Introduction to botany
Gymnosperms are seed plants that do not produce flowers. The word
from the words gymnos meaning naked and sperm meaning seed, the name is so because of the fact that seeds of
not surrounded by a protective wall which exists in the angiosperms. However, usually when the seeds of gymnosperms are
immature they are enclosed within and protected by modified leaves or
a cone. In flowering plants or angiosperms, the ovary wall or fruit encloses the seeds, whereas in
gymnosperms there is no equivalent structure; hence, the interpretation of the seeds as "naked" or not enclosed.
Diversity and OriginThere are between 700 and 900
species of Gymnosperm.
It is widely accepted that the gymnosperms originated in the late Carboniferous Period. Early characteristics of seed plants
were evident in fossil progymnosperms of the late Devonian period around 380 million years ago.The gymnosperms are the most
ancient seed plants; they appear to have arisen from fern ancestors in the Devonian Period. Cycads retain the most primitive
characters of the extant seed plants. Gnetophytes are considered from morphological and molecular evidence to share a common
ancestry with the flowering plants
Conifers are by far the most abundant gymnosperms with around 600 species. Cycads are the next most abundant group with about
130 species. Approximately 75 - 80 species of Gnetales exist and only one species of Ginkgo remains today.
Classification of gymnosperms :- Gymnosperms have been variously classified by different workers from time to time.By the
beginning of the present century, Engler (1897), Coulter and Chamberlain (1910), Engler and Prantl (1926), Rendle (1930) and
others recognized gymnosperms as a primary division of Spermatophyta . They divided the group into classes or orders of
coordinate rank, viz. (1) Pteridospermae (Cycadofilicales) (2) Cycadales (3) Bennettitales (4) Cordaitales (5) Ginkgoales (6)
Coniferales and (7) Gnetales.Between 1950~1981, gymnosperms are included within Division Tracheophyta:- the vascular plants
The class essentially included the conifers and their allies (by which term is meant "related species of plants"), including
several groups of extinct plants known only from fossils. In these earlier classification schemes, the "naked seed" plants were
clearly set off from the other classes of higher plants (that is, the ferns and flowering plants), essentially as they are
today. However, fossil evidence suggests that the angiosperms evolved from a gymnosperm ancestor, which would make the
gymnosperm taxon paraphyletic. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor
and inclusive therefore of all descendants of that common ancestor. So, while the term gymnosperm is still widely used to
distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms, plant species once treated as gymnosperms
are distributed among four groups given equal rank as divisions within the Kingdom Plantae.The four division into which
gymnosperms are classified represent four evolutionary lines. These groups are:
Habit and habitat :- The gymnosperm is represented by a perennial, evergreen, woody plant. Most of
them are trees and some are shrubs.A few gymnosperms may be lianas or climbers. There are no herbs.Gymnosperms include one of
the world's tallese trees - Sequoia
sempervirens (the Giant Red Wood Tree) measuring about 125 meters in height and 30 meters in girth.
are distributed worldwide, with a majority, particularly the conifers, in temperate and subarctic regions. Cycads and
gnetophytes are mainly tropical to subtropical.
General features of Gymnosperms :-Morphology (external structure of
- The sporophyte plants
have a persistent tap root system.In Cycas there is symbiotic relationship between Cyanobacteria and root as a result coralloid
and in Pinus between roots and fungi produces mycorrhizic roots.
- The aerial trunk is branched or unbranched and woody.
Majority of gymnosperms have branched stem (except in Cycas and Zamia). In Pinus branches are of two types :-
- Long shoots or branches of unlimited growth
- dwarf shoots or branches of limited growth bearing clusters of
number of needle shaped leaves and collectively called as spur.
- Leaves are different in form and arrangement.
- They are both simple and compound ranging in size from a minute scale to leaves a few meters long.
Arrangements of leaves usually spiral, they may also be whorled as in Cedrus or opposite decussate as in Cupressaceae,
Welwitshchia and Gnetum.
- Scale leaves are known as microphyllous whereas larger leaves are known as megaphyllous and
their vascular supply always leaves a leaf gap in the stem stele.
- Venation may be parallel (Agathis and Welwitschia),
reticulate (Gnetum) or dichotomous (Ginkgo) or as in most genera there may be a single vein.
- Leaves may be
dorsiventral or isobilateral, amphistomatic( leaf having stomata on both surfaces)or hypostomatic (leaf having stomata mostly
on the underneath ) with sunken stomata.
Anatomy(internal structures of plant):-There is a
well-developed vascular system of xylem and phloem and have true roots, stems, and leaves.All gymnosperms are woody plants.
- Root :- vascular cylinder is di-to polyarch, xylem exarch. Main elements of xylem are tracheids. Phloem is composed of
cells and lacks companion cells.
- Stem :-. Their stems are similar in structure to woody dicots except that most
gymnosperms produce only tracheids in their xylem, not the vessels found in dicots.Vascular bundle of stems are collateral,
endarch or mesarch, open and arranged in a ring. Normally xylem
lacks vessel elements and the phloem lacks companion cells exception- Gnetales. Mature metaxylem and secondary xylem elements
have bordered pits of various types. Wood or secondary xylem of two
types namely, manoxylic and pycnoxylic. In manoxylic type (as in Cycas) wood is less, porous and soft. There is a large cortex
and pith and parenchymatous rays are wide. In pycnoxylic type (as in Pinus) wood is dense or compact with small xylem rays and
reduced pith and cortex.
- Primary growth is growth originating in the apical meristems of the
shoots and roots - results in an increase in length.
- Secondary growth is growth derived from secondary or lateral meristems - results in an increase in girth; example of
secondary growth- trees (wood and bark)
Results from the activity of two lateral meristems
Vascular cambium- forms secondary xylem (essentially all tracheids in gymnosperms; tracheids and vessels in angiosperms) and
secondary phloem .
- The vascular cambium produces secondary xylem to the inside and secondary phloem to the
- The cork cambium produces pelloderm to the inside and phellen (cork) to the outside. All of the tissues
outside the vascular cambium comprise the bark.
- Thus, the typical gymnosperm stem is a eustele with central pith
substantial amounts of secondary xylem, which in turn is enclosed by bark (Raven, Evert, & Curtis, 1981).
- New, living
wood produced by the vascular cambium towards the inside of the trunk is termed
sapwood. The main function of sapwood is to conduct water and minerals throughout the plant. Sapwood also stores reserves made
within the leaves.
- The water conducting
cells of gymnosperms are tracheids. However, the tube-like xylem cells only become fully functional water conducting cells
after they lose their
protoplasm and die.
- Earlywood consists of tracheids that have a wide diameter with thin
walls; whereas latewood consists of tracheids with smaller diameters and thick walls.
- The older wood in the center of
the stem is termed heartwood.
- Medullary rays connect the cental pith to the cambium. Rays are made primarily of
parenchyma cells and function to carry sap radially through the plant.
- The rays of conifers are usually very thin,
being one to
two cells wide.Some conifers possess tubular passages in
their wood called resin canals.
As woody plants grow in diameter a new layer of wood is produced each year by the cambium. This layer is called an annual ring
or growth ring.
Dendrochronology - The study of growth rings in trees and aged woods.
Example :-In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and
more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is
called an annual ring. By counting the rings (dark bands of summer xylem in pine wood), the age of a tree can be determined.
- In Cycas, vascular bundles are arranged in a
horse-shoe shaped manner or look like an inverted omega (U). They are diploxylic having both centripetal and centrifugal xylem.
- In Ginkgo petiole receives two vascular bundles and each half of lamina is supplied by one bundle which later dichotomizes.
Gnetum, the petiole shows an arc-shaped arrangement.
- Mesophyll may be differentiated into spongy and palisade parenchyma (Cycas,
Gnetum) or undifferentiated (Pinus).
- Transfusion tissue is a prominent feature. Leaf surface protected by a thick
sometimes by an additional waxy layer.
- Resin canals occur in all conifers (except Taxus) and mucilage ducts in cycads
Ginkgo and latex tubes in Gnetum.
- Vasculature of petiole quite variable.
Reproduction :-Vegetative reproduction are rare in gymnosperms but vegetatively propagating bulbils are known in
- Plants are heterosporous(micro- and megaspores). They may be monoecious (Pinus) or dioecious (Cycas).
- Reproductive structures are borne in cones or strobili that are either staminate (male) or ovulate (female) except in
where ovules are borne on loose megasporophylls.
- Sporangia are borne on fertile leaves or leaf-like structures called
microsporophylls (in male cone) and megasporophylla (in female cones) which are arranged spirally around a central axis.
- The female cones produce eggs, which are
contained in ovules. Female cones have a
sticky resin that “catches” the pollen released by
the male cone.
- Microsporangia(Male cone):- Microsporangia are borne on abaxial or lower surface of microsporophylls
(the modified leaves) and usually take several years to develop.Male cones are
normally smaller than female cones.
The male cones produce sperm, which are contained in pollen grains. They may be numerous and grouped in sori (Cycas) or
reduced to two (Pinus).
- Microsporogenesis :- Each microsporophyll bears a microsporangium in which the
microspores are produced.
- Within each sporangium meiosis occurs in many cells; many haploid spores are produced.
Microspores are produced in tetrads.
- Development of male gametophyte:- Still inside the microsporangium, each
microspore divides and grows to form a four-celled (four nuclei, ) microgametophyte (male gametophyte,) also known as pollen
which contains two sperm nuclei.Each cell of the microgametophyte, including the two sperm, is haploid. The microspore or
pollen grain at the time of shedding may have only one prothallial cell (Cycas) or two prothallial cells (Pinus), the former
being liberated at three-celled stage and latter at four-celled stage. Also, there is no antheridium in which the sperm are
produced. A grain of pine pollen also has two large air sacs to make it buoyant in the wind
Megasporangium or ovule is borne on adaxial or upper surface of megasporophyll or ovuliferous scale and are generally
orthotropous. Young megasporangium consists of :-
- Integument accessory structure surrounding ovule; 2n tissue
generally 3 layers: outer fleshy layer; middle sclerenchymatous layer and inner
fleshy layer; integument forms seed coat
- Nucellus parenchyma forming the main body of ovule; functionally similar to
megasporangium of lower plants; in mature seeds, the nucellus will become
compressed and essentially replaced by the cellular female gametophyte.
- Micropyle small opening at the upper end of nucellus which provides entrance for
endosporic male gametophytes (pollen grains). A space between the micropyle and the megasporangium is called the pollen
chamber. Both micropyle and pollen chamber have important roles in the pine life cycle.
- Megasporogenesis:-Only one cell in each megasporangium goes through meiosis. In addition, only one haploid megaspore is
produced by meiosis in each megasporangium.
- Development of female gametophyte:-The single megaspore develops into a
multicellular megagametophyte. Within the megagametophyte are several archegonia, each with an egg. The development of female
gametophyte is monosporic (except in Welwitschia and Gnetum where it is tetrasporic). It is permanently retained inside the
megaspore wall and dependent on parent sporophyte for its nutrition. The archegonia in female gametophyte may be two (in
Ginkgo) to many (as in many conifers). Archegonia are quite large and elongated and lack neck canal cells. Often the ventral
canal cell too, is eliminated. Gametophytic cells around the archegonia develop into a nutritive layer or jacket. However, the
nutritive layer and archegonia are absent in Welwitschia and Gnetum.
Pollination and Fertilization
- The microspores or pollen grains( partially developed, endosporic, male
borne by wind and enter the ovule directly through the micropylar canal.
- The transfer of pollen from the male cone to
female cone is pollination.
- When an ovule is receptive, a drop of sticky fluid is produced and fills the pollen
fluid exudes through the micropyle.
- Pollen sticks in the fluid exuding from the micropyle. As the fluid dries, it and
pollen are drawn in through the micropyle.
- The micropyle grows shut, entrapping pollen in the pollen chamber.
- The male
gametophyte (pollen grain) consists of two cells. One small and is called a generative cell. The other, larger cell is a tube
cell. The generative cell will later divide to produce two sperm.
- The pollen germinates and a tube digests through the megasporangium and into the megagametophyte. Eventually the pollen
grows close to an archegonium and ruptures, releasing a sperm into the egg. The fused sperm and egg is the diploid zygote, the
first cell in the sporophytic phase of the life cycle.
In all gymnosperms except Welwitschia, Gnetum, and Sequoia sempervirens, seed
development begins with a series of free nuclear divisions at the start of
embryogeny. Later embryo becomes cellular after wall formation and gradually differentiates into a suspensor, shoot apex,
cotyledons, hypocotyl and radicle. Polarity is endoscopic with the shoot end directed away from the micropyle. An immature
sporophyte is developed within the ovule and is surrounded by the megagametophyte. The immature sporophyte is an embryo. While
this is happening the outermost layer of the ovule, the integument, develops into the seed coat. Mature embryo is
differentiated into root, stem and leaves. .
Polyembryony:-In most gymnosperms a common feature in embryogeny is polyembryony - formation of several embryos in a
single gametophye inyoung seed. Usually from
fertilization of more than one archegonia or from cleavage polyembryony where
cells of developing embryo become separated and each give rise to a new (usually 4)
embryo.The mature seed normally has only one embryo. The young embryo draws its nutrition from the endosperm which develops
before fertilization and is haploid (n). Endosperm develops from female gametophyte that has absorbed the food from
nucellus.Histological maturation of various layers of integument continues and the stony layer becomes an extremely hard,
resistant shell which effectively encloses and mechanically protects the female gametophyte and the embryo.
The fertilized egg (zygote) develops into an embryo which is contained within the seed.
A seed is a matured ovule. The matured ovule consists of a seed coat, megagametophyte, and embryo. The seed coat is diploid
tissue from the original parent sporophyte. The megagametophyte is a haploid plant that developed from a megaspore produced by
the original sporophyte. The embryo is a new diploid sporophyte
The detached seeds of all gymnosperms (except for cycads and Ginkgo) remain dormant for sometime undergoing a resting period.
Gymnosperms being mostly temperate plants (except for cycads, Gnetum and Ephedra) growing in tropics and subtropics show very
little activity in the development of reproductive structures during winter. The activity is renewed in the spring. Under
favourable conditions the embryo resumes growth and after rupturing the seed coat develops into a new sporophyte plant, thus
completing the life cycle. .
Gymnosperms are plants with naked seeds (no fruit). Angiosperms are plants in which the seeds are enclosed
within a fruit.
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