- Books Name
- ACME SMART COACHING Biology Book
- ACME SMART PUBLICATION
- CBSE Class 11
The term gymnosperm was used by Theophrastus (300 Be) in his book Enquiry into plants.
They are popularly called naked seeded vascular plants. Goebel called them as phanerogams without ovary.
It is smallest group of plant kingdom.
General characters of gymnosperms:
Majority of gymnosperms are perennial woody plants forming either shrubs or trees. Ephedra is a climber. Some of these are very large and live for thousands of years, e.g., Sequoia.
Unlike bryophytes and pteridophytes, in gymnosperms the male and female gametophytes do not have an independent free living existence.
Tap root system is present. Cycas also has special azeotropic N2 fixing coralloid roots having association with Anabaena cycadae (BGA). Pinus root shows symbiotic relation with ectomycorrhiza.
The leaves may be simple (Pinus) or compound (Cycas). These leaves are well adapted to withstand extremes of temperature, humidity and wind, e.g., Needle leaf of Pinus. Scale leaves may also be present.
Stem is woody and branched (Pinus, Cedrus) or unbranched (Cycas).
These are heterosporous. There occurs two types of sporophylls; microsporophylls and megasporophylls. The two types of sporophylls are usually aggregated to form compact cones (strobili) i.e., pollen cones (male cones) and seed cones (female cones).
Microsporophylls do not show distinction of filament and anther.
The micros pores develop into a male gametophyte which is highly reduced and is confined to only a limited number of cells. This reduced male gametophyte is called pollen grain. The development of pollen grains takes place within the microsporangium.
Megasporophylls are not rolled like carpels. A distinction of ovary, style and stigma is absent.
Ovules (integumented. megasporangium) lie exposed on the megasporophyll. Each ovule is surrounded by a 3-layered single integument (Bitegmic in Gnetum).
Female gametophyte contains archegonia (Absent in certain members of order Gnetales)
Pollination is direct as a stigma is absent and the pollen grains directly reach the micropylar end of ovules. Pollination is usually accomplished by wind (anemophily).
Male gametophyte produces only two male gametes or sperms. Generally, one of them is functional.
External water is not required for transport of male gametes. Instead, a pollen tube is formed by the male gametophyte for effecting fertilization (siphonogamy).
Seeds contain a food laden tissue or endosperm for future growth of embryo into seedling. This tissue represents the female gametophyte, hence haploid.
Like pteridophytes, xylem does not possess vessels, except in some Gnetales. Phloem is without companion cells. Albuminous cells perform the function of companion cells. Sieve tubes are also not formed as the sieve cells are not arranged end to end in rows.
Vascular tissues are arranged into vascular bundles just like angiosperms.
Foliage leaves do not have lateral veins. Transfusion tissue (hydrostereom) occurs internally for lateral transport.
Wood is soft and homoxylous but members of Gnetales bear vessels in xylem.
Types of wood :
(i) Manoxylic :
Soft wood, vascular tissues with medullary rays, commercially less important, e.g., Cycas.
(ii) Pycnoxylic :
Compact wood without or with narrow medullary rays, commercially more important, e.g., Pinus.
(iii) Monoxylic :
With single persistent cambium ring and bundles, e.g., Pinus
(iv) Polyxylic :
With many persistent cambium rings and bundles, e.g., Cycas
Gymnosperms are, therefore, those seed plants in which the ovule remains exposed over the surface of the megasporophylls both before and after fertilisation.
1. Smallest gymnosperm = Zamia
Largest gymnosperm = Sequoia
2. The three generations in seed are:
- (i) Testa, tegmen and perisperm represent parental sporophyte
- (ii) Endosperm represents female gametophyte;
- (iii) Plumule, radicle, suspensor and cotyledons (embryo) represent future sporophyte.
3. Polyembryony: It is the formation of more than one embryo inside a single seed. It was reported by Leeuwenhoek in oranges. Simple polyembryony is due to fertilisation of many eggs, e.g. Pinus ovule has 2-8 archegonia. Cleavage polyembryony is true polyembryony and very common. It is due to splitting of embryo tissue. Adventive polyembryony is the formation of extra embryos directly from diploid cells (e.g., rosette cells) other than embryonal cells.
4. Order Gnetales consists of Gnetum, Ephedra and Welwitschia. These are nearer to flowering plants in having flower like arrangement of sporophyll and possessing primitive vessels in xylem so wood is called heteroxylous. Plants of Cycadales and Coniferales are commonly called Cycads and Conifers respectively.
5. Cycas, Ginkgo and Metasequoia are living fossils.
6. Ginkgo biloba (Pagoda tree or Maiden hair tree) is oldest living fossil and it is connecting link between cycades and conifers.
7. Largest ovule (found in Cycas revoluta)
8. Largest male cone (in C. circinalis)
9. Largest male gamete (sperm) in Cycas
10. Largest female gamete (egg) in Cycas
11. Independent, free living, photosynthetic gametophyte is not found in gymnosperms and angiospersm.
12. Gymnosperms are divided into four orders-Ginkgoales, Gnetales, (e.g. Ephedra, Gnetum, Welwitschia), Cycadales and Coniferales. Ginkgoales is represented by maiden hair tree (Ginkgo biloba) where Ginkgo biloba happens to be only living member, rest all are extinct. So it is also called as living fossil.
i. Edible Seeds. Seeds of Pinus gerardiana called chilgoza are edible.
ii. Timber. Gymnosperms possess soft wood. The same is used in preparation of light furniture , plywood, packing cases, match sticks, railways sleepers, etc, e.g., Cedrus deodara.
iii. Resin. Resin is a semifluid secreted by special resin canals. It solidifies on exposure to air. Therefore, it plugs the places of injury. It helps in sealing female cones after pollination. Resin is commercially extracted and distilled to obtain turpentine and resin. Resin is used in water proofing, sealing joints and preparation of writing paper. Turpentine is used as solvent in paints, polishes and wax, e.g., Pinus.
iv. Ephedrine. Drug ephedrine is obtained from Ephedra. The drug is used in curing respiratory elements including asthma.
v. Sago. A starchy food sago is obtained from stem of Cycas revoluta which is thus also called as sago palm.
vi. Canada balsam. A mounting agent used in preparation of permanent slides is obtained from Abies balsamaea.
vii. Cedar wood oil. Useful in microscopy is obtained from Juniperus virginiana.
viii. Taxol. Anticancerous chemical obtained from Taxus.
Life Cycle of Pinus
Pinus is a coniferous gymnosperm.
It is a large tree having pyramidal or excurrent shape like a "Christmas tree". The plant body is a sporophyte having stem, leaves and roots.
The main stem is straight. Stem branches are dimorphic i.e., of two types, long and dwarf. Long shoots show unlimited growth and bear only scale leaves. The dwarf branches possess 1-5 needle-like foliage leaves which are surrounded at the base by a sheath of scare leaves.
Pinus has a horizontally spreading tap root system with peg-like downwardly directed roots present at intervals.
Finer roots are of two types-normal (with root hairs and root cap) and mycorrhizal. The mycorrhizal roots occur near the soil surface.
They are devoid of root hairs and root cap. Mostly Boletus (fungus) forms an ectomycorrhiza with roots of Pinus.
The sporophytic plant body does not multiply vegetatively. It is a monoecious tree. It produces microsporophylls and megasporophylls in two types of cones, male and female.
Male or Pollen Cones
These are non-woody structures occurring sub-terminally in clusters on lower long branches.
These cones are homologous to dwarf shoots. Each male cone has a short stalk, a central axis and a number of spirally arranged microsporophylls.
A microsporophyll bears two oblong, parallel microsporangia on its lower surface. In a microsporangium, the diploid microspore mother cells undergo meiosis and form haploid microspores or pollen grains.
The pollen grains are released from the male cones at 4 celled stage, having two prothallial cells, one generative cell and one vegetative cell and dispersed by air currents.
They form yellow clouds in the pine forests called as 'sulphur shower'. A pollen grain has two air sacs or wings for making it light, which are formed by its exine
Female or Seed Cones
These are initially green but become woody and brown at maturity.
They develop in groups of 2-6 on upper long branches of the tree. These are homologous to long shoots.
Each female cone has a long stalk and a central axis on which are borne a number of spirally arranged paired scales. The lower of the pair is called bract scale while the upper scale is ovuliferous scale.
The ovuliferous scale bears two ovules towards the basal region on the upper side.
Each ovule has a 3-layered integument with a terminal wide, oblique pore or micropyle and nucellus (= megasporangium). In the middle of nucellus, a megaspore mother cell differentiates.
It forms four haploid megaspores by meiosis but only one survives.
The functional megaspore gives rise to female gametophyte called endosperm.
The female gametophyte bears 1-8 archegonia. An archegonium has a short neck and a large venter.
Neck canal cell is absent. Venter canal cell is ephemeral. The venter contains a large egg or oosphere.
Pollination is anemophilous (by air) and direct. The formation of male gametes occur after pollination.
The pollen grains pass into the opened micropyle and rest on the tip of nucellus, where they are caught in the mucilagenous pollination drop oozed out by terminal part of nucellus.
Here each pollen grain germinates and forms a pollen tube (siphonogamy) but further growth is stopped due to arrival of winter in the first year.
Fertilization occurs after 13 months from the time of pollination. In the spring of next year, fertilization occurs.
Pollen tube grows and pierces an archegonium.
One male gamete or male nucleus fuses with the oosphere to form diploid zygote or oospore. The oospore forms an embryo while the ovule matures into a seed.
Part of the upper surface of the ovuliferous scale is peeled off alongwith the seed to form its wing.
A female cone takes about 26 months for reaching maturity. It then opens to release winged seeds which are dispersed by air. After falling on a suitable soil, each seed gives rise to a new plant.
(a) Development of embryo is meroblastic in Pinus.
(b) The number of cotyledons in Pinus ranges from 3 -18.
(c) There are 4 tiers in proembryo, namely basal embryo tier, suspensor tier, rosette tier and upper tier.
(d) Development of sporangium in Pinus is eusporangiate.
(e) Pinus shows polyembryony by three methods -simple, cleavage and adventive.
Salient Features of Cycas (Sago Palm)
Cycas is called Palm-fern. It bears crown of leaves on unbranched stem called caudex like palms. It also shows circinate ptyxis and presence of ramenta like ferns.
Roots are of two types-normal tap root and ageotropic coralloid roots. Latter are formed in form of coralline masses which bear blue green algae in their cortical cells, e.g., Anabaena, etc. They help in nitrogen fixation.
Leaves are of two types-scale and foliage, both arising in whorls. Every year almost one is added.
Old leaves fall and leave characteristic leaf scars on the stem. Foliage leaves are leathery and pinnately compound. Scale leaves are brown, and membranous.
Stem of Cycas shows manoxylic and polyxylic wood.
Rachis shows presence of diploxylic vascular bundles arranged in shape of inverted omega (w).
Transfusion tissue replaces lateral veins in the leaves for lateral transport of food.
Plants are dioecious. Male cones are found but female cones are absent (loose megasporophylls). Largest egg, sperm and ovule are found in Cycas. Ovules are orthotropous.
Sperms in Cycas are multiflageliate (= multiciliate). Pollen tube is formed in Cycas, but is haustorial in nature. Thus, Cycas shows both siphonogamy and zooidogamy.