2021 Solved Old Paper (BOT

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Difference between Gymnosperms and Pteridophytes:-

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Cycadeoidea:-

1. Morphological Features of Cycadeoidea:-

> The Cycadeoid trunks were short, stout, spherical to sub-spherical and un-branched or branched. The trunks and leaves of many of its species show remarkable resemblance with those of living Cycads. Some of the species were short while others (Cycadeoidea jenneyana) attained a height of 3 to 3 .6 metres.

> The trunk generally attained a diameter of about 50 cm, and had many, persistent, rhomboidal leaf bases. A compact crown of Cycad-like, large, pinnately compound leaves was present at the apex. The leaflets had many parallel veins.

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Glossopteris:-

Systematic Position:-

Kingdom:- Plantae

Division:- Gymnospermae

Order:- Glossopteridales

Family:- Glossopteridaceae

Genus:- Glossopteris

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Male strobilus:-

> In Gnetum, the male cones are compact and slender axis-like structures, which are up to 6 cm in length. Each cone is generally a penicle that is either solitary and axillary or fascicled at the apex. The axis of the strobilus bears opposite and connate bracts. Bracts are arranged one above the other to form collars or cupules. There are 6-25 cupules are present, each cupule, staminate flowers are arranged in several definite rings, usually 3 to 6 in number.

> Each mature microsporophyll consists of a stalk bearing two unilocular microsporangia (anthers). The stalk is invested at the base by a perianth.

> In Gnetum ula, 2-4 anthers are there which are formed from microsporangial initial cells. It divides periclinally and anticlinally to form a primary cell wall, tapetal cell, and sporogenous cells. Sporogenous cells later differentiate into spore mother cells. Microspore mother cell forms microspores (pollen grains) through meiosis.

> The wingless pollen grains are liberated by the longitudinal dehiscence of the anthers.

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Geological time scale:-

> Earth’s history is approximately 350 million years old.

> Geologists have used two major units for subdividing the geological history of the earth these are- time and strata.

> On the basis of time the geological history of earth has been divided into five Eras namely Archaeozoic, Proterozoic, Palaeozoic, Mesozoic and Coenozoic.

> Eras have been divided into periods and periods into epochs.

> Similarly on the basis of strata, the geological history of the Earth is divided into system,series, stage and zone.

> The various eras and their age along with plant groups evolved during that period have been described in the given figure.

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Reproduction in Gymnosperms:-

> Strobilus or cone is the reproductive structure of gymnosperms. Both male and female strobili can be present on the same (seen in Pinus) or on different trees (seen in Cycas). Gymnosperm plants are heterosporous. They produce different spores, which are the haploid microspores and megaspores.

> Male Cones:- The male strobili or male cones have microsporophylls, which have the microsporangia that produce the haploid microspores. Some of these microspores develop into male gametes called the pollen grains, while the rest degenerate.

> Female Cones:- The megasporophylls form a cluster and are called as the female strobili or cones. They bear the ovules having the megasporangium. Thus produces the haploid megaspores and a megaspore mother cell.

> The megaspore mother cell undergoes meiotic division produces four megaspores. One of these megaspores develops into the multicellular female gametophyte. The female gametophyte also has two or more archegonia, which are the female sex organs.

> Fertilization:- When the pollen grains are released from the microsporangium, they get dispersed through wind and reach the female cones. The pollen grain develops a pollen tube, which grows towards the archegonium.

> The discharge of male gametes happens near the mouth of the archegonium. The fusion of the male and female gametes occurs. After fertilization, a zygote develops to form the embryo and the ovule forms the seed. The following figure shows the life cycle of a gymnosperm.

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Stem Genus Pentoxylon Sahnii:-

> Pentoxylon sahnii and Nipanioxylon guptai are the stem genera of “Pentoxyleae”. The stems of Pentoxylon sahnii attained a diameter from 3mm to 2 cm. The stem has always been reported in association with the leaves called Nipaniophyllum.

> Presence of five steles in a cross- section of the stem has been the main reason for giving the name Pentoxylon to the genus. Many short lateral shoots or dwarf shoots were also present on the stem.

> Five steles occupied greater part of the stem in a cross-section. Each stele had its own cambium. The cambium was uniformly active in the young stems, but at maturity more secondary tissue developed towards the centre, and thus the secondary wood appeared eccentric.

> Primary phloem and primary xylem were present towards outer and inner sides of the cambium, respectively. Six steles have also been observed by Sahni (1948), although rarely. According to Vishnu-Mittre (1953) the number of steles varied along the length of the stem.

> There were present five much smaller bundles just alternating with the main bundles of the stem i.e. five steles. Each such bundle had a large amount of secondary wood. These were probably the leaf trace bundles.

> Medullary rays of the main steles were uniseriate, and they lacked ray tracheids, wood parenchyma and resin canals. The secondary wood resembled greatly with that of Araucaria. Uniseriate or bi-seriate bordered pits were present on the radial wall of tracheids.

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Taxus:-

Systematic Position:-

Division:- Coniferophyta

Class:- Coniferopsida

Order:- Taxales

Family:- Taxaceae

Genus:- Taxus

External Morphology:-

> Taxus baccata, commonly known as ‘Yew’, is an evergreen tree attaining a height of 9-20 metres with a massive trunk. The stem is profusely branched and remains covered with a thin brown-coloured bark.

> It differs from Pinus in not possessing dimorphic branches. All the branches are of unlimited growth and form a very dense canopy, thus making Taxus a shade-providing tree. Only the green leaves are present on the vegetative branches.

> The leaves are linear, small, only 2-3 cm. long and spirally arranged. Each leaf possesses a single strong vein and recurved margins. The upper surface is dark green while the lower surface is pale or rusty red in colour.

> The apex is sharply pointed mainly because of accumulation of silica. This sharply pointed apex may cause death of catties eating these leaves. Each leaf is shortly stalked. The stalk broadens into a flat persistent base which shows a slight twist. The scaly leaves present on the fertile shoot are opposite and decussate. Taxus possesses a long and well- developed tap-root. The roots are deep-feeders and highly branched.

Internal Structure:-

a. Stem:-

> In transverse section the stem is irregular in outline and resembles Pinus in structure. It is surrounded by a thickly cuticularised single-layered epidermis. Inner to the epidermis is parenchymatous cortex having some tannin-filled cells. It is followed by endodermis and sclerenchymatous pericycle.

> The young stem shows a ring of conjoint, collateral, open and endarch vascular bundles enclosing a distinct pith in the centre. The protoxylem consists of spiral tracheids, and the phloem contains sieve cells with sieve plates and phloem parenchyma. Companion cells are absent.

> The cambium is persistent and develops a thick vascular cylinder due to secondary growth. The cambium cuts secondary phloem towards outer side and secondary xylem towards inner side. The secondary wood is devoid of resin canals and wood parenchyma. Its tracheids show uniseriate bordered pits only on their radial walls.

> The tracheids also show spiral thickenings. The medullary rays are uniseriate and homogeneous but in Taxus baccata they are sometimes bi-senate. The wood is strong and dense. Due to the presence of tertiary spirals the wood is elastic in nature. Phellogen may develop in the older stems showing extrastelar secondary’ growth.

b. Root:- Except that of the absence of resin canals, the root anatomy of Taxus resembles very much with that of Pinus. The root is diarch.

c. Leaf:-

> The leaf is dorsiventral. It shows xerophytic characters. Upper and lower epidermal cells are rectangular in shape and thickly circularized. The cuticle is comparatively thin on the lower surface. The stomata are of sunken type and restricted only to the lower epidermis. They are haplocheilic in development.

> The mesophyll is differentiated into palisade and spongy-parenchyma. The palisade is generally two-layered. Only one vascular bundle is present in the mid-rib region. Enclosed by a distinct endodermal layer or bundle sheath the collateral vascular bundle contains phloem towards the lower side and xylem towards the upper side.

> Transfusion tissue is present on both the sides of the vascular bundle. Resin canals are generally absent. The xerophytic characters of the leaf include the presence of thick cuticle, sunken stomata, transfusion tissue and differentiation of mesophyll into palisade and spongy parenchyma.

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Gymnosperms:-

> Gymnosperm:- “Gymnos” = necked and “Sperma” = seeds.

> The word Gymnosperm was first used by Theophrastus, a pupil of Aristotle in his famous book

“Enquiry into Plants”. He used this term in all those plants having unprotected (without covering) seeds.

Classification of Gymnosperms:- Sporne (1965) classified gymnosperms into 3 classes, 9 orders and 31 families.

The classes include:

i. Cycadospsida

ii. Coniferopsida

iii. Gnetopsida

General Characters of Main classes:-

Class I – Cycadopsida:-

> Plants are palm-like or fern-like.

> Compound, frond-like pinnate leaves.

> Manoxylic wood.

> Sperms are motile.

> Flower like structures are absent. Strobili are simple.

> Example:- Cycas, Zamia.

Class II – Coniferopsida:-

> Tall trees with simple leaves of varied shape.

> Wood is pycnoxylic.

> Cone like strobili are present.

> Motile sperms are absent (except Ginkgo biloba).

> Example:- Pinus.

Class III – Gnetopsida:-

> Shrubs, trees and lianas.

> Leaves are elliptical or strap-shaped, simple, opposite or whorled.

> Motile sperms are absent.

> Wood contains vessels.

> Strobili is called as inflorescence.

> Flower like structure with perianth is present.

> Example:- Gnetum, Ephedra.

Morphology and Reproduction:-

1. Occurrence:- Gymnosperms are found all around the planet. Most of them are present in the temperate and subarctic regions of the northern hemisphere.

2. Size:- Gymnosperms include only tall, perennial trees. The smallest gymnosperm is Zamia pygmaea, and the tallest is Sequoia sempervirens. Gnetum ula is a woody climber.

3. Plant body:- The plant body is sporophyte and divided into root, stem, and leaves.

4. Habit:- Gymnosperms are mainly evergreen and woody plants. Most of the gymnosperms are trees and shrubs. A few of them may be liana or climbers.

5. Life Span:- As gymnosperms do not produce fruits or flowers, it takes a longer time in fertilization and development. For example, a pine tree can have a lifespan of over \(100\) years.

6. Roots:- They have a fibrous root or taproot system.

7. Stems:- They are woody plants, so they have erect stems. Stems of gymnosperms can be either unbranched or highly branched.

8. Leaves:- Leaves may be simple, needle-like (Pinus, Juniper, etc.) or compound like palms. (Cycas), with a thick cuticle.

There are two main types of leaves:

a. Foliage leaves:- These are green and photosynthetic.

b. Scale leaves:- These are on the nodes and short branches, brown in colour and non-photosynthetic.

9. Xylem:- The xylem carries water and minerals to different parts of the plant and provides support. Vessels are absent in the xylem of gymnosperms. Gnetum has vessels in the secondary wood.

10. Phloem:- In most of the cases, the sieve tube, companion cells are absent in the phloem. Instead, they have sieve cells to conduct food. Albuminous cells are present in pine in place of companion cells.

11. Flower:- They do not produce flowers.

12. Microsporophyll:- In gymnosperms, microsporophyll is hard leafy triangular clusters of male cones that bear microsporangia that contain microspores.

13. Megasporophyll:- In gymnosperms, megasporophyll bears a female gamete, megasporangia containing only one megaspore. Male and female cones may be borne on the same plant (Pinus, monoecious) or different plants (Cycas, dioecious).

14. Pollination:- Pollen grains are carried from male cone to female cone by wind (Anemophily).

15. Gametophyte:-

a. Male gametophytes:- They are formed from haploid microspores. Male gametophytes or pollen grains are extremely reduced to a limited number of cells.

b. Female gametophytes:- They are highly reduced and are formed from haploid megaspore. There are integuments, so the ovules are called integumented megasporangium. All gymnosperms (except Gnetum) are unitegmic.

16. Endosperm:- They have haploid endosperms.

17. Seeds:- The seeds are naked in gymnosperms and borne in cones that are not visible till maturity.

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Origin of Gymnosperms:- The major question, whether gymnosperms had one, two, three or more ancestral origins, or whether they represent a monophyletic, di-phyletic, tri-phyletic or polyphyletic group, remains still to be solved.

1. Monophyletic Origin of Gymnosperms:-

> Several botanists, including Beck (1960), believe that gymnosperms are of monophyletic origin. Plants grouped under Progymnospermopsida (e.g. Aneurophyton, Tetraxylopteris, Protopitys, Archaeopteris, Callixylon) are of definite significance in the monophyletic origin. Several characteristics of Cycadophyta are combined by these plants with those of Coniferophyta.

> The Progymnospermous plants also show intermediate steps in the evolution of megaphylls from

branch systems. Progymnosperms, of course, show certain anatomical characters associated commonly with gymnosperms, and a full gymnospermous status may be granted to them.

> In-spite of some definite evidence the confirmation of the theory of monophyletic origin of

gymnosperms depends upon the discovery of some more fossil genera from Devonian or Silunan penods and reinvestigation of some already available fossil plants of these periods.

> Sporne (1965) has also mentioned that “the evidence which is available at the moment seems to favour the view that gymnosperms are monophyletic. Confirmation must, however, await the discovery of further fossils from Devonian, or even Silurian deposits.”

2. Di-Phyletic Origin of Gymnosperms:-

> Some definite correlation between the anatomy of wood, form of leaf and structure of seed of two

major groups of gymnosperms (i.e. Cycadophyta and Coniferophyta) have lead some botanists to conclude that gymnosperms are of di-phyletic origin. In the members of Cycadophyta. the wood is manoxylic, and it remains associated with the large-sized leaves(megaphylls) and radial symmetry in the seeds.

> On the other hand, in the members of Coniferophyta, the wood is pycnoxylic, and it remains associated with the small-sized leaves (microphylls) and bilateral symmetry in the seeds. Due to these peculiarities of these two major groups, gymnosperms appear to be of di-phyletic origin.

3. Tri-Phyletic Origin of Gymnosperms:-

> Greguss (1972), in the latest edition of his book entitled “Identification of Living Gymnosperms on the Basis of Xylotomy”, pleaded for a tri-phyletic origin of gymnosperms. He opined that on the basis of Xylotomy (wood-anatomy) three well-defined evolutionary series may be traced among the existing

gymnospermous taxa.

> All these three series among existing gymnosperms have definite correlations with three types of

pteridophytes, another tri-phyletic group:

i. Cycadales (Cycas), Ginkgoales (Ginkgo), Araucariaceae, Podocarpaceae and probably Taxales showing correlations with Pteropsida of Pteridophytes.

ii. Cupressaceae showing correlations with Sphenopsida of Pteridophytes.

iii. Pinaceae and Taxodiaceae showing correlations with Lycopsida of Pteridophytes.

Evolutionary Trends of Gymnosperms:- On the basis of the available studies of several fossil groups, although it appears that the gymnosperms constitute a heterogenous group, yet it is possible to delineate several over-all trends in the evolution in this group.

1. Vascular System:-

> The primary vascular system exhibits a narrow range among gymnosperms. Majority of the

gymnosperms are monostelic, and only some exhibits polystelic condition. Some of the members also possess co-axial evolutionary cylinders of secondary wood which develops from the anomalous activity of cambium.

> A change from the cauline to foliar nature has been the main evolutionary trend in the stem anatomy of different members of gymnosperms. The primary wood is suppressed in this change from cauline to foliar nature.

> During this suppression the primary wood passes through various stages such as:

i. Solid protostele

ii. Pith surrounded by mesarch strands

iii. Development of endarch strands adjacent to the secondary wood

iv. Mixed xylem

v. Parenchyma

> During the entire process of these changes a gradual transference of function from primary to

secondary wood takes place. The leaf trace system, which was associated with the primary wood in the

earliest types, now becomes “more and more closely associated with the secondary wood”.

> In members of Cordaitales, Ginkgoales and Coniferales there is a definite increase in the secondary

wood. In Gnetales, however, vessels replace tracheids, though not completely but only partly.

2. Leaves:-

> The microphylls and megaphylls are the two different kinds of leaves found in gymnosperms. The

microphylls are small and usually possess one or two parallel veins. The megaphylls are quite large, frond-like leaves and possess well-branched veins.

> The gymnosperms of Coniferophyta-line possess microphylls while the members of Coniferophytaline bear megaphylls. Ginkgoales, however, possess neither microphylls nor megaphylls but wedge-shaped or fan-shaped leaves of almost normal size.

> Majority of the recently reported fossil members of gymnosperms indicate their affinities more with

the microphylls than with the megaphylls. Several botanists consider Cycadophytes as more primitive than Coniferophytes. This indicates that megaphyllous leaves are more primitive than microphyllous leaves.

> Much evolutionary significance has not been attached with the leaf anatomy of gymnosperms. A little

consideration has, however, been given to the xeric conditions of these members. It is supposed that

inappropriate condition of water by the tracheids has been the major reason for the xeric condition among gymnosperms.

3. Reproductive Organs:- Some important events of the evolution of reproductive organs among gymnosperms may be listed as under:

> Probably, all reproductive organs in the beginning were stachyspermous (i.e. reproductive organs

borne on stem). Some gymnospermous genera have managed to stay as stachyspermous even today.

> During the course of the evolution, the structures bearing reproductive organs progressively became

more leaf-like, i.e. phyllospermous. The sporophylls were thus originated.

> In Ptendosperms, the sporophylls were leaf-like.

> A From the compact sporophylls developed the cone. The Cordaitales were the first to produce monosporangiate cones. The tendency of bearing mono-sporangiate cones is also retained in Ginkgoales and Coniferales.

> The mono-sporangiate cones or strobili evolved into bisporangiate cones or strobili during course of

evolution. Gnetales (e.g. Welwitschia) possess bisporangiate strobili.

> In Cordaitales, Ginkgoales and Coniferales, the sporophylls are distinctly attached to the axis, and,

therefore, the strobili, in these groups are simple. On the other hand, compound strobili are produced

in some more advanced groups such as Gnetales.

4. Microsporophylls:-

> In Cycadophytes, the microsporophyll’s are pinnate and peltate. They bear sori only on their lower

surface. Due to these characters the microsporophyll’s of Cycads are very primitive. In different members of different groups of gymnosperms, a reduction of long sporophyll to a short discoid structure and then also to a filament is observed, and these all are the features of evolutionary significance.

> Some other such features include:

i. Formation of individual sporangia instead of synangia

ii. Fast elimination of annulus from the fern-like appearance.

5. Ovules:-

> Two evolutionary trends are visible in the ovules of the gymnosperms. These are:

i. Degeneration of the outer fleshy layer

ii. Appearance of two integuments

> In most primitive type of gymnospermous ovules, the integument is single and it is also free from the

nucellus. Three distinct layers are discernible in the integument, viz. outer fleshy layer, middle stony layer and inner fleshy layer. Two vascular strands supply the ovule, of which the outer strand enters the outer fleshy layer and the inner strand enters the peripheral region of the nucellus.

> In majority of the gymnosperms the integument remains fused with the nucellus in most of its part,

except the micropylar region, where the nucellar beak is formed and the trilayered integument is not reduced at all.

> The outer fleshy layer develops quite conspicuously in the ovules of Cycadophyta-line. In the ovules

of the Pinaceae-line, the outer fleshy layer is represented by a young ovule. Instead of one, two integuments are present in the ovules of Gnetales, and, therefore, it exhibits an example of a second evolutionary trend in the gymnospermous ovules.

6. Male Gametophyte:-

> The pollen grains of most of the fossil gymnosperms, except of Coniferales and Taxales, are almost

uniform in possessing a layer of parietal cells surrounding the centrally located spermatogenous region. The layer of parietal cells probably represents an antheridial jacket.

> In the early stages of the evolution of gymnosperms, the vegetative prothallial cells have been in such

a low number that the male gametophyte might be treated as a reduced form of an antheridium. A reduction in the antheridial jacket and the simultaneous formation of a pollen tube have been observed between the fossil gymnosperms and the extant forms (i.e. members still in existence).

> A homology, probably exists between the tube cell of the modern gymnosperms and the antheridial

jacket of Palaeozoic gymnosperms The male gametophyte development appears to be quite uniform within a family.

> In Pinaceae, two senescent primary prothallial cells are produced from the embryonal cell of the

microspore. This embryonal cell functions as an antheridial initial and results in the formation of a peripheral tube cell and a generative cell. A periclinal division in the generative cell gives rise to an outer spermatogenous cell and an inner sterile cell. Two male gametes are produced by the division of the spermatogenous cell.

> In Taxaceae, Taxodiaceae, Cupressaceae and Cephalotaxaceae there is no prothallial cell, and the

function of the antheridial initial is performed directly by the embryonal cell.

> In Araucariaceae and Podocarpaceae, the prothallial cells show secondary proliferation. The ontogeny of male gametophyte in Araucariaceae resembles greatly with Pinaceae. In Araucariaceae and Podocarpaceae. however, the generative cell divides anticlinally and not penclinally. Very little proliferation of primary prothallial cells is observed in Podocarpaceae in comparison with that of Araucariaceae.

> In Gnetales (e.g. Gnetum and Welwitschia), the prothallial cells are generally absent The function of

the antheridial initial is performed by the embryonal cell. A tube cell and a generative cell are formed.

> Division of the generative cell results in the formation of a spermatogenous cell and sterile cell. The

spermatogenous cell divides and form two male gametes.

> The male gametes or sperms are ciliated and motile in Cycas. Probably the same condition existed in

Ptendospermales, Cycadeoideales and Cordaitales. In Coniferales and Gnetales, however, naked nuclei are present in place of motile and ciliated ones. Pollen chamber also disappears simultaneously. The sperms are carried away by the pollen tubes.

> In Ephedra, however, a deeply situated pollen chamber is present. This is, however, an indicative of a

secondary origin. A well- developed archegonial neck is also present in Ephedra.

7. Female Gametophyte:-

> In gymnosperms, the female gametophyte is a massive, multicellular body. It serves the dual purpose

of bearing the archegonia and providing nourishment to the young embryo.

> Major evolutionary steps in the female gametophyte include:

i. Free-nuclear divisions,

ii. Vacuolation,

iii. Process of wall-formation resulting into the formation parietal tissues, and

iv. Process of wall-formation extending towards the central region and resulting into the development of endosperm.

> In majority of Cycadales, Ginkgoales, Coniferales and some Gnetales (e.g. Ephedra), the early

development of female gametophyte involves a fairly uniform plan. Alveoli also appear in members of almost all these groups but the pattern of alveoli vanes. The archegonia develop in the female gametophyte.

> In both pteridophytes and gymnosperms, the general structure of the archegonium is almost similar.

An egg cell, a ventral canal cell, a few neck cells and a venter are present in an archegonium of both the

groups.

> The gametophyte is, however, parasitic on sporophyte in gymnosperms while it is free-living and

green in pteridophytes. Neck canal cells are absent in gymnosperms while they are present in the archegonia of ptendophytes.

> These differences indicate some evolutionary trends showing a reducing capacity of gametophyte for

its independent existence. On the other hand, several simulates show homologies in the structure of the female gametophyte of both ptendophytes and gymnosperms.

Q. From Which Type of Earliest Gymnosperms Developed the Modern Gymnosperms?

Ans. The earliest gymnosperms, responsible for the evolution of this entire modern group, probably had the following characteristic features, according to Sporne (1965):

i. Stems with primary solid wood, also perhaps possessing some amount of secondary wood.

ii. Plants perhaps had little distinction between leaf and stem.

iii. Pollen-bearing organs were borne fully exposed at the tips of green photosynthetic branch- lets.

iv. Seeds were also fully exposed.