Introduction :
Gymnosperm (Gk. Gymnos = naked ; sperma = seed) are the plants with exposed or naked seeds or ovules
Term Gymnosperm given by Theophrastus.
Study of Gymnosperm known as Gymnospermlogy.
Gymnosperm & Angiosperm are collectively included under spermatophyta i.e. seed bearing plants.
All gymnosperm are vascular plants. Therefore vascular tissue present i.e. xylem & phloem. Xylem lack vessels & phloem lack companion cells.
Exceptionally in xylem of Gnetum, Epheddra, Welwitschia true vessels are present.
In gymnosperm vascular bundle is Conjoint - Collarateral - Endarch - Open.
In the vascular bundle cambium is present therefore secondary growth takes place in gymnosperms, so that Gymnospoerms are woody plants.
Distribution :
Gymnosperm are very limited in distribution. They are mainly found in cold regions. In India Gymnosperm are found on Himalayan mountains . Xerophyte founds on slopes of mountain & cold region therefore gymnosperm are xerophyte.
Habit :
Living gymnosperms are mostly perennials, evergreen, arboreal and woody plants. They grow as wood trees, bushy shrubs or rarely as climbers (e.g., Gnetales). None of them are herbs or annuals.
External features
(i) The plant body is sporophyte and differentiated into root, stem and leaves.
(ii) The plant possess well developed tap root system. In some cases the roots are symbiotically associated with algae (e.g., Coralloid roots of Cycas) or with fungi (e.g., Mycorrhizal roots of Pinus).
(iii) The stem is erect, aerial, solid, woody and branched (unbranched in Cycadales) but almost tuberous in Zamia.
(iv) The leaves may be microphyllous or megaphyllous.
Gymnospermous wood
(i) Manoxylic wood : Cambial activity is short lived, cortex and pith are broad, parenchymatous rays are broad, wood is soft and commercially useless. e.g., Cycas.
(ii) Pycnoxylic wood : Cambial activity is long lived, cortex and pith are reduced, parenchymatous rays are few, wood is hard and compact, wood is commercially most important and used as good quality timber. e.g., Pinus.
LIFE CYCLE OF GYMNOSPERM :
1) In Gymnosperms main plant body is sporophyte (diploid). Gymnospoerms are dioecious. i.e. male & female plants are separate.
2) All Gymnosperms are heterosporus. At the time of reproduction two types of spores are formed.
Microspores - Male
Megaspores - Female
3)These two types of spores are formed in different sporangia.
⇒ Microspores are formed in Microsporangia, Microsporangia also term as pollen sac.
⇒Megaspores are formed in Magasporangia. megasporangia also term as ovule.
4)Both type of sporangia are formed on different sporophylls.
⇒Microsporangia are formed on Microsporophyll. It is known as stamen.
⇒Megasporangia are formed on Megasporophylls. It is known a carpel.
5)Both types of sprophylls are found in groups & form male cone (strobilus) & female cone.
Gymnosperm’s cone are just like flower of angiosperm.
Carpels of Angiosperm & Gymnosperm are different to each other.
Carpel of Gymnosperm :
Carpel of gymnosperm is less modified leaf it keeps its identity.
Ovule of gymnosperm develops openly on carpel ovule doesnot enclose in any structure. Therefore carpel of gymnosperm is termed as open carpel.
Carpels of Angiosperm :
Angiosperm’s carpel divide into three part ovary, style stigma.
Ovule enclosed in ovary. Therefore angiosperm’s carpel termed as closed carpel. i.e. angiosperm carpel is well modified leaf. Sprophyll lost its identity.
6) Meoisis takes place in cells of microsporangium & megasporangium and form microspore & meagaspore respectively.
7) In Gymnosperm (included angiosperm) germination of spores is endosporic i.e. germination of spore akes place with is sporangia.
8) Male gametophyte form after germination of microspore. Male gametophyte is also known as pollen grain. Male gametophyte forms male gamete.
9) In lower gymnosperm, male gametes are motile & multiciliate but in higher gymnosperm are angiosperm male gametes are non motile due to the absence of cilia and flagella. They (higher gymnosperm) do not require water for fertilization.
10) Female gametophyte form after germination of megaspore.
Female gametophyte of gymnosperm contains two type structures.
Archegonia - If form egg.
Endosperm - It provides nutrition for the development of embryo.
11) Pollen grain (male gametophyte) reach at the ovule by pollination. Pollen grains reach at the micropyle (ovule) by wind. Anemophilly occur in gymnosperm.
FERTILIZATION -
Two types of fertilization take place in gymnosperm
(1) Zoodio - siphonogamy - This type fertilization occurs in lower gymnosperm. Male gamete is motile and contained in pollen tube.
(2) Siphonogamy - This type of fertilization occur in higher gymnosperm. Male gamete is non motile and transferred to female gamete (egg) by pollen tube. After Pollination male & female gametes are fused & form a diploid zygote.
VARIATION BETWEEN GYMNOSPERMS and ANGIOSPERMS
In gymnosperm single fertilization takes place so that single zygote form through fertilization. In Angiosperm double fertilization takes places so that two product are formed are formed after it (i) Zygote (ii) Endosperm.
In gymnosperm endosperm form before fertilization so it is haploid but in angiosperm endosperm are formed after fertilization so endosperm of angiosperm is triploid.
DEVELOPMENT OF ZYGOTE -
(1) Embryo formed by the development of diploid zygote. After embryo formation ovule term as seed. (Seed → Ovule + Embryo)
Ovule is a sporangia. Embryo enclosed ovule because development of spore is endosporic.
Seeds are not formed in pteridophyta because germination of spore is exosporic i.e. embryo develops outside the sporangia.
Note :- Polymbryony occurs in gymnosperm i.e. a single seed develops many embryo.
There is two type Polyembryony
(A) Potential polyembryony - Many embryo form by fertilization of many archegonia.
eg. Cycas - Single ovule of Cycas contains two archegonia. Fertilization occurs in both archegonia so two zygote form in a ovule. So two embryo form. Potential polyembryony is
not true polyembryony because a zygote form only one embryo.
(B) Cleavage polyembryony - Many embryo are formed by the cleavable in zygote so it is true polyembryony.
eg. Pinus
(2) Embryo enclosed is seed. Seed absorbs water & bursts. Now embryo germinate and form a new diploid plant.
COMPARISION OF LIFE CYCLES
Life cycle of Gymnosperm & angiosperm is diplontic because gametophytic generation is short lived. Gametophyte is very reduced & depend on its sporophyte.
Antheridia is absent in gymnosperm & angiosperm i.e. pteridophyte is last group of antheridia.
But Archegonia is also absent in angiosperm. So gymnosperm is last group of archegonia.
Evolution of archegonia starts form liverworts. It is well develops in moss. Gymnosperm is last group of it. So in this group it is very reduced.
Archegonia of liver worts - NCC = 4 - 6 VCC = 1 Egg cell = 1
Archegonia of Moss - NCC = 6 - 18 VCC = 1 Egg cell = 1
Archegonia of Pteridophyta - NCC = 1 - 2 VCC = 1 Egg cell = 1
Archegonia of Gymnosperm - NCC = Absent VCC = 1 (Short lived) Egg cell = 1
Note : neck of archegonia of Ephedra is longest.
During evolution Gametophyte becomes reduced & sporophyte becomes well-develop
Gametophyte
Very reduced - In Angiosperm
Well developed - In Moss
Sporophyte
Very reduced - In Thallophyta (Only zygote)
Well developed - In Angiosperm
Classification :
Robert Brown (1827) recognised the gymnosperms as a group distinct from Angiosperms. However, Bentham and Hooker (1862-83) in their 'Genera Plantarum' placed them between Dicotyledonae and Monocotyledonae, Chamberlain (1934) divided gymnosperms into following two sub-classes.
(i) Cycadophyta
(ii) Coniferophyta
(i) Sub-class I. Cycadophyta :
These are characterised by the presence of unbranched stem and large foliage leaves. Internally, the stem has large pith and cortex but the wood is relatively small. It includes following 3 orders.
Order 1. Cycadofilicales : It is a group of fossil plants. These plants resembled with ferns, hence they were given the name Pteridospermae (i.e., seed bearing ferns). e.g., Lyginopteris, Medullosa.
Order 2. Bennettitales or Cycadeoidales : It is also a group of fossil forms. These plants resembled with modern cycads. e.g., Cycadeoidea, Williamsonia.
Order 3. Cycadales : It includes both living and fossil forms. e.g., Cycas, Nilssonia, Zamia.
(ii) Sub-class II. Coniferophyta :
These are characterised by long profusely branched stem and simple small leaves. In stem the amount of wood is much more than cortex and pith. It includes following four orders.
Order 1. Cordaitales : All the members of this order are extinct. e.g. Cordiates, Dadoxylon.
Order 2. Ginkgoales : All the members of this order, except for Ginkgo biloba are extinct. Ginkgo biloba is a medium sized tree with branched stem and bilobed leaves. Because of the resemblance of the leaves of this plant with those of Adiantum (maiden hair fern), the name Maiden hair tree has been given.
Order 3. Coniferals : The order includes both fossils and present day forms. e.g., Pinus, Cedrus, Sequoia.
Order 4. Gnetales : Gnetales are modern group consisting of living forms. The order differs from other gymnosperms in the presence of vessels in the xylem. e.g., Ephedra, Gnetum, Welwitschia.