MOSSES
Mosses are small, green and simple land plants belonging to phylum Bryophyta and class Musci. They are found growing on damp places. A common example of mosses is Funaria hygrometrica commonly found growing on old blocks, block fences, tree trunks and rock surfaces. They grow close together in large numbers forming velvet-like carpets.
The plant (gametophyte) consists of a false stem (because it lacks xylem and phloem) and root-like rhizoids (rootlets) also lacking xylem and phloem. The false stem is erect with leaf cells arranged on it spirally or alternately. The erect nature of the stem is partially due to turgor pressure (fluid pressure) in the cells (Turgor is like inflating a balloon with air which makes it to stand erect). The rhizoids (rootlets) which grow from the stem help to anchor the plant to substratum and also absorb water and nutrients from soil. The whole plant is capable of absorbing water and nutrients.
Alternation of Generation
The life cycle of bryophytes and pteridophytes demonstrate a clear phenomenon of alternation of generations. Two clear organisms, a haploid gametophyte generation and a diploid sporophyte generation, alternate in their life cycles. The haploid generation is the gametophyte (gameto – gamete; phyton = plant) as it produces gametes from mitotic division resulting in haploid gametes. The haploid gametes fuse to form a diploid zygote which germinates into next generation resulting into diploid sporophyte (sporo = spore; phyton = plant). Spore formation is by meiosis resulting in haploid spores which give rise to haploid gametophyte.
Life cycle of Funaria hygrometrica
When a gametophyte has attained maturity, and ready to reproduce, sex organs develop on top of the stem on the middle of the leaves. Male and female organs develop on separate branches as indicated in the diagram. Male organs called antheridia produce coiled biciliate antherozoids (spermatozoa) while the female organ called archegonium produces one large ovum (egg) at its base. The archegonium releases certain chemical that attracts antherozoids to it. After heavy rainfall, water covers the plant, antherozoids are then released. Antherozoids then swim in water into the archegonium of the female plant within the tufts. One antherozoid then fuses with egg in the archegonium. A diploid zygote is then formed on top of the female plant which then grows into a diploid sporophyte. This constitutes the sporophyte generation. The sporophyte reproduces asexually to produce haploid spores by meiosis.
The sporophyte generation consists of sporangium which is made up of seta and capsule. The capsule contains spores. When the spores mature the capsule breaks off to liberate haploid spores. The spores are tiny and light and float in the air. They are dispersed by wind. When a spore lands on suitable substratum it germinates into a gametophyte.
The gametophyte (moss plant) which produces gametes (antherozoids and ova) represents the gametophyte generation while the sporangium which produces spores represent sporophyte generation.
The sporophyte generation depends upon gametophyte generation for most of its nutrients.
Economic importance of mosses
- Mosses help to break down rock surfaces into soil in which other plants use.
- Their death add humus to the soil.
- They prevent soil erosion since they grow together in large numbers forming carpet on the ground.
