngbut at maturity it becomes very hard and brown in colour.

Development of sporangia:

• The development of micro and megasporangia is of lepto-sporangiate type.The leptosporangiate differ from other ferns in having a sporangium with ajacket layer one cell in thickness. There is a definite number of spores withina sporangium. The sporangium develops from a single initial cell and thetapetum differentiates from a single internal cell of a developingsporangium.

Dehiscence of Sporocarp and Liberation of Spores:The decaying of the wall of the sporocarp takesplace due to bacterial action and thus thesporangia and spores are liberated. The sporocarpbursts open only in water. The gelatinuous ringabsorbs water and extends greatly through theopen margins of the sporocarp thus dragging outsori along with it. The gelatinous ring bears twoalternating rows of sori. The delicate mucilage wallof the sporangia (micro-or mega) opens in waterand the spores (micro-or mega are liberated whichgerminate soon

Gametophytic Phase:

• The microspores and the megaspores are the unit of male and female gametophytesrespectively.

• They germinate to produce the respective gametophyte in the following ways:

Development of male gametophyte:

• The microspore is the initial stage in the development of male gametophyte. Eachmicrospore is a unicellular, uninucleate, thick walled globose and haploid structure.

• The cytoplasm is surrounded by inner wall called endosporium and outer wall calledexine or exosporium. The microspore germinates just after its liberation. The firstdivision is in a lenticular plane to form a small lens shaped prothallial cell and a apicalcell.

• The apical cell divides transversely to form 2 equal antheridial initials. Each antheridialinitial divides periclinally to form an outer first jacket cells (initial) and inner wedge-shaped sister cells. The inner cell further divides by periclinal division to form a secondsmaller jacket cell and a large outer cell.

• The large cell again divides by a periclinal wall to form an outer or peripheral third jacketlayer and a central cell or the primary androgonial cell. Each primary androgonial celldivides to form 16 androcytes (Sperm mother cells) and finally metamorphosises intoantherozoids. In all 32 antherozoids are produced.

Development of Female Gametophyte:

• The megaspore is an oval or elliptic structure, the wall of which imbibes water and expands toform a gelatinous mass around the megaspore. About two or three hours later of the beginning ofthe spore germination, the nucleus divides which is soon followed by a transverse division.

• This transverse wall develops in between the denser cytoplasm of the protuberance and theremaining watery cytoplasm. The small cell lies within apical protuberance. The lower large cell isnutritive or prothallial cell and occupies the remaining large portion of the megaspore. The largeprothallial cell is quite vegetative and does not divide further.

• This large cell functions as store house of food material. The small cell situated within the apicalpapilla develops into the gametophyte proper.

• All the four segments divide by anticlinal divisions several times giving rise to a one-celled thick vegetative tissue. The archegonial initial divides periclinally forming a primary cover cell and central cell. The primary cover cell further divides by two successive anticlinal divisions forming four quadrately arranged neck initials.

• Each neck initial divides by an oblique wall forming a short neck comprised of two tiers of four cells each. Simultaneously, the central cell divides into two cells, a small primary canal cell and a large primary venter cell. In M. drummondii and some other species the primary canal divides producing two small canal cells.

• In all species the primary venter cell divides into two cells, a small venter canal cell and a large spherical oosphere or egg. On the maturation of the archegonium the neck canal cell or cells and the venter canal cell disintegrate and the archegonial neck remains open.

Fertilization:

Many antherozoids are attracted towards the funnel shaped structureof megagametophyte. The antherozoids enter gelatinous envelope andreach to the archegonium. Only one antherozoid enters the egg’scytoplasm and fuses with the egg nuceus to form zygote.

Development of Embryo into new sporophyte:

The zygote is the mother cell of the next sporophytic generation. Thefirst division of the zygote is vertical followed by a transverse divisionresulting in the quadrant stage (four-celled stage) of the embryo.Subsequent development of the upper two cells forms the root andleaf, whereas the lower two cells give rise to the foot and shoot apex