The first plants to colonize land were the bryophyta. They are generally thought to have evolved from green algae.
The bryophyta are poorly adapted to live on land and are mainly confined to damp shady places (Fig. 9.1)
Fig. 9.1 A moss bug, lacking rigid supporting tissue, bryophytes are low-profile plants they are most common in damp habitats.These plants are devoid of specialized conducting (xylem and phloem) and strengthening tissues. only the process of diffusion and osmosis helps in the transportation of water and minerals as well as in transportation of prepared food and other substances. The plant body is with a proper cuticle, or has 2 very thin one. The water is absorbed by the general surface of the plant. The bryophyta are said to be the amphibians of the plant world because they cannot live away from water. They need water for reproduction (Fig. 9.2).
Fig. 9.2Mosses often grow at wet places as seen here in a small water fall.
The bryophytes are non-vascular flowerless plants. These plants show a regular alternation of heteromorphy (morphologically different) generation. They have a dominant independent free living gametophyte. This may be tabloid as in many liverworts or is differentiated into structures resembling to stem, leaves and absorbing and anchoring organs, rhizoids, as in mosses and some liverworts. The gametophyte produces a sprophyte, which is a less conspicuous generation, partially or totally dependent upon the gametophyte for its nutrition. The sporophyte generally consists of foot, seta and capsule.
The sporophyte is diploid (2n) which produces in sporangia one kind of haploid spores (i.e. it is homosporous) by meiosis. The spores germinate and give rise to gametophyte which is also haploid. Multicellular male and female exe organs i.e. antheridia and archegonia respectively, are born on gametophyte either on same or different plants. These sex organs are multicellular and protected by a sterile covering of cells (Fig. 9.3).
Fig. 9.3Sex organs, male (antheridium) female (archegonium)of a bryophytic plant
Gametes are produced by mitosis. Male gameters produced within antheridia are called antherozoids; antherozoids are motile and always produced in large number . female gametes formed within archegonia are termed as eggs. A single egg is formed in each archegonium. Fertilization takes place in water. Antherozoids (n) are attracted towards archegonia (n) chemotactically. A single antherozoid fuses with an egg(n) thus accomplishing fertilization which results in the formation of the diploid zygote (2n). the zygote is retained within the female sex organ (archegonium) for some time. After a resting period the zygote develops through mitotic divisions into a diploid embryo. The embryo ultimately develops into a sporophyte which is also diloid.
The entire development of sporophyte thus takes place within the gametophyte plant body. Even which the sporophyte is fully developed it remains attaché d to the gametophyte for nourishment and protection because it does not contain chloroplasts and is unable to perform photosynthesis. There is an alternation of generations in the life cycle of bryophytes i.e. multicellular haploid gametophytic (gamete producing) generation (Fig. 9.6). it is a very important phenomenon, which provides continuous genetic variabilities and selection for the best genetic make up for survival and adaptation in the changing environment (s)(as explained in a later section).
An view of the above mentioned discussion, bryophyta can therefore be defined more precisely as plants with the distinguishing characters as follows:
“Vascular system absent; gametophyte dominant; saprophyte attached of gametophyte; homosporous.”