Question

How does the appearance and nutritional status of the gametophyte differ between bryophytes and ferns and the rest of the vascular plants?

Answer 1

BRYOPHYTES

The dominant and independently living phase of the life cycle is gametophyte.

These are green, photosynthetic, independent. It is thalloid in Riccia,Marchantia. It is foliose in Porella. It is differentiated in to rhizoids, stem like axis and leaf like structures in funaria (mosses). Rhizoids may be unicellular or multicellular, Branched or unbranched. No vascular tissues.Plant body is parenchymatous.

During germination, spores give rise to the gametophyte directly (liverworths) or to juvenile gametophytes called protonema (mosses). Buds on protonema form adult gametophyte. In T.S., the thallus shows ventillated upper portion which is photosynthetic and assimilatory region. In anthoceros, gametophyte is simple, lobed, dorciventral. Green cells have single cholorplast with many pyrenoids. Thallus has unicellular rhizoids. In class bryoxida, gametophyte has juvenile, prostrate, branched filamentous stage resembling green alga known as protonema and adult upright stage known as gametophore which develops from protonema as a lateral bud.

Gametophore is differentiated to root, stem and leaf like structures. Rhizoids are multicellular and branched. Leaves are spirally arranged on the stem. The stem of gametophore is internally differentiated into epidermis, cortex and central strand.

Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.  

Protonema, in hepaticopsida and anthocerotopsida is globose.  In bryoxida, it is differentiated into filamentous like structure with leaf-like structures and chlorophyl.

PTERIDOPHYTA

In pteridophyta, gametophyte is reduced multicellular prothalli (gametophyte). Due to water requirement, cool, dampy and shady places for their growth, they are restricted to narrow geographical regions. Prothalli are always dioecious in heterosporous species and usually monecious in homosporous species. It bears female sex organs - archegonia and male sex organs - antheridia. Prothallus is green and with chlorophyl can manufacture its own food material.  

In lycopsida, prothallus is slightly elongated. In sphenopsida, gametophyte has lobes and it has marginal meristem. In pteropsida, prothallus is heart-shaped, single layer and produces unicellular rhizoids.

Antheridia produces biflagellated male gametes and archegonia are complex, well developed and generally contain one egg cell, one venter canal cell and six to many neck canal cells.

VASCULAR PLANTS

In other vascular plants, gymnosperms, pollen grains and the egg cells, they cannot manufacture their own food material.

GYMNOSPERMS

In gymnosperms, integumented megasporangia ovules are present on megasporophylls. Ovules are unitegmic. Bitegmic in gnetum and anatropous in pinus. Nucellar cell act as a megaspore mother cell. By reduction division, it produces four spores. The megaspore undergo pre-nuclear divisions and form heploid endosperm which is the food material for the embryo.

ANGIOSPERMS

In angiosperms also, male reproductive gametes, pollen grains and ovules cannot produce food material as they are not green in color. In gymnosperm and angiosperm, gametophyte stage is not dominant.

Triploid endosperm is the food material for the embryo. Stigma, style and ovary are the structures in carpels. and pollen grains are male reproductive structures.