Bryophyta Learning Materials

Bryophytes: Introduction: Bryophyta (Mosses, Liverworts, and Hornworts) is a group of the simplest and primitive non-vascular embryophytes. These plants occupy a position intermediate between the green algae and pteridophytes. The group is represented by about 960 genera and 24,000 species, occurring on most kinds of substrata wherever there is sufficient moisture for their vegetative and reproductive phases. Bryophytes may be terrestrial or aquatic. General Characteristics: Gametophytic and Sporophytic Phases: Both phases are present in their life cycle. Bryophytes exhibit heteromorphic alternation of generations. The gametophytic phase is predominant, independent, green and freely branched, whereas the sporophytic phase is short-lived and completely dependent on the gametophyte. Plant Body: The plant body is gametophytic, non-vascular, thalloid or foliose, prostrate, and thalloid, but in mosses, the plant body is erect, slender, and leaf-like. The stem axis is frequently differentiated. Roots: True roots are absent and function of anchorage and absorption is performed by filamentous structures known as rhizoids. Tissues: The tissues are cellular and can be simple or multi-cellular and branched. Water Conducting Tissue: The members of the order Marchantiales have well-differentiated scales in addition to rhizoids, which protect the growing region of the thallus and help in the absorption of water. Internal Structure: The plant body is composed of parenchymatous cells and lacks xylem and phloem, the internal conducting tissue. Rhizoids perform the function of absorption and anchorage. Reproduction: Both vegetative and sexual methods are present. Vegetative multiplication takes place by decay and death of the older parts of the thallus, by adventitious branches or by special structures like tubers and gemmae. Sexual Reproduction: The sexual reproduction is of oogamous type. The sex organs are multi-cellular and considerably more complex than those of thallophytes. Sex Organs: The male sex organ is known as antheridium. The antheridia are stalked, lobed, or somewhat elliptical. They have an outer sterile one-cell thick jacket with numerous spermatocytes or antherozoids. The antherozoids are biflagellate, spindle-shaped, or somewhat elliptical. They have an outer sterile one-cell-thick jacket with numerous spermatocytes or antherozoids. The antherozoids are biflagellate, spindle-shaped, and have a motile biflagellated antherozoid. (10) The female sex organ, known as archegonium, is flask-shaped, consisting of a basal swollen venter and somewhat more slender elongated upper part called neck. The venter and neck are surrounded by a jacket of sterile cells. (11) Water is necessary for fertilization. Antherozoids enter the archegonium, fuse with the egg, and form a diploid zygote. (12) The zygote does not have any resting period but divides immediately after fertilization. (13) The first division of the zygote is always transverse and exosporic cell gives rise to the embryo. The next few divisions lead to the embryonic development. The embryo develops into the venter of the archegonium. (14) Depending upon the division, the embryo forms a simple structure without rhizoids (stem or leaves) or completely differentiated gametophyte with sporophyte or positively structured, differentiated into foot, seta, and capsule. In some members, however, the sporophyte is represented either only by a capsule (Ex: Riccia) or by foot and capsule (Ex: Corsinia). (15) The sporogenous cell present in the capsule, form haploid spores after reduction division. All spores are similar in shape and size. (16) Spores are non-motile and they disseminate exclusively by air. (17) Under favorable conditions, the spores either form a filamentous protonema which directly gives rise to a young gametophyte (Riccia, Marchantia) or forms a protonema as in mosses. (18) In bryophytes, sex is determined either by morphologically distinct sex chromosomes or by environmental factors. Origin of Bryophytes: Because of the soft plant body, bryophytes fossil record is very meager and more recent. Most of the fossils available resemble very much with the present-day forms. Hence, the fossil record can hardly throw any light on the origin of bryophytes. However, comparative morphology of living forms, their analogy, and phylogeny have been helpful in this regard. (ii) Algal Origin of Bryophytes: According to Ingler (1943), most primitive algae ancestors of Bryophytes show prominent resemblances in the following features: Primitive terrestrial forms. Bryophytes and Pteridophytes have possibly evolved from primitive algae along different lines. However, also supported the evolution of Bryophytes: Bryophytes show resemblance with green algae in their mode of division and photosynthetic pigments like chlorophyll a, b, and carotenoids like lutein and violaxanthin. Starch as a reserve food material. Cellulose and pectose in the cell wall basis. Fritsch (1945) based comparative study of various algal groups concluded that algae resemble Bryophytes in the following features: Heterotrichous habit with parenchymatous structure and erect elements showing apical growth region. Groove reduction of vegetative body and differentiation of conductive tissue in aerial parts. According to Ripp, amongst green algae group Bryopsidales are the only plants which have been postulated with erect system as the heterotrichous algal ancestor. Smith (1955) embryology supported algal origin of Bryophytes. According to Smith, Bryophytes have originated from the Chlorococcales (Chlorophyceae): The primitive forms are those which became established in areas with plenty of water during drought from available water. Structural Adaptations: Structural adaptations led to the establishment of the massive plant body in water reducing surface to volume ratio and increasing surface absorbing tissue of the thallus to volume and prevent undue drying up. Since these plants grew in moist shady habitats, there is no need for cutinized covering. Absorbing tissues and thin cuticle. The extraordinary abundance of moisture has explained the reduction of cuticle as a characteristic of algae. According to bold: Primitive forms of algae and primitive Amphibious plants have evolved from common stock of simple green marine algae found on land. According to Fritsch and Smith considered heterotrichous green algae to be their ancestor. Chaetophorales as their ancestors. (ii) Algal Origin of Bryophytes: According to Ingler (1943), most primitive algae ancestors of Bryophytes show prominent resemblances in the following features: Primitive terrestrial forms. Bryophytes and Pteridophytes have possibly evolved from primitive algae along different lines. However, also supported the evolution of Bryophytes: Bryophytes show resemblance with green algae in their mode of division and photosynthetic pigments like chlorophyll a, b, and carotenoids like lutein and violaxanthin. Starch as a reserve food material. Cellulose and pectose in the cell wall basis. Fritsch (1945) based comparative study of various algal groups concluded that algae resemble Bryophytes in the following features: Heterotrichous habit with parenchymatous structure and erect elements showing apical growth region. Groove reduction of vegetative body and differentiation of conductive tissue in aerial parts. According to Ripp, amongst green algae group Bryopsidales are the only plants which have been postulated with erect system as the heterotrichous algal ancestor. Smith (1955) embryology supported algal origin of Bryophytes. According to Smith, Bryophytes have originated from the Chlorococcales (Chlorophyceae): The primitive forms are those which became established in areas with plenty of water during drought from available water. Structural Adaptations: Structural adaptations led to the establishment of the massive plant body in water reducing surface to volume ratio and increasing surface absorbing tissue of the thallus to volume and prevent undue drying up. Since these plants grew in moist shady habitats, there is no need for cutinized covering. Absorbing tissues and thin cuticle. The extraordinary abundance of moisture has explained the reduction of cuticle as a characteristic of algae. According to bold: Primitive forms of algae and primitive Amphibious plants have evolved from common stock of simple green marine algae found on land. According to Fritsch and Smith considered heterotrichous green algae to be their ancestor. Chaetophorales as their ancestors. Reproduction in Riccia: Riccia reproduces by vegetative and sexual methods. (a) Vegetative Reproduction: Vegetative reproduction of Riccia takes place by the following methods: By Progressive Death and Decay of Older Parts of the Thallus: Vegetative propagation takes place continuously by the continuous progressive death and decay of the older basal parts of dichotomous aerial branches. When the older basal parts of dichotomies die and rot away, the growing apex continuously grow and give separate branches. The growing tip survives the dry period of one season, then grows to form new thalli. By Adventitious Branches: Vegetative growth of Riccia also takes place by the formation of adventitious branches from the lower surface of the thallus. The branch later separates from the parent thallus and forms a new plant. By Rhizoids: In some species of Riccia, lobes develop on the thallus. These perennating structures can easily survive a period of drought and can germinate in favorable seasons. It has been found that a new gametophyte develops from parts of the rhizoids. The tips of the rhizoids divide repeatedly to form a mass of elongated rhizoid cells. These cells further grow to form a new thallus. (b) Sexual Reproduction: Sexual reproduction takes place by means of well-developed sex organs. The male reproductive organ is the antheridium and the female the archegonium. Riccia is mostly monoecious or homothallic, but some are also dioecious or heterothallic. Sex Organs Develop Singly: They are formed once acropetally in a linear row in upper epidermal furrows. At first, they are superficial, but at later stages of development they become deeply embedded in the thallus and are formed by the overarching thallus. The Cavity: The cavity formed is called an antheridial and archegonial chamber and are open on the upper surface by narrow cylindrical pore. Antheridia: The mature antheridium is globular or pear-shaped with a single-layered jacket (coat) enclosing a number of androcyte or mother cells. Each androcyte mother cell divides diagonally to form two androcyte or antherozoid cells. Each androcyte cell forms a biflagellated sperm or antherozoid.