The life cycle of fern (Dryopteris) clearly shows the alternation of generation. The gametophytic stage $(n)$ alternates with the sporophytic stage $(2 n)$ figure given shows its complete life cycle.
Prothallus The prothallus of the fern is multicellular, free living, thalloid, haploid and autotrophic structure. It develops from the spores produced by sporophyte after reduction division. These spore germinate with a germtube with an apical cell and forms a filament of 3-6 cells and one or two rhizoids at the base which later develops into gametophytic plant.
The male and female gametophytes of pteridophytes and gymnosperms different from each other are
| Male Gametophyte of Pteridophyte | Male Gametophyte of Gymnosperm |
|---|---|
| A distinct male gametophyte may not be present. | A male gametophyte is always present. |
| It contains an antheridium. | An antheridium is not found. |
| Male gametes are flagellated. | Male gametes may or may not be flagellated. |
| Male gametes reach the female gamete by swimming in a film of water. | Male gametes reach the female gamete through a pollen tube. Water is not required. |
| Female Gametophyte of Pteridophyte | Female Gametophyte of Gymnosperm |
|---|---|
| A distinct female gametophyte may or may not be present. | A distinct gametophyte is always present. |
| It is largely independent.. | Female gametophyte does not leave the parent plant. |
| It is not enclosed is an ovule. | It is enclosed inside an ovule. |
Mycorrhiza (Myco's $=$ fungus, rize $=$ roots) is a symbiotic association between fungus and the roots of vascular plants. The fungus colonizes the roots of the host either intra or inter cellularly. It helps in the nutrient absorption from soil for the plant. Mycorrhizal associations are present in conifers, i.e., Pinus, Cedrus, Abies and Picea.
Coralloid roots is develop in Cycas. It is produced in clusters at the base of the stem and protrudes out over the ground. It is dichotomously branched and greenish in colour. It contains algal zone in cortex. This algal zone contains blue green algae like Anabaena and Nostoc which grow in symbiotic association with coralloid roots.
Gametophyte is a haploid multicellular adult stage in a bryophyte's life cycle. It bears male reproductive structure (antheridia) and female reproductive structure called archegonia, and thus produce haploid gametes antherozoids (male gamete) and egg cell (female gamete) respectively.
In mosses, liverworts and hornworts the gametophyte is a dominant form and thus most familiar phase of life cycle of the bryophyte. The moss gametophyte originate from a haploid spore. Initial phase of growth forms protonema in moss.
The protonema further develops into a main plant body of bryophyte which is thallus like prostrate and erect, attached to substratum by unicellular or multicellular rhizoids. They may possess root like, leaf like or stem like structures.
With the help of a schematic diagram describe the haplo diptontic life cycle pattern of a plant group.
Haplo diplontic life cycle (is also referred as diplohaplontic, diplo biontic or dibiontic) shows multicellular diploid and haploid stages as dipicted in the following figure.
Life Cycle of a Pteridophyte The life cycle of a pteridophyte consists of two morphologically distinct phases
(i) The gametophytic phase
(ii) The sporophytic phase
These two phases come one after another in the life cycle of a pteridophyte. This phenomenon is called alternation of generation. The gametophyte is haploid with single set of chromosomes. It produces male sex organs antheridia and female sex organs archegonia.
(i) The antheridia may be embedded or projecting type. Each antheridium has single layered sterile jacket enclosing a mass of androcytes.
(ii) The androcytes are flask-shaped, sessile or shortly stalked and differentiated into globular venter and tubular neck.
(iii) The archegonium contains large egg, which is non-motile.
(iv) The antherozoids after liberation from antheridium, reaches up to the archegonium fuses with the egg and forms a diploid structure known as zygotes.
(v) The diploid zygote is the first cell of sporophytic generation. It is retained inside the archegonium and forms the embryo.
(vi) The embryo grows and develop to form sporophyte which is differentiated into roots, stem and leaves.
(vii) At maturity the plant bears sporangia, which encloses spore mother cells.
(viii) Each spore mother cell gives rise to four haploid spores which are usually arranged in tetrads.
(ix) The sporophytic generation ends with the production of spores.
(x) Each spore is the first cell of gametophytic generation. It germinates to produce gametophyte and completes its life cycle.