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Section B

Division - Bryophyta (Mosses)

Characteristics of Mosses :-Mosses are mostly-terrestrial bryophytes.Mosses are found in a range of habitats, although moist and shady habitats are more common. Mosses are often epiphytes.

  • The dominant phase of the moss life cycle is the gametophyte (haploid).
  • The plant is called a thallus, they may be erect or prostrate (axis along the ground).
  • Mosses have radial symmetry, in that a cut down the long axis of an individual gives two similar halves.
  • The gametophyte has a stem like axis with spirally arranged leaves, which are known as phyllids . Mosses attach to their substrate with multicellular rhizoids .
  • Moss leaves are variable in shape.Leaves usually consist of a single cell layer and are traversed by a midrib that is always more than one cell in thickness.The phyllids of mosses such as Mnium may be a single cell thick, but with a midrib with hydroids and leptoids. Polytrichum have a pad of cells and filamentous strands of photosynthetic cells. The margins of the leaves are often toothed, the teeth pointed or rounded.
  • It lacks xylem and phloem.The plant body may have conducting tissue.
  • The xylem-like water-and-mineral-conducting tissue is called hydroid. The phloem-like sugar-and-amino-acid-conducting tissue is called leptoid.
  • All mosses have a sporic (diplohaplontic) life cycle that is oogamous.

Gametophyte Characters:-

Spore Germination and Protonemata :- Moss life cycle begins when haploid spores are released from a sporophyte capsule and begin to germinate. In the majority of mosses, germination is exosporic, i.e., the spore wall is ruptured by the expanding spore protoplast after its release from the capsule and prior to any cell division. However, in some mosses, e.g. Andreaea, Drummondia, and Leucodon, germination is precocious and endosporic, meaning that cell divisions occur prior to spore release and spore wall rupture, respectively. There are variations in patterns of germination of moss ( K. Nehira 1983). In most mosses, a highly branched filamentous, uniseriate protonema are formed.

Cell specialization occurs within the protonema as a result two types of filaments are formed:-

  • a horizontal system of reddish brown,anchoring filaments (rhizoids), called the caulonema
  • upright, green filaments, the chloronema.

Each protonema can spread over several centimeters, forming a fuzzy green film over its substrate. Usually this protonemal stage is short-lived, but in a few taxa, e.g., Buxbaumia it persists as the vegetative phase of the plant.

Formation of bud apical cells:- As the protonema grows, target cells usually on the caulonema generate bud initials that will ultimately divide by sequential oblique divisions to form bud apical cells. This initiates the growth of the leafy gametophore or shoot stage of the moss.

Shoot Morphology and Habit:-

  • The leafy shoot continues to grow by mitotic division of its obovoidal to fusiform apical cell and surrounding meristem.
  • Divisions occurring in the apical cell form spirally arranged derivatives, each of which will give rise to a single leaf and a portion of the stem.
  • The angle of divergence between successive derivatives is responsible for the spatial arrangement of the leaves or phyllotaxy of the shoot.
  • Mature leaves of few mosses are clearly ranked; e.g., the leaves of Fissidens and Bryoxiphium are in two rows, a 1/2 phyllotaxy
  • Fontinalis and Tetraphis have leaves aligned in three rows, a 1/3 phyllotaxy.
  • In most mosses, however, the leaves are spirally distributed, with 2/5 and 3/8 phyllotaxies being most common (W. Frey 1971; B. Crandall-Stotler 1984).
  • The peristomate or true mosses (Superclass V) on the basis of position of the perichaetia and subsequent sporophytes have traditionally been divided into two broad morphological groups:-
    1. Acrocarps :-Acrocarps are characterized by erect or ascending shoot systems that are either unbranched or only sparingly branched. Branching is typically sympodial with the branches morphologically comparable to the determinant main shoot from which they arise. Perichaetia are differentiated at the tip of the main or primary shoot and terminate its growth, so further plant growth occurs only if a branch is produced below the perichaetium; such branches are called subfloral innovations
    2. Pleurocarps :-. Pleurocarps are generally characterized by creeping shoot systems, with extensive lateral branching.In such systems, the indeterminant main stem may be morphologically distinct from the secondary and tertiary level branches that arise from it (C. La Farge 1996). Perichaetia in pleurocarps are produced at the tips of very short, basally swollen lateral branches that are very short, morphologically distinct from the vegetative branches.
  • Cladocarpic mosses produce perichaetia at the tips of unspecialized lateral branches that display the same heteroblastic leaf series as the vegetative branches. Such branches are themselves capable of branching, and these mosses are neither acrocarpic nor pleurocarpic.
  • Pleurocarps form a natural, monophyletic lineage of true mosses (B. Goffinet and W. R. Buck 2004), but cladocarpy has evolved in several different lineages.
  • The main stems of Sphagnum (Superclass II) display a furcate or dichotomous branch architecture (H. A. Crum 1984).

Rhizoids:-Mosses are anchored to their substrates by filamentous, often branched, reddish brown rhizoids, except Takakia and Sphagnum. The rhizoids (As in caulonemata) are multicellular with oblique cross walls; their walls are smooth or roughened with papillae.

  • Most rhizoids are slender and only sparingly branched (micronematal type) arise from any of the epidermal cells of the stem.
  • But others are larger in diameter and extensively branched (macronematal type) and is associated only with branch primordia.

They function primarily as anchoring structures . Rhizoids are not major sites of water and nutrient uptake, but can enhance capillary movement of water along the outer surface of the stem (M. C. F.Proctor 1984).

Stem Anatomy:-. In many mosses, the stem is anatomically complex, consisting of a differentiated epidermal layer, a cortex, and a central strand of thin-walled, hydrolyzed water conducting cells, called hydroids.

Leaves:-Considerable variation in the arrangement and structure of moss leaves provides some of the most morphologically useful characters for species identification.

  • Leaves typically arise from all sides of the stem, most commonly exhibiting a spiral phyllotaxy, but distichous and tristichous arrangements can also be found.

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  • Isophyllous:-The mature leaves of a given shoot are usually all similar in size and shape.
  • Anisophyllous:- but there are taxa that are anisophyllous, with either dorsal or ventral leaves decidedly smaller than the lateral leaves.
  • Except for a few taxa like Fissidens, leaves are attached to the stem along broad transverse lines.

Sexual reproduction :-

  • For sexual reproduction, the moss gametophyte produces gametangia. The male and female gametangia may be on the same thallus (homothallic or monoecious) or on separate gametophytes (heterothallic or dioecious).
  • Both the antheridium and archegonium have a sterile jacket of cells, which better protects the gametes against desiccation in the terrestrial environment.
  • Antheridium :- The antheridium consists of a stalk, a sterile jacket, and spermatogenic tissue. The antheridium sterile jacket has a cap cell which disintegrates when turgor pressure rises.By mitotic division of haploid spermatogenic tissue inside the sterile jacket haploid flagellated sperms are formed. Water is required for transfer of the motile sperm to egg.Most antheridia are in terminal disk-shaped clusters to facilitate water capture for sperm transfer. Sperms are chemotactic and swim through free-water up a concentration gradient of the chemotactic agent to find the open archegonium.The first drop of water landing in the cup causes the cap cell of the anteridium to burst providing an opening for sperm into the drop of water. Filaments of cells found between the antheridia, called paraphyses, swell up with water and squeeze the antheridia to help expel sperm into the water of the splash cup. The next raindrop to land in the splashcup will splash out a solution containing sperm. These will swim through a film of rainwater to fuse with the egg.
  • Archegonium:-The archegonium consists of a stalk, a venter surround the egg, and a long neck. The neck is filled with canal cells. The sterile jacket has a cap cell which disintegrates when turgor pressure rises. All cells of the archegonium, including the egg cell, are produced by mitosis of haploid gametophyte cells. The disintegrating neck and ventral canal cells provide chemicals involved in sperm chemotaxis to fuse with the egg. After fusion of egg and sperm zygote is formed which diploid.
  • After fertilization, the sporophyte grows out of the archegonium, and nutrients for the developing sporophyte are provided by the gametophyte.
  • Meiosis in the capsule produces haploid spores. When spores are mature, the lid of the capsule, called the operculum, opens. Due to changes of humidity a row or rows of hygroscopic teeth, the operculum, open and release spores.

The gametophyte plant is produced by the germination of a haploid spore. As a spore germinates, it produces a branched filament of photosynthetic cells called a protonema. This branching filament is similar to a green alga.The protonema produces a caulonema filament which can produce either a leafy moss gametophyte or a hard, dry bulbil for asexual reproduction.The moss gametophyte produces male and female gametangia. The sperm and egg fuse in syngamy.

Sporophyte of Moss :- Syngamy of the egg and sperm produce a zygote within the archegonium. This zygote undergoes mitosis to produce an embryo, again retained within the archegonium. Finally, the embryo matures into a sporophyte. Diploid sporophyte is typically not photosynthetic and so is parasitic (dependent) on the gametophyte for its nutrition.The sporophyte consisting of :-

  • A sporangium (capsule) :- Sporogenous tissue forms around the columella, and spore mother cells undergo meiosis to form tetrads of haploid spores. At the top of the capsule is the cap-like operculum beneath which is a double row of triangular peristome teeth. The teeth are attached to a thick-walled annulus around the upper end of the sporangium. When the sporangium is mature, the operculum breaks off, and the peristome is left holding the spores in place. The teeth are very sensitive to humidity (i.e. hygroscopic) and when wet or very humid weather occurs, they bend into the capsule, when dry, they straighten out and lift some spores out with them. The spores are then distributed by air currents, and later they germinate into protonemae.
  • A seta (stalk) :-
  • A foot :- Foot remains embedded in the gametophyte tissue. The continued attachment of the sporophyte to the gametophyte allows the sporophyte to absorb most of its needed nutrients from the gametophyte. A seta or stalk :- which elevates
  • the sporangium, or capsule.
  • Typically, a portion of the gametophyte, called the calyptra , protects and covers the developing capsule.The haploid hairy calyptra of Polytrichum is quite elaborate and a contrasting pink color covering the entire sporophyte capsule.

Division - Hepatophyta (Liverworts)

Occurrence :-Liverworts are odd little plants that appear as small, flat green patches attached to the ground, although they may form large masses in favorable habitats such as moist, shaded rocks or soil, tree trunks or branches and a few even grow directly in water.

Size :-Liverworts are the simplest of the living plants, and range in size from minuscule, leafy filaments less than 0.02 in (0.5 mm) in diameter, to plants exceeding 8 in (20 cm) in size.

General Characteristics of Liverworts :-

  • Liverworts are made up of flat, lobed thalli (singular=thallus, it looks like a liver).
  • There are two kinds of liverworts based on body form: thallose and leafy.
  • Liverworts are made up of flat, lobed thalli (singular=thallus, it looks like a liver). Liverworts lack specialized conducting tissues, cuticles, and stomates, and their rhizoids are always unicellular. The gametophytes arise directly from spores in most species. Most liverworts (75%) have nine chromosomes in their haploid cells. There are two kinds of liverworts based on body form: thallose and leafy.