Plant Kingdom Introduction

Introduction to the Plant Kingdom - 1

The Plant Kingdom comprises a large and varied group of organisms that have the

following characteristics in common. All plants are:

• Eukaryotic

• Photosynthetic

• Multicellular with tissue development

• Sexually reproducing

Who are the Plants?

The general classification of plants uses the following features to divide plants into

four broad groups and a number of phyla:

• Presence or absence of vascular tissue

• Dispersal by spores or by seeds

• Seed plants are distinguished further by the protection of the seed.


The Plant Phyla


The Life History of all plants involves an alternation of a haploid phase (the

Gametophyte) that produces gametes in protective structures called gametangia,

with a diploid phase (the SSporophyte) that produces sporangia. To start our

discussion of plants, we will review the alternation of generations, first presented in

Biology 201. Variations in life history are essential to classification in the Plant

Kingdom.

Plant Life History - The Alternation of Generations

Sperm (n) + Egg (n)

|

Zygote (2n)

Growth by mitosis produces:

|

Embryo (2n)

|

Sporophyte (2n)

|

Sporangium (2n)**

Meiosis in Sporangium produces:

|

Spore (n)

Growth by mitosis produces:

|

Gametophyte (n)

| |

Antheridium (n) Archegonium (n)

| |

Sperm (n) Egg (n

* Variations in Life History Patterns

• Sporangia: Homosporous or Heterosporous

• Prevalence (predominance) of sporophyte or gametophyte. More complex

plants = more sporophyte dominance

• Independence or dependence of Sporophyte or Gametophyte on alternate

generation. More advanced plants have a dependent gametophyte.


Evolutionary Trends in the Plant Kingdom

Most plants are terrestrial. Plants evolved from green algae ancestors (Charophyceae)

that successfully inhabited land. As we have discussed, plants must obtain their

nutrients from both air and soil, and much of their structure relates to the need to

maximize obtaining nutrients and water, minimizing water loss and protecting the

plant from damage.

To this end, to survive on land, plants evolved:

• Protective surfaces to minimize desiccation - cuticle

• Gas exchange to circumvent the cuticle –stomata

• Increase in photosynthetic surface area - leaves

• Increase in dimension with conducting and support tissues

• Reproductive strategies that work without water transport of gametes

All plants disperse to new locations via single-celled spores or by seeds. Spore walls

contain ssporopollenin, a protective coating that provides protection against decay

and dehydration. Seeds are protected by a seed coat, a multicellular layer of thick-

walled resistant cells.


Non-Vascular Plants

The non-vascular plants include the mosses, hornworts and liverworts. Because they

lack vascular tissue they are small in vertical stature, and typically grow in clumps or

masses. Most lack a cuticle and many are capable of withstanding long periods of

desiccation. They attach to their substrate with “rhizoids”, but absorb water and

minerals through all surface cells. Some have air pores for diffusion of gases.

Some Bryophytes have a thallus (flattened-shaped) structure and others are tiny,

three-dimensional with an axis and “leaves”, called phyllodes, attached. They have

apical meristems. All Bryophytes disperse by spores and water is required to

transport the motile sperm for fertilization. The gametophyte generation is dominant.

The three non-vascular plant phyla are:

Hepatophyta

Anthocerophyta

Bryophyta

Vascular Plants

Vascular plants exhibit a number of advances that promote survival on land.

• Protection of above-ground body surfaces with a cuticle or surfaces tissues with

cell walls with impermeable layers minimizes water loss, often mentioned in

Biology as a significant problem for plants.

• The evolution of conducting tissue provided a means for plants to become large

in dimension and volume. Vascular tissue provides support for the plant as well

as conducting water and solutes throughout the plant body. Dependence on

diffusion of water and nutrients affects plant size, as seen in the Bryophytes.

• The vascular plants all have a dominant sporophyte generation. They produce

multiple numbers of sporangia on each sporophyte and the spore-dispersing

vascular plants can disperse many airborne spores which means plants can

occupy more locations rapidly.

• Transport of sperm in many plants requires water. Motile sperm requiring water

for transport limits plants ability to sexually reproduce.

• Gametophytes are greatly reduced in size as well as duration relative to the

sporophyte in the vascular plants. As vascular plants become even more

complex, sperm are retained in the male gametophyte, the pollen grain, freeing

sexual reproduction from water dependence.

• In the seed plants, the seed (embryo sporophyte surrounded by nutrients and

seed coat) replaces the haploid single-celled spore as the dispersal unit.

These adaptations to the terrestrial environment have made vascular plants the

predominant vegetation in most parts of the world. While abundant, Bryophytes are

rarely predominant.


To summarize, Vascular Plants have:

• Body plan along a supporting axis (the stem)

• Vascular tissue

• Xylem promotes (allows) increase in size

• Sporophyte generation independent and assimilative

• Gametophyte often dependent on the sporophyte

• Cutin and/or suberin produced on aerial parts to minimize dehydration

• Generally large surface area/volume ratio

• Allows land survival with less surface H2O

Classification of the Vascular Plants

There are seven (or nine) extant vascular plant phyla and at several phyla comprised

of extinct vascular plants. We use the fossil record of the extinct vascular plants to

trace our plant ancestors. Moreover, long-extinct precursors of today’s vascular

plants are the source of most of the world’s coal deposits.

Evolutionary Features of Vascular Plants


Review of Features used in Classifying Vascular Plants

Vascularization

• Protostele, siphonostele and eustele vascular patterns in stems

• The protostele is a solid core of stem vascular tissue from which leaves

diverge without leaving a stem vascular gap.

• The siphonostele has a central pith surrounded by a cylinder of vascular

tissue. In some plants that have siphonosteles the vascular strands that go

to leaves (the leaf traces) result in a leaf gap of parenchyma cells.

• The eustele, found in most seed plants, is comprised of discreet vascular

bundles surrounding the pith. Leaf trace gaps are found in the eustele.

• Microphylls vs. Megaphylls for leaves

• Microphylls contain one vascular strand, probably evolving from small

projections, or enations located along stem tissue.

• Megaphylls have a branching system of veins, and probably evolved from

side branches of stems, in a webbing process, in which tissue containing

chlorophyll filled in spaces between nearby branches.

Microphylls

Megaphylls


Leaf trace patterns and nodes in protostele and siphonstele

Dispersal Method

Spore (single cell) vs. seed

Sperm transport

H2O vs. pollen grain

Types of Sporangia

Plants that produce one type of sporangium are called hhomosporous. Other plants

produce two different sporangia, megasporangia and microsporangia and are

heterosporous.

A plant that is hhomosporous has one type of sporangium. However homosporous

plants can produce one type of spore that grows into a gametophyte that contains

both male and female reproductive structures (the antheridium and archegonium,

respectively) or a homosporous plant can produce two types of spores in its

sporangia. When two types of spores are produced, one grows into male

gametophytes and the second grows into female gametophytes.

Heterosporous plants always have separate sporangia. The microsporangia produce

only microspores that grow into male gametophytes, producing only antheridia.

Megasporangia produce megaspores that grow into female gametophytes, producing

only archegonia. The megasporangia and microsporangia may be found on the same

sporophyte individual, or on separate sporophyte individuals.


The Vascular Plant Phyla

Spore-dispersing Vascular Plant Phyla

Rhyniophyta – Extinct

Zosterophyllophyta - Extinct

Trimerophytophyta - Extinct

Lycophyta (Lycopodiophyta)

Lycopodiae

Selaginellae

Isoetae

Pteridophyta

Equisetales (Equisetales)

Psilotales (Psilotales)

Ferns (Pterophyta)

Ophioglossales – Eusporangiate

Marattiales – Eusporangiate

Filicales – Homosporous and Leptosporangiate

Salviniales – Heterosporous and Leptosporangiate

Marsileales – Heterosporous and Leptosporangiate

Seed-dispersing Vascular Plant Phyla

Progymnospermophyta - Extinct

Pteridospermales

Cordaitales

Bennettitales

The Gymnosperm Phyla

Cycadophyta

Ginkgophyta

Coniferophyta

Gnetophyta

The Angiosperms

Anthophyta

Monocotyledones

Eudicotyledones

Plus

Magnoliidae, including several orders

Nymphaeales – Water Lilies

Illiciales – Star Anise

Amborellales

Archaefructales – Earliest Anthophyte Fossil


Generalized Life History of the Vascular Plants

Spore (n)

|

Gametophyte (n)

|

| |

Antheridium (n) Archegonium (n)

| |

Sperm (n) Egg (n)

(motile) |

| |

|

Zygote (2n)

Growth by mitosis produces a multicellular

|

Sporophyte (2n)

|

Sporangia (2n)

|

| |

If Homosporous If Heterosporous

| |

| | |

Sporangium Megasporangium Microsporangium

| | |

Meiosis Meiosis Meiosis

| | |

Spores (n) Megaspores (n) Microspores (n)

Growth by mitosis from single-celled spore produces a multicellular

| | |

| | Megagametophyte Microgametophyte

| | (Often retained in

| | the Sporophyte)

| |

| |

Two One

Separate Gametophyte

Gametophytes with both Antheridia

(Male and Female) and Archegonia present

Documents

Plant Kingdom Introduction