BIO102: Plant Diversity - staff.oouagoiwoye.edu.ngstaff.oouagoiwoye.edu.ng/uploads/412_COURSES_Plant_Diversity_24430.pdf · ovule. The ovule contains a tissue called nucellus, which

BIO102: Plant Diversity

OLABISI ONABANJO UNIVERSITY Bup Oyesiku PhD

www.bupoyesiku.net ~ 1 ~ Department of Plant Science

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DIVERSITY OF GYMNOSPERMS

Cedar Pine

Gnetae

Cycad Ginkgo

Conifer

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Contents Topic: Diversity of Gymnosperms ...................................................................................................... 3

Objectives ..................................................................................................................................... 3

Reading Assignment ...................................................................................................................... 3

Lecture .......................................................................................................................................... 3

Introduction ...................................................................................................................................... 4

Main Content .................................................................................................................................... 4

Background of Gymnosperms ........................................................................................................ 4

Reproduction in Gymnosperms ..................................................................................................... 5

Vegetative ................................................................................................................................. 5

Sexual........................................................................................................................................ 5

Classification of Gymnosperms ...................................................................................................... 7

Characteristics of Gymnosperms.................................................................................................... 8

Diversity of Forms in Gymnosperms............................................................................................... 8

Conifer forms .......................................................................................................................... 10

Cycads forms ........................................................................................................................... 10

Gnetae forms .......................................................................................................................... 11

Ginkgos forms ......................................................................................................................... 12

Cycads forms ........................................................................................................................... 12

Habitats of Gymnosperms ........................................................................................................... 12

Ecology of Gymnosperms ................................................................................................................ 13

Economic uses of Gymnosperms ..................................................................................................... 13

Conclusion ....................................................................................................................................... 14

Summary ......................................................................................................................................... 14

References/Further readings/Links .................................................................................................. 14

Test Yourself Questions (TYQ).......................................................................................................... 15





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Topic: Diversity of Gymnosperms

Objectives

In this lecture we will learn the followings:

Clear idea of the features that separate Gymnosperms from other plants

Skills in identifying them in our environment

Importance of the Gymnosperms

Contributions to our lives

Reading Assignment

In addition to the discussion in this lecture you should read current textbooks in the

library and visit: ilearn.bupoyesiku.net; [email protected]

for the online lecture.

Lecture

In previous lectures we discussed about the diversities of members of algae,

bryophytes, fungi lichen and monera. In our today’s lecture, we are going to consider

seed plants (gymnosperms and angiosperms). To position our thoughts in line with

the previous lectures on spore plants we are going to see the transition from the

spore plants to seed plants. Be patient and try not to get bogged down by new

terminologies. An important point to keep in mind is the transition from spore plants

to seed plants, looking back at the lectures on bryophytes and fungi, lichens and

pteridophytes.


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Introduction

Gymnosperms (Greek=naked seeds) are the first group of seed producing plants.

They comprise less than 5% of the whole plant kingdom. They won the celebrity

price for longevity. The two oldest living celebrity gymnosperms include the Great

Basin Bristlecone Pine of Nevada estimated to be 5,000 year old and the Giant

Sequoia estimated to be 3,000 year old. It is obvious that gymnosperms would have

been among the first trees to grow after the Noah’s flood. One other reason that

gymnosperms are the first land seed plants is that animals were rare then and the

gymnosperms rely on wind for pollination. Another reason is that gymnosperms are

capable of recovering quickly when experience adverse conditions and they produce

many seeds than their flowering sisters, which we will discuss next lecture. Unlike

angiosperms they do not have true flower and do not disperse fruits. Examples to be

discussed here include conifers, cycads, Ginkgos and gnetae. Like other plants they

primarily trap solar energy directly from the atmosphere and convert it to food. They

contribute to the advanced terrestrial ecosystems.

Main Content

Background of Gymnosperms

Gymnosperms are the first vascular seed plants with distinct vascular bundles

(xylem and phloem). They reproduce by naked seeds thus they are called vascular

seed plants. It is important to know that the seed plants are probably descendants of

certain extinct members of pteridophytes called Progymnosperms. It is now obvious

that gymnosperms evolved from either extinct or advanced pteridophytes. In

addition, Gymnosperms probably evolved from the first vascular non-seed plants

because they both share multicellular sex organs.





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Reproduction in Gymnosperms

Gymnosperms are one of the two groups of seed plants. The other one is the

flowering plants (Angiosperms). You will recollect that going up the evolutionary

ladder from bryophytes and pteridophytes we came across very significant

differences in plant reproduction. In bryophytes and pteridophytes spores are

commonly involved in the reproduction of new individuals. But in gymnosperms and

angiosperms seeds are commonly involved. An important point to keep in mind is

looking back at the spore producing plants. The process of reproduction in all plants

follows basic stages. The gametophytes, male and female, produce antheridia and

archegonia, which fused together during fertilization to form zygote, which in turn

developed into sporophytes. Mature sporophyte contains diploid spore mother cell

that undergoes meiosis process to form haploid male and female spores. The

mature spores undergo mitotic divisions to produce new male and female

gametophytes, which complete the basic life cycle.

Vegetative

There are no obvious types of vegetative reproduction in Gymnosperms. But

generally the branch of few gymnosperms when contact with suitable substrate and

favorable conditions are at hand the branch can take root and grow, although very

rare.

Sexual

The reproductive structures of gymnosperms are contained within strobili, or cones.

Most species are dioeciously; individuals of plant produce microsporangiate or

megasporangiate strobili or cone. Microsporangiate contain the haploid reproductive

spores (Fig 1.1). The microsporangia undergo meiotic division to produce the haploid

pollen grains. When shed from the microsporangia, the pollen grains (equivalent of

male gametophytes in spore producing plants) consist of five cells within the pollen

grain wall.





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Megasporangiate (Fig 1.2) consist of oppositely paired bracteoles enclosing an

ovule. The ovule contains a tissue called nucellus, which is protected by delicate

envelope called integument. The nucellus cell divides meiotically to produce a row of

haploid cells (megaspores). Of the many megaspores produced only one greatly

enlarged and undergoes multiple mitotic divisions to produce between 500 to 1,000

naked nuclei after which cell walls begin to form around each nucleus. The top end

of each cell extended to form archegonium structure, which contain a haploid egg

cell.

Before fertilization process starts, some cells of the nucellus degenerate to a sticky

liquid that escape through a tiny opening (mycropyle) of the integument to form

pollination drop at the tip of the extended integument. The pollen is largely transfer

by wind. The sticky drop attracts the air borne pollen grains as the drop dries it pulled

the pollen grains inward into the nucellus. The pollen grains germinate and the pollen

tube penetrate the archegonium. A mature male gamete fuses with a mature egg

Figure 1.1 Microsporangium

Figure 1.2 Megasporangium





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nucleus resulting into fertilization. Following fertilization, eight or more diploid nuclei

with cellulose cell walls are produced. Of these diploid nuclei only one develops to a

mature seed, the rest form the nutritive tissue enclosing the seed.

Classification of Gymnosperms

Gymnosperms are the smaller group of the two sub-division of Spermatophyta

(Phanerogams). In this lecture we shall adopt the classification according to

Chamberlain (1935)

Under the current trend in classification, gymnosperms are sub-divided into two

members, Cycadophytes and Coniferophytes (Fig1.2). We have one living genus in

Cycadophytes and three living genera in Coniferophytes.

Spermatophyta

Angiosperm Gymnosperm

Cycadophytes

1). Cyadofilicales (extinct)

2). Cycadeoidales (extinct)

3). Cycadales (extant)

Coniferophytes

1. Corditales (extinct)

2). Coniferales (extant)

3). Ginkgoales (extant)

4). Gnetales (extant)Figure1.2 Classification of Gymnosperms

(Chamberlain, 1935)





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Characteristics of Gymnosperms

Gymnosperms are characterized as seed plants with naked ovules. They are the

softwoods of the plant kingdom. Many usually have needle-like leaves (Pine) and

remain evergreen throughout the year. It will be of interest to you that few

gymnosperms are in the habit of dropping their leaves as you can find in the genus

Ginkgo. Other characteristics are outlined below.

Most leaves are needle-like, fan-like, strap-like and palm-like

Sporophyte is the dominant plant (independent of gametophyte)

Gametophyte is reduced to a cone

Cone contain sporangia that produce the spores

A whole plant differentiated into root and shoot (stem, leaves, flower, seed)

Vines are rare in gymnosperms

Wood lack large diameter vessel elements unlike Angiosperms

Xylem cells are uniformly small vessel except in the order Gnetales

Phloem lack complementary sieve tube elements and companion cells

Simple, primitive, unisexual flowers

Heterosporous spores (microspore pollen and megaspore embryo)

Pollination is by wind and rarely by animals or insects

Absence of ovary cause absence of fruit

Diversity of Forms in Gymnosperms

Gymnosperms are commonly included in the following forms, Conifers, cycads,

ginkgoes, gnets, pines, spruces. Others include cedars and firs (Fig1.1).





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Gymnosperms have two alternating plant bodies the gametophyte and sporophyte.

And for the first time we are mentioning seeds producing plants. Gymnosperm forms

are distributed round the whole world. Cycads and gnetae dominate the tropical and

sub-tropical regions. The temperate and subarctic regions are dominated by

conifers. Most gymnosperms are shrubs, trees and woody vines. Their leaves occur

oppositely or in whorls of three.

At this point it is important for you to acquire the necessary skill to distinguish one

form from another.

Cedar Pine

Gnetae

Cycad Ginkgo

Conifer

Figure1.3 Varieties of forms in gymnosperms





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Conifer forms

Conifers form the largest group of living gymnosperms. Cedar (genus Cedrus) is a

coniferous tree closely resembles Spruce in terms of cone structure.

Other conifers that share similar botanical features, which are but not true cedar

include Thuja, Chamaecyparis and Juniperus. Common and obvious characters of

cedar include the followings

Persistent, cross (decussate) flat scale-like spray leaves

Reddish and vertically peeling bark

Woody and fleshy cones

You may wish to go online to view the images it is important you do so.

Cycads forms

Cycads ranked the second largest living gymnosperms. Encepharlatos barteri is an

indigenous species commonly called West African cycad or bread starch. Cycads

are distinctly identified by the following characteristics

Stout, erect, unbranched, palm-like tree

Pinnate forming crown leaves

Erect single or multiple terminal cone (microsporophylls)

Rosette of terminal carpels (megasporophylls)






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Gnetae forms

Gnetae ranked the third largest living gymnosperms. Gnetae contain three genera

that are widely distributed all over the world from the deserts to the rainforests. The

genera are Ephedera, Gnetum and Welwitschia. It is important to know that the only

character that places these genera under the Gnetales is their similar reproductive

structure. It is only in Gnetum you find vines. Another similarity is the complete

absence of archegonia structure which is typical of other gymnosperms but absent in

Gnetum, Welwitschia and angiosperms. As we can see gnetae have many features

in common with the sister angiosperms. This is of a great scientific interest; they are

the only gymnosperms that have similar vessels with angiosperms. The cones,

leaves structure and venation of gnetae resemble that of angiosperms flowers.

Another interesting finding is double fertilization; two sperm nuclei uniting with an

egg nucleus instead of one sperm nucleus uniting with an egg nucleus, as found in

other gymnosperms and spore producing plants. This condition of double fertilization

in Gnetum is an indication of transition from single fertilization to double fertilization

found in angiosperms today. Hence the many similarities between gnetae and

angiosperms is an indication that gymnosperm is an ancestor of the angiosperms.

E. barteri young leaves

Encepharlatos barteri

Zamia





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Ginkgos forms

Ginkgos ranked the least living gymnosperms on earth. Ginkgo is a genus of strange

gymnosperm. Chinese name for this plant is yínguǒ (銀果) meaning “silver fruit”.

Ginkgo biloba is the only living “fossil” species endemic to China and it is popularly

known as the maidenhair tree. The similar recognizably fossils exist 270 Million

Years Ago (MYA). The popular name “maidenhair tree” is because the leaves

resemble the leaves of a pteridophyte, maidenhair fern, Adiantum.

Cycads forms

Cycads are distinctly identified by the following characteristics

Long-lived large trees (20 - 35 m) estimated to be 2,500 year old

Angular tree crown

Fan-shaped leaves with dichotomous (repeatedly fork in two) radiating veins

Branches bear regularly spaced leaves at short internodes

Reproductive structures form in leaves that clustered at the tip of short shoots

Female structure produce two ovules, one or both developed into seeds after

fertilization

Male structure produce cones containing motile sperms


Habitats of Gymnosperms

Habitats of Gymnosperms are widely distributed all over the world from extremely

dry deserts to extremely moist rainforests. They are resilient to great diverse habitats

conditions. For example, many cycads survive in habitat with poor nutrient and able

to source for needed nitrogenous compound through symbiotic association with a

Cyanobacterium. Generally Gymnosperms are found in the following geographical

zones of the world.

Northern temperate forests





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Equatorial regions

High Mountains (China)

Ecology of Gymnosperms

All cycads have coral-like roots and are of ecological interest. These roots contain a

Cyanobacterium that traps nitrogen gas from the atmosphere and fix into nitrate for

its own and other plants use. This may be a great advantage as many of the

gymnosperms grow in nutrient poor habitats. Gymnosperms can also be important

plants in ecosystem.

Economic uses of Gymnosperms

Gymnosperms have great economic and aesthetic values. Current trend in the use

of gymnosperms have stimulated many interests. One unique aspect of them is that

many gymnosperms produce resin in their trunks and branches to repair bruises or

cut done by man and animals on it. A warning is that the resin is highly toxic if

ingested and also a powerful medicine. It is insoluble in water and quickly hardened

when contact with air. Resins are source of gloss, lacquers polishes, varnishes,

solvents, and cleaners. It is important you know that resins produce by the sister

angiosperms is usually a mixture of resin with gums, mucilage, waxes, latexes and

oils.

Many species of Conifers are important industrial materials for making pulp wood,

pencils and music instrument. The tone-wood used for many musical instruments is

from conifers (cedar and spruce).

Leaves of many members of Cycadales are traditionally used in medicine. Cycad

spp. and Zamia spp. (smallest gymnosperm) are known as good source of quality

starch. Cycad revolute seeds and stem are used for production of gymnosperm





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wine. For ornamental purposes the Ginkgo biloba trees are used for beautification of

gardens, parks and road sides. Thuja occidentalis for its great variations in colors,

shape and size, particularly make it useful for screens and hedges in gardens and

parks.

Conclusion

At the end of this lecture you must have learnt how to recognize, and where to find

Gymnosperms. Focus on the economic usefulness of certain groups of the living

gymnosperms.

Summary

Having study this lecture note, you should:

Have some understanding of forms of Gymnosperms

Have a better idea of the characteristics that separate Gymnosperms from other

groups of plants

Be ready to get going with identification of the members on the field

Understanding contributions of Gymnosperms to our lives

References/Further readings/Links





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Test Yourself Questions (TYQ)

Matric No Department Attendance-Code Session/Level

AC_____________

Question TRUE FALSE

1. Almost all Gymnosperms are aquatic plants

X

2. Gymnosperms are advanced spore plants

X

3. Animals and insects are major agents of pollination in gymnosperm

X

4. Progymnosperms is an assumed ancestor of gymnosperm X

5. Fan-like leaves are found in ginkgoes X

6. Ginkgos are endemic to China X

7. The female gametophyte of gymnosperm consist of pollen grains

X

8. Ovule is naked in gymnosperm X

9. A fertilized ovule always develop into fruit X

10. Nucellus is a nutritive tissue X

Comment:


Documents

BIO102: Plant Diversity - staff.oouagoiwoye.edu.ngstaff.oouagoiwoye.edu.ng/uploads/412_COURSES_Plant_Diversity_24430.pdf · ovule. The ovule contains a tissue called nucellus, which