Diversity of Seed Plants and their Systematics
Gymnosperms I
CHARACTERISTICS FEATURES OF SEED PLANTS WITH (ANGIOSPERMS) AND
WITHOUT (GYMNOSPERMS) FRUITS.
Dr. NUPUR BHOWMIK
Department of Botany
University of Allahabad
Senate Hall
Allahabad – 211002
neelamalld@yahoo.com
Date of submission: 27/06/2006
Keywords
Endosperm, seed, saprophyte, inflorescence, pollen tube, synergids, carnivorous, megasporangium,
stigma, nectaries, Coniferophyta, nucellus, microsporophylls, ovule, archegonium, Pteridosperms,
epiphyte, parasite, prothallial cells, integument, dendrochronological, cretaceous, Devonian.
The spermatophytes or "seed bearing" plants have embryo enclosed within the seed and remains
protected by the seed coat. Reserve food material is found either in the cotyledons or in the endosperm
which normally surrounds the embryo. There are two main groups of "seed bearing" plants:
gymnosperms and angiosperms.
The gymnosperms are naked seeded plants where the ovules and seeds are unprotected and are not
enclosed in the fruit but are freely exposed on open megasporophylls. The angiosperms are close
seeded plants, where the seeds are enclosed in the fruit. They are regarded as "flowering plants" and
are the highest evolved forms amongst plants. There are two broad groups of angiosperms the
dicotyledons and the monocotyledons. In the dicotyledons the embryo of the seed bears two
cotyledons whereas in monocotyledons the embryo of the seed bears only one cotyledon.
Adapting Cronquist et al. (1966) angiosperms have been placed in the Division Magnoliophyta and
the Division is divided into two classes, the Magnoliopsida and Liliopsida (Cronquist, 1981)
corresponding to dicotyledons and monocotyledons. Besides cotyledon number there are some
additional distinctions between the two classes :
Class Magnoliopsida
Class - Liliopsida
(Dicotyledons)
(Monocotyledons)
1.
Two cotyledons in embryo
(exception-several species of
Ranunculaceae, Umbeliferae
and
Gesneriaceae
having
single cotyledon)
Single cotyledon
2.
Vascular bundles of
arranged in a cylinder.
stem
Vascular
bundles
commonly
irregularly disposed in stem or in
one or more rings as seen in
transverse sections.
3.
Herbaceous and woody forms.
Woody species with vascular
cambium
producing
secondary xylem and phloem.
Mostly herbaceous, some woody
forms with specialized cambium
giving rise to secondary vascular
bundles.
4.
Leaves with reticulate
venation. Ultimate veins
ending freely in mesophyll.
Leaves frequently with "parallel"
veins. Veins fused near tip of leaf,
leaves also have small
interconnecting veins.
5.
Flower parts in fours or fives
or
their
multiples
or
indefinite. Certain floral parts
greatly reduced in numbers or
absent in many species.
Floral parts basically in threes or
multiples of three.
Angiosperms or "flowering plants" (Magnoliophyta) form the largest group of the plant kingdom
including about 300-400 families, 8000 genera and more than 300,000 species. They are considered to
be the highest evolved plants on earth arising in the Mesozoic (Cretaceous) Age and forming the
dominant vegetation today. The group includes crop plants (eg. wheat, rice, maize etc.) that form the
basic food supply of the world. They are necessary for human survival in the form of wood for
buildings, drugs and fibres. They are found growing almost everywhere in each possible type of
habitat and climate. They occur in deep lakes, deserts, in sea beds and even on high mountain peaks.
Species of Opuntia growing under acute desert conditions can survive without water whereas the
aquatic Hydrilla is extremely sensitive to dry conditions. Some species of "river weed" family or
Podostemaceae grow attached to rocks in swift flowing streams or waterfalls, while some are marine
like Rhizophora found growing near sea water and forming a part of "mangrove vegetation".
They exhibit tremendous variations ranging from small stemless free floating plants like the minute
sized Wolffia microscopica or the attached thalloid aquatic angiosperm of Podostemaceae family
having variable shape and size often resembling algae, lichens or liverworts. The plant body of
Podostemads consists of thallus, secondary shoots with leaves and anchoring organs like haptera or
root hairs. Besides these thalloid forms angiosperms also include some of the tallest trees of the world
like species of Eucalyptus from Australia.
Angiosperms may be annuals, biennials or perennials. They may be herbs, shrubs, trees, climbers,
twiners and lianas. They may be epiphytes, parasites, saprophytes, symbionts or even insectivores.
Some species of angiosperms have adapted intelligent means to survive in their otherwise hostile
environment. To cite the example of a few-the Pebble plant (Lithops sp.) growing in the harsh Namib
desert of Africa has leaves that mimic their surroundings to avoid attack of prey. The plant has two
rounded, succulent leaves which not only reduce surface area for evaporation but also make it difficult
to be identified among the gravel and pebbles. The colour of the plant varies from grey or blue, to
yellow, orange or brown just like the surrounding pebbles and thirsty grazers simply don't see it.
(Plate-1, Figure-4)
Others like the fescue grass and silverweed establish their dominance by increasing their territory year
after year. They do so by putting out traveling stems that advance horizontally close to ground. This
habit of the fescue grass not only makes it one of the longest lived of all herbaceous plants but also
one of the largest too. (Plate-1, Figure-3)
Angiosperms can also boast of having the tallest of all seed-bearing plants like the Australian
mountain ash -Eucalyptus regnans. The gigantic noble tree formed great forests in the hills of Victoria
and Tasmania. They reached heights above 325 feet. A fallen trunk of the species was measured to be
435 ft. long. (Plate-1, Figure-1)
Flowers of many angiosperms arise in inflorescences and the inflorescence of a species of the arum
family is a monster. If the inflorescence may be regarded as a single flower then the titan arum
(Amorphophalus sp.) could be the largest flower in the world. Found growing in central Sumatra in
Indonesia, the inflorescence has a trumpet- shaped spathe from the centre of which projects a spadix.
The spathe is 3 feet across and 4 ft. long and the spadix is 9 ft. tall. The inflorescence produces a
pungent aroma (smelling mildly of rotting fish) to attract pollinators. The giant "flower" arises from a
corm which is a modified underground stem. The seedlings produce a leaf which when full grown
may be 20 ft. tall and forms an umbrella-like canopy 15 ft. across. (Plate-2)
Compared to the titan arum which has the biggest flower like inflorescence, the fig (Ficus glomerata)
has the distinction of bearing several hundred small flowers inside a marble sized green sphere. The
inflorescence (hypanthodium) which are produced in bunches that arise directly from the fig trunk.
(Plate-3, Figure-3) Each green sphere has a small hole opposite the stalk. This is the entrance to the
inflorescence. The flowers nearest the entrance are male flowers and deeper down are female flowers.
Amongst the many fresh water aquatic angiosperms the giant Amazon water lily is quite spectacular.
The plate-like giant leaves, 6ft. across have upturned rims and a single plant may produce forty to
fifty such leaves that float on the surface rarely allowing others to grow alongside or below it. (Plate4, Figure-2) Like the leaf the lily's flower is also outsize running more than a foot in diameter. Its
petals are initially pure white in colour and open in the evening. (Plate-4, Figure-3) The flower emits a
rich perfume for a particular species of beetle which pollinate the flowers. The beetles feed on the
glistening drops containing sugar and starch and while leaving carry the pollen grains to other flowers.
The colour of the flower turns pink on the next night. (Plate-4, Figure-1)
Out of the many effective means of seed dispersal seen in angiosperms a Brazilian tree Hura sp.
commonly called as monkey's dinner bell, throws out its seeds with a great explosion from the pods
sometimes at distances over forty feet. Some seeds are small like cotton seed covered with tuft of hair
which can make them fly for miles by catching the wind.
Some trees disperse their seeds by sea, as in coconut palm. The seed has a hard shell that contains
everything needed for a long voyage. Inside is the food rich endosperm and outside is the fibrous
mesocarp which keeps the fruit afloat. (Plate-1, Figure-2)
While normally animals turn to plants for food there are rare instances where plants growing in
nutrient impoverished environments- bogs, mountains slopes washed by heavy rains, moorlands etc.
feed on animals to fulfill their nutritional requirements. The simplest vegetable carnivore is
Heliamphora or marsh pitcher (Plate-3, Figure-1) which has a simple trap. The foot long leaves are
curled lengthwise and joined at margin to form a tall vertical tube. The top of the midrib region
enlarges to form a reddish rimmed hood which has numerous nectar producing glands. Flies and
mosquitoes attracted by the sweet fragrance of nectar alight on the hood and move down into the tube.
The tube is covered with long, slippery downward pointing hairs. The base of the tube has water into
which the flies and mosquitoes drown. Bacterial decay dissolves the resulting soup.
The shape of the pitcher plant traps varies in different species and where they develop on the plant.
The Phial pitcher has globular traps, whereas those of Raffles' pitcher are a foot deep. Low's pitcher is
a climber and dangles its traps in the air, whereas hairy pitcher rests its six inch traps on the ground.
(Plate-5)
Besides pitchers and trumpets leaves of some animal hunting plants may be converted into active
traps as in Sundews (Drosera) (Plate-6) and Venus' fly trap (Dionea). (Plate-3, Figure-2) Plants may
be climbers or rosette plants. The leaves in Drosera are covered with hairs with each hair bearing a
shining drop of fluid at the tip. The fluid contains not only glue but also digestive substances that
dissolve the insect's body.
Another animal hunting plant is the Bladderwort (Utricularia). (Plate-7, Figure-3) It is aquatic and
lacks roots. Their traps, the bladders from which they get their name, are tiny transparent capsules.
There are glands on the inner surface of capsules which are able to absorb water. Each capsule has a
small lid fringed with sensitive bristles. The tightly fitting lid opens if a small water creature
accidentally touches the bristles. Water rushes in carrying the little organisms with it.
All angiosperms may not be autotrophs some may develop close relationship with other plants by
parasitizing other plants through roots or cling directly on to trunk or branch of a tree. The mistletoe
(Loranthaceae) family (Plate-8, Figure-3) has over a thousand different species most of which live in
tropics and are parasites. They may form tall trees, or may be dwarf members. Besides the mistletoes
a very common leafless plant parasite is Dodder (Cuscuta sp.) (Plate-8, Figure-2) which is a climber
and totally dependent upon its host. But the most spectacular of all plant parasites is Rafflesia growing
on a host vine in the forests of Borneo and Sumatra. The orange coloured flower is 3 ft in diameter believed to be the largest in the world, and when fully open emits a repulsive smell of rotting flesh.
(Plate-9)
Some angiosperms are known to inhabit hostile terrains like the Arctic circle, harsh deserts, mangrove
swamps etc. The plants evolve different forms for better adaptation to their new circumstances. Where
cold remains a major danger, a furry blanket helps keep off the chills as is seen in a Himalayan plant
Saussurea (Brahm kamal) which protects its leaves within the mound of fur. There is a hole at the top
through which pollinating bees reach the flowers. (Plate-8, Figure-1)
Angiosperms in the mangrove swamps of the tropics deal with the difficult problem of their roots
becoming drowned twice a day. (Plate-7, Figures-1 & 2) Where the river reaches the sea, plants
anchor themselves into the sticky mud by means of their roots and support themselves with numerous
arching prop roots which grow close together. Oxygen supply to the water logged roots takes place
through lenticels present in arching prop roots. While the adult trees become considerably stable with
the help of prop roots the seedlings find difficulty in anchoring themselves in the moving surface of
the mud. The seeds germinate while they are still attached to their parent (vivipary). A long green
stem develops that hangs vertically downwards, becomes detached and develops tiny rootlets to
anchor itself in the mud. These are but only a few interesting features among myriads exhibited by
some of the fruit bearing seed plants.
Salient Features of Angiosperms :
1.
The sporophyte is the dominant and long lived phase in the life cycle of angiosperms. They
may be trees, shrubs, herbs or climbers etc. The plant body is differentiated into roots, stems
and leaves. The sporophyte shows unlimited growth by means of apical and lateral meristem
which produce secondary vascular tissues (monocots lack the type of cambial activity
commonly present in dicots.) The root system in well developed and roots may be diarch,
triarch, tetrarch or polyarch. Primary xylem is exarch. Stem stele is eustelic and show
secondary growth in most dicots.
2.
The vascular system comprising of xylem and phloem are highly evolved. The xylem
possess vessels in addition to tracheids. Among primitive vesselless forms are Degeneria,
Drimys, Austrobaileya, Tetracentron, Trochodendron, Pseudowinteria, Sarcandra and
Nympheales. The phloem is composed of two types of characteristic cells - the sieve tube
elements and the companion cells. The phloem cells are placed in an end to end series with
sieve plates being confined to end walls.
3.
The sporophytes are heterosporous producing two kinds of spores - microspores (pollen
grains) and megaspores.
4.
The reproductive structure is called as the flower and one of the most definitive organs of the
flower is the carpel which resembles a megasporophyll in function but bears ovules (or
megasporangia) enclosed within its hollow basal portion called as ovary. Each ovary
terminates in a stigma which serves as a receptive structure for pollen grains.
The flower consists of the terminal part of the floral axis called as the receptacle to which are
attached the floral parts. Flowers are bisexual or sometimes unisexual. The basal outermost
segments of the flower are the sepals collectively called as calyx. Inner to the calyx is the
corolla consisting of a variable number of petals. Inner to the corolla are the androecium
comprising of the stamens. Each stamen consists of a fertile distal part termed as anther inside
which pollen is produced and a sterile proximal part called as filament. The innermost
segment of the flower consists of one or more separate carpels. Carpels may be fused into one
unit called a pistil. They form the gynoecium. A single carpel or the fused carpels are
composed of three regions - the stigma, style and ovary. Inside the ovary are enclosed ovules
which become seeds in a mature ovary (fruit). Ovules are attached to the inner wall of ovary
by strips of specialized tissue termed placentae. Vascular supply to the flower closely
resemble that typical of a vegetative shoot.
5.
The functional megaspore is permanently retained within the nucellus or megasporangium of
ovule.
6.
Mode of pollination is frequently zoophilous or sometimes anemophilous. The flower is
attractively designed, coloured, often scented and may have nectar secreting glands. All these
phenomena are connected with zoophily. Pollen grains deposited on the stigma of the carpel
never get inside the microphyle of ovules and only pollen tubes travel through the stylar
canal and enter the ovules.
7.
Spore dimorphism results in the production of two types of gametophytes - male and female
gametophytes. The gametophytes are endosporic and reduced.
The male gametophyte is extremely reduced consisting of two cells (a generative cell and
vegetative cell) at the time of dispersal and after germination of pollen grain the pollen tube
consists of the tube nucleus and two non-ciliated male gametes.
The female gametophyte may be of monosporic type (developing from one megaspore out of
the four megaspores produced after meiosis of megaspore mother cell) or bisporic or
tetrasporic type resulting from elimination of wall formation between the first two or four
daughter nuclei of the megaspore mother cell. The female gametophyte or embryo-sac lacks
archegonia, has no extensive development of vegetative tissue and is eight nucleate. The eight
nuclei arise when the megaspore undergoes three mitotic divisions. After first division each of
the two nuclei moves to opposite poles and after the third division there are four nuclei at
each pole. The embryo-sac has an egg apparatus comprised of three cells (a central egg cell
flanked on either sides by two synergid cells) towards the micropyle, three antipodal cells at
the chalazal pole and two polar nuclei in the central cell.
8.
The non-motile male gametes are carried by the pollen tube into the embryo sac where one
of the gametes fuses with the egg cell nucleus to form the zygote and the other fuses with the
two polar nuclei (fused together to form secondary nucleus) to form the primary endosperm
nucleus.
9.
This occurrence of "double fertilization" or "triple fusion" is an entirely novel phenomenon
in angiosperms.
10. The endosperm is a post-fertilization nutritive tissue. It may be diploid to polyploid.
11. The whole of the zygote (fertilized egg) forms the embryo and the first division of the zygote
is followed by wall formation and free nuclear divisions are almost absent. Polarity of
embryo is endoscopic where root apex of the embryo is directed towards the micropyle.
12. The embryo is enclosed within the seed (fertilized ovule). Seeds are enclosed within the
ovary wall which develops into the fruit.
13. Polyembryony is generally absent in angiosperms.
Gymnosperms are seed plants that compose a major component of the temperate forests of the world.
They are widely appreciated for the great range of variation in habit and shapes. They have some of
the tallest trees known in the plant kingdom. Sequoia sempervirens, commonly called the Californian
or "Coast Redwood" because of its red coloured bark and red heart wood, reaches a height of 112
metres, and a diameter of 5 metres (See Pl. 10 & Pl. 11). Another species, Sequoiadendron giganteum
from California, U.S.A. commonly called "Big Tree" or the Wawona Tree, is a little shorter in length
about (87m) but attains a diameter of more than 10 metres (See Pl. 12).
The age of tallest tree is estimated from 400 to 800 years. The oldest redwood on record, which has a
diameter of 3.7 metres across is estimated to be 2,200 years. However, the distinction of being the
oldest living tree on earth has now gone to a species of Pinus, P. aristata now called as P. longavea.
A tree of this species is growing at 3275 m. above sea level in east Nevada-Utah State boundary (See
Pl. 13). The age has been dendrochronologically shown to be 4900 years old (Foster & Gifford, 1974).
Other estimates of age of the oldest tree of this species are mentioned as their being as old as 6000
years old (Sahni, 1990).
However, all gymnosperms are not tall and gigantic. There are some small herbaceous looking forms
also, like Zamia pygmaea and Stangeria paradoxa (See Pl. 14) or the two parasitic forms, Parasitaxus
(= Podocarpus) ustus (See Pl. 21) and Gnetum trinerve. While the former parasitises on Dacrydium
taxoides, the latter is a parasite in Cinchona forests of Chimboraza (Pearson, 1929). A species of
Zamia, Z. pseudoparasitica is exceptional among all gymnosperms in being epiphytic (See Pl. 15). A
few gymnosperms may be lianas or climbers.
They are mostly evergreen, but some such as Larix and Taxodium are deciduous (See Pl. 16). The
plants are mostly xerophytic but plants such as Taxodium are also known to grow in swamps or
marshy places in Mexico and Florida. (See Pl. 17)
The gymnosperms include ancient lines of seed-bearing plants whose evolutionary history dates back
to the Upper Devonian, roughly about 375 million years ago. One of the extinct gymnosperms
consisted of an assemblage of plants whose foliage was fern-like in general appearance yet it
possessed a primitive type of seed. This group of plants was named "seed ferns" or "pteridosperms".
Undoubted gymnosperms had come into existence during the Lower Carboniferous (about 355 million
years back) and had reigned over the plant kingdom by virtue of their dominance right upto the
Cenomanian (about 96 million years from now) till the advent of angiosperms at the end of the
Cretaceous.
The surviving gymnosperms no longer dominate the plant kingdom but can still claim to own about
70 genera and 800 species distributed in various parts of the world. They include the newly described
genera Chigua and Wollemia. While Chigua belonging to Zamiaceae of cycads was described by
D.W. Stevenson in 1990(See Pl. 18), the genus Wollemia assigned to Araucariaceae of conifers is the
most recently described genus of conifers but has some features that are very different from those of
any known living conifers. Leaves are light green and have complex and unusual structure. Trees are
30m high and about 1 m in diameter, monoecious, with green seed cones and brown, cylindrical male
cones at tips. The plant of Wollemia nobilis was discovered in 1994 and described in 1995 from
Wollemi National Park, New South Wales, Australia. (See Pl. 19). The largest group is the
Coniferophyta, which includes pines, firs, cedars, spruces and junipers which form extensive forests
of northern and southern hemispheres. It contrast, living representatives of the Ginkgophyta and
Cycadophyta are veritable "living fossils", the former being represented by the sole living member
Ginkgo biloba (See Pl. 20) existing in wild state in certain mountains in southeastern China and the
latter are relicts from the past age being confined to limited areas in tropics and sub-tropics. The last
group of living gymnosperms is the Gnetophyta represented by Welwitschia, Ephedra and Gnetum.
Welwitschia is one of the most remote relicts of the past. The single species W. mirabilis is native to
coastal desert of Namibia and South Angola of S.W. Africa and survives largely on dew. Plants are
very long lived, perhaps 5000 years. Plants are dioeceous, male and female structures are borne at the
base of a pair of strap shaped leaves that grow throughout the life of the plant from the base and are
sometimes 60ft long. The leaves are tattered being blown continuously back and forth by desert winds
and they lie curled in untidy heaps around the stunted trunk. The root is a large, conical tap root.
Gymnosperms are highly valued for their timber. They are also sources of starch, resins, essential oils,
drugs, edible nuts etc. Many are cultivated as ornamental plants ("palms") and are found in gardens all
over the world while some conifers are sold as Christmas trees. Some gymnosperms like the Ginkgo
are held in high esteem and are found growing around Buddhist temples in China and Japan. A few
gymnosperms are medicinally important like Taxus commonly called as Yews are usually large
shrubs, sometimes trees. The single seeds are ovate and hard enclosed in a fleshy red aril. All parts of
Taxus are poisonous except the fleshy red arils. Taxus baccata, the common yew, offers the heaviest
wood amongst soft woods. Wood is durable, oily and decorative. Taxol obtained from T. brevifolia is
effective against cancer. Taxine, a tonic principle is present in leaves, shoots and seeds.
Another taxon (Ephedra) growing mostly as shrubs, lianas or may become tree-like have are small
scale like leaves arranged in pairs on a wiry stem. Plants dioecious and do not bear conspicuous seed
"cones". Alkaloid ephedrine is extracted from green branches of E. gerardiana, E. sinica, E.
equisetine
Gymnosperms (gymnos = naked, sperma = seed) include plants whose ovules are naked and freely
exposed for pollination. They are borne on microsporophylls, scales, or comparable structures. In
angiosperms (angios = closed, sperma = seed) ovules are enclosed in a carpel and usually completely
closed at the time of pollination.
The sporophyte is usually arborescent comprising of large or small woody trees or shrubs. Few may
be lianas or climbers. Most gymnosperms are evergreen but some are deciduous such as Larix and
Taxodium(See Pl. 16). The plants are mostly xerophytic.
Gymnosperms show the following features:
1.
The plants have a long lasting tap root system. The vascular cylinder is di- to polyarch, xylem
exarch. Main elements of xylem are tracheids. Phloem is composed of sieve cells and lacks
companion cells. Some forms exhibit additional symbiotic relationship between roots and
algae in coralloid roots (Cycas) and between roots and fungi in mycorrhizic roots (Pinus).
2.
The sporophyte shows unlimited growth of aerial trunk by means of apical and lateral
meristem which produce secondary vascular tissues.
3.
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 :- (a) long
shoots or branches of unlimited growth (b) dwarf shoots or branches of limited growth
bearing clusters of variable number of needle shaped leaves and collectively called as spur.
Vascular bundle of stems are collateral, endarch or mesarch, open and arranged in a ring.
4.
Secondary growth very pronounced. Secondary vascular tissues consisting mainly of
tracheids and sieve cells. Normally xylem lacks vessel elements and the phloem lacks
companion cells. But both vessels and companion cells present in members of 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.
5.
Leaves are diverse 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 (See Pl. 21) and Gnetum. Scale leaves are microphyllous whereas larger leaves
are 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 or hypostomatic with sunken stomata. 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 cuticle
and sometimes by an additional waxy layer. Resin canals occur in all conifers (except Taxus)
and mucilage ducts in cycads and Ginkgo and latex tubes in Gnetum. Vasculature of petiole
quite variable. 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. In Gnetum, the petiole shows an arc-shaped
arrangement.
6.
Vegetative methods of reproduction are rare in gymnosperms but vegetatively propagating
bulbils are known in Cycas.
7.
Plants are heterosporous. 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 Cycas 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.
8.
Microsporangia are borne on abaxial or lower surface of microsporophylls. They may be
numerous and grouped in sori (Cycas) or reduced to two (Pinus). Microspores are produced in
tetrads.
9.
Megasporangium or ovule is borne on adaxial or upper surface of megasporophyll or
ovuliferous scale and are generally orthotropous. Young megasporangium consists of a
nucellus which is surrounded by a sheath or integument and inside the nucellus is single
functional megaspore. A narrow passage above the nucellus in the integument is the
micropyle.
10.
Microspore and megaspore germination in situ, producing micro-and megagametophytes
which are not autotrophic. Microspores have partly developed endosporic male gametophytes
when they are transferred to micropyle of ovule. 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.
11.
The microspores or pollen grains are borne by wind and enter the ovule directly through the
micropylar canal. The micropyle in almost all gymnosperms secretes a sugary exudate called
the "pollination drop" which not only receives the pollen grains but also transports them to
nucellus of ovule.
12.
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.
13.
Fertilization is effected by means of a pollen tube (siphonogamy). The multiflagellate
spermatozoids or male cells within the pollen tube have no specialized means of locomotion
and occur within the ovule. The pollen tube bursts liberating two large flagellated sperms into
the cavity (archegonial chamber) directly above female gametophyte. One sperm fertilizes the
large egg cell giving rise to a zygote (2n).
14.
At the beginning of embryogeny, zygote shows free nuclear division in all except
Welwitschia, Gnetum and Sequoia sempervirens. 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. Embryo
remains contained within the seed developed from the ovule. Mature embryo is differentiated
into root, stem and leaves.
15.
In most gymnosperms a common feature in embryogeny is polyembryony with young seeds
having more than one embryo. It may be simple polyembryony when more than one
archegonium is fertilized producing several zygotes or cleavage polyembryony when single
zygote gives rise to multiple embryos due to cleavage or splitting of embryonal layer. The
mature seed normally has only one embryo.
16.
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.
17.
During last phases of embryogeny, the nucellar tissue of ovule becomes disorganized and
frequently persists only as a paper-like cap of dry tissue at micropylar end of the seed.
18.
Gymnosperm ovules and seeds are unprotected and not surrounded by an ovary wall, hence
true fruits like that of angiosperms are not formed.
19.
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.
20.
The detached seeds of all gymnosperms (except for cycads and Ginkgo) remain dormant for
sometime undergoing a resting period.
21.
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.
22.
Polyploidy is rare in gymnosperms, except in Juniperus (partly) Sequoia and Ephedra (partly)
where gametic number is multiple of the basic number of the families. Sequoia is the solitary
gymnosperm which is hexaploid. Ephedra is the only genus where polyploidy is common.
Although angiosperms and gymnosperms are both seed bearing plants, the latter are considered to be
less advanced than angiosperms because their ovules and seeds are unprotected. Besides this,
gymnosperms fail to grow in varied habitats, show slow growth, do not freely propagate vegetatively,
xylem generally lacks vessels; no self pollination because cones are unisexual, seed dispersal by wind,
man or birds.
Suggested readings:
1. GREEN PLANTS Their origin and Diversity - by Peter R. Bell & Alan R. Hemsley (2000)
Cambridge University Press.
2. AN INTRODUCTION TO GYMNOSPERMS, CYCAS AND CYCADALES - by Divya Darshan
Pant (2002) Birbal Sahni Institute of Palaeobotany, Lucknow, India.
3. GYMNOSPERMS - by S.P. Bhatnagar & Alok Moitra (1996) New Age International Limited.
4. MORPHOLOGY AND EVOLUTION OF VASCULAR PLANTS - by E.M. Gifford, & A.S.
Foster (1989) W.H. Freeman & Co., New York.
5. MORPHOLOGY OF VASCULAR PLANTS - by D.W. Bierhorst (1971), The Macmillan Co.,
New York.
6. MORPHOLOGY OF GYMNOSPERMS - by J.M. Coulter & C.J. Chamberlain, (1917),
University Chicago Press, Chicago.
7. GYMNOSPERMS: STRUCTURE AND EVOLUTION - by C.J. Chamberlain (1935), Chicago
Univ. Press, Chicago.
8. STUDIES IN GYMNOSPERMOUS PLANTS. CYCAS - by D.D. Pant & B. Mehera (1962),
Central Book Depot, Allahabad.
9. LIVING INDIAN GYMNOSPERMS. Pt. I (Cycadales, Ginkgoales and Coniferales) - by M.B.
Raizada and K.C. Sahni (1960) Ind. For. Rec. (N.S.) Bot. 5: 73-150.
10. THE PRIVATE LIFE OF PLANTS - by David Attenborough (1995), BBC Enterprises, Ltd.
Woodlands, London.
11. CONIFERS - THE ILLUSTRATED ENCYCLOPEDIA. Vol. I & II - by D.H. van Gelderen &
J.R.P. van Hoey Smith (1996), Timber Press, Inc. Orgon, U.S.A.
12. A TEXTBOOK OF BOTANY - by S.N. Pandey, S.P. Mishra and P.S. Trivedi (2004). Vikas
Publishing House Pvt. Ltd., New Delhi.
13. A TEXTBOOK OF BOTANY - ANGIOSPERMS by B.P. Pandey. S. Chand and Company Ltd.,
New Delhi.
14. PLANT ANATOMY by A. Fahn (1990). Butterworth Heinemann Ltd. Linacre House, Jordon
Hill, Oxford OX2 8DP.
15. AN INTEGRATED SYSTEM OF CLASSIFICATION OF FLOWERING PLANTS by A.
Cronquist (1981), Columbia University Press, New York.
Plate 10
Sequoia sampervirens, Coast Redwood, Native to Northern California, U.S.A. (Reproduced
from “The Private Life of Plants” by David Attenborough, 1995)
Plate 11
A
B
Sequoia sampervirens, Coast Redwood. A: Showing remarkably small cones of this giant tree.
B: Giant trees (All photographs reproduced from “Conifers- The Illustrated Encyclopedia” by
D.M. Van Gelderen and J.R.P. Van Hoey Smith, 1996)
Plate 12
A
B
Sequoiadendron giganteum. A: Trunks of two gigantic trees growing in Mariposa Grove,
California, U.S.A. B: A tree in a U.S. National Park with the nick name “Grizzly Giant.”
(All photographs reproduced from “Conifers- The illustrated Encyclopedia” by D.M. Van
Gelderen and J.R.P. Van Hoey Smith, 1996)
Plate 13
Pinus longavea, the longest living organism on earth growing at 10,000 ft. in White mountains of Eastern
age of some has been estimated at about 4,600 years. (Photograph reproduced from “The Private Life
Attenborough, 1995)
Plate 14
A
B
C
D
Zamia pygmaea. A: Showing leaf details. B: Showing female cones
Stangeria (paradoxa) eriopus. C: Forest form showing leaf details. D: Forest form showing
habitat. (All Photographs reproduced from “The Cycads” by Loran M. Whitelock, 2002)
Plate 15
A
B
C
Zamia pseudoparasitica. A, B: Male and female cones. C: Showing epiphytic habit in
habitat near El Cope, Panama. (All Photographs reproduced from “The Cycads” by Loran
M. Whitelock, 2002)
Plate 16
A
B
C
D
Larix decidua. A, B: Deciduous trees, also called as a “witches” broom C: Same plant as
above in winter. D: A tall and slender tree with pendulous branches. The tree is very
valuable for its timber. (All photographs reproduced from “Conifers- The illustrated
Encyclopedia” by D.M. Van Gelderen and J.R.P. Van Hoey Smith, 1996)
Plate 17
C
B
A
Taxodium distichum. A: The Bald Cypress or Swamp Cypress in its native habitat along
the Alatamaha river, Georgia, U.S.A. B: Showing male inflorescences and female cones.
C: The respiratory “knees” of this species, as growing in Breukelen, The Netherlands.
(All photographs reproduced from “Conifers- The illustrated Encyclopedia” by D.M. Van
Gelderen and J.R.P. Van Hoey Smith, 1996)
Plate 18
A
B
C
Chigua restrepoi. A: Showing an entire male plant. B: Showing leaf details. C: Showing
male cones. (Figure and photographs reproduced from “The Cycads” by Loran M.
Whitelock. 2002)
Plate19
B
C
A
Wollemia nobilis. A: Showing habit, shoots with male and female cones and a seed on
ovuliferous scale. B: Showing the warty bark. C: A young specimen with typical needles.
(Photographs reproduced from “Conifers- The Illustrated Encyclopedia” by D.M. Van
Gelderen and J.R.P. Van Hoey Smith, 1996 and figures courtesy D.K. Chauhan)
Plate 20
B
A
C
D
Ginkgo biloba. A: A specimen of the Maidenhair Tree, growing in Sendai, Japan, said to be 1,000
years old. B: Another tree, in winter. C: Showing male inflorescence. D: Showing the fruits and
autumn colours of leaves. (All Photographs reproduced from “Conifers- The illustrated
Encyclopedia” by D.M. Van Gelderen and J.R.P. Van Hoey Smith, 1996)
Plate 21
A
B
C
D
Welwitschia mirabilis. A: A large plant growing at the shore of the Atlantic Ocean, Namibia. B:
A female specimen of this species showing the inflorescences.
Parasitaxus ustus. C: The only known parasite conifer, seen in its habitat in New Caledonia,
where it grows on roots of the only known host, Dacrydium taxoides. D: A small shrub of the
same species producing coppery red leaves that are scale-like and triangular in shape. (All
photographs reproduced from “Conifers- The illustrated Encyclopedia” by D.M. Van Gelderen
and J.R.P. Van Hoey Smith, 1996)