Introduction to botany
No rated * * * * *
Bryophytes structure and reproduction
Section " A"
Bryophytes (nonvascular Plants) are the only embryophytes (plants that produce an embryo) whose life history includes a
(haploid) stage.They are an ancient and diverse group of non-vascular plants.They comprise three main taxonomic groups: mosses
(Bryophyta), liverworts (Marchantiophyta or Hepatophyta) and hornworts (Anthocerotophyta) which have evolved quite
separately.They are not
considered to have given rise to the vascular plants but they probably were the earliest land plants (Qui & Palmer, 1999). Like
the rest of the land plants, they evolved from green algal ancestors, closely related to the Charophytes.
Most bryophytes have erect or creeping stems and tiny leaves, but hornworts and some liverworts have only a flat thallus and
leaves.Worldwide there are possibly 10,000 species of mosses, 7000 liverworts and 200 hornworts.
Habitats :- Small in size, but they can be very conspicuous growing as extensive mats in woodland, as cushions on
walls, rocks and tree
trunks, and as pioneer colonists of disturbed habitats.
Classification of Bryophytes:-
Although the bryophyte is used as a collective term for all of these -Bryophyta(mosses), Hepatophyta
(liverworts), and Anthoceratophyta (hornworts).
General Characteristics of Bryophyta (Liverworts, Hornworts and Mosses)
- All of these are land plants
(terrestrial) with some aquatic forms.
- They are very small. The sporophyte and gametophyte have very different
morphologies (heteromorphic generations) and the sporophyte is usually partly dependent on the gametophyte.
- Photosynthetic, non-vascular plants
- Plant body is either :-
- Thalloid and attached to the substratum by hair-like structures called rhizoids (true roots are absent) or
- is differentiated into stem-like (caulalia) and leaf-like structures (phyllids), true stems and leaves lacking.
- Cuticle and stomata are absent.
- The bryophytes show alternation of generations - the haploid
gametophyte (producing gametes for sexual reproduction) alternates with diploid sporophyte (producing spores for asexual
- Gametophytes homothallic or heterothallic.
- The gametophyte generation is dominant, conspicuous and independent.
- The female sex organ is the archegonium.
- The male sex organs are antheridia.
- The ovum remains in the archegonium and spermatozoids swim to it by
- Although bryophytes are land plants, they are still dependent upon water for fertilization, as the sperm
swim in a water film.
- The sporophyte is attached and dependent upon the gametophyte for nutrition i.e. is
parasitic on the gametophyte
- The diploid sporophyte usually consists of a basal foot, an elevating seta and a
terminal sporangium - the capsule
- Spores are produced as a direct result of meiosis.
- Spores dispersed by a
mechanism which ensures dispersal in dry weather only.
- These plants (in either generation) lack specialized cells for the transport of
materials (vascular tissue). Absence of vascular tissue limits bryophytes to moist habitats and small size.
General life cycle :-
- Archegonia ;- Archegonia are stalked, multicellular, flask-shaped female sex organs.
- Archegonia are consisting of
an elongated upper portion called neck and lower swollen portion -venter.The neck consists of an axial row of cells called
neck canal cells surrounded by a sterile jacket.The venter also made up of a 1-2 layer-thick wall of sterile cells which
encloses a larger egg cell or the ovum and the smaller ventral canal cell just above the egg.
- At maturity, the tip of
the archegonium opens and the neck canal cells as well as the ventral canal cells
disintegrate, opening the neck for the entrance of the antherozoids.
- Antheridia consist of rounded structure
consisting of a single layered jacket surrounding a central mass of cells - androcytes.
- Each changes into slender
- The antherozoids are released when the antheridium ruptures, thus allowing them to swim freely in a water film.The
antherozoids enter through the open necks and fuses with egg to form diploid zygote.
- After, divisions of
zygote a multicellular embryo is formed, which is nourished by the gametophyte.
- The embryo grows & forms a mature
within which sporogenous tissue will form spore tetrads, which in turn are released as the spores, forming either the
gametophyte, or the protonema, which in turn forms the typical gametophyte.
Characters of Division- Bryophyta(Mosses) , Division- Marchantiophyta or Hepatophyta(Liverworts) and
Division - Anthocerotophyta(Hornworts)
|| Filamentous, forming many buds
|| Globose, forming one bud
||Globose, forming one bud
| Gametophyte form
|| Leafy shoot
|| Leafy shoot or thallus; thallus simple or with air chambers
|| Simple thallus
| Leaf arrangement || Leaves in spirals
|| Leaves in three rows || Not Applicable
| Leaf form || Leaves undivided, midvein present.
|| Leaves divided into 2+ lobes, no midvein
|| Not Applicable.
| Special organelles || None
|| Oil bodies
|| Single plastids with pyrenoids.
| Water conducting cells ||Present in both gametophytes and sporophytes
|| Present only in a few simple thalloid forms
|| Brown, multicellular
|| Hyaline, one-celled.
|| Hyaline, one-celled
| Gametangial position || Apical clusters
|| Apical clusters (leafy forms) or on upper surface of thallus
|| Sunken in thallus, scattered |
| Stomates || Present on sporophyte capsule.
|| Absent in both generations
|| Present in both sporophyte and gametophyte.
| Seta || Photosynthetic, emergent from gametophyte early in development
|| Hyaline, elongating just prior to spore release
| Capsule || Complex with operculum, theca and neck; of fixed size
|| Undifferentiated, spherical or elongate; of fixed size
|| Undifferentiated, horn-shaped; growing continuously from a basal meristem.
| Sterile cells in capsule || Columella.
|| Spirally thickened elaters
|| Columella and pseudoelaters.
| Capsule dehiscence
|| At operculum and peristome teeth
|| Into 4 valves || Into 2 valves.
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
- 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
- All mosses have a sporic (diplohaplontic) life cycle that is oogamous.
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:-
Each protonema can spread over several centimeters, forming a fuzzy green film over
substrate. Usually this protonemal stage is short-lived, but in a few taxa, e.g., Buxbaumia it persists as the
phase of the plant.
- a horizontal system
reddish brown,anchoring filaments (rhizoids), called the caulonema
green filaments, the chloronema.
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
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.
- 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:-
:-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
- 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.
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
- 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.
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).
- 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.
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.
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.
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.
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
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
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,
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
- 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 :-