Chapter 2: Plant

Diversification

Bryophytes:

Ancestral green algae evolved and led to the diversification of land plants.

This phylogeny occurred through plant morphology, biochemistry, and genetics.

From one ancestral green alga species, 10 phyla of extant plants arose. These

ten phyla can be broken down into two categories: Bryophytes (nonvascular

plants), and Vascular plants.

The bryophyte clade is composed of three phyla of non-vascular plants:

phylum Hepatophyta (liverworts), phylum Anthocerophyta (hornworts), and

phylum Bryophyta (mosses).

Liverworts are described as

“thalloid” because of the flattened

shape of their gametophytes. Some

liverworts are called “leafy” because

their gametophytes have stemlike

structures with leaflike parts.

Hornworts have grass like

sporophytes and there is a

sporangium which extends along its

length and releases spores at the

tip of its “horn”. Gametophytes

grow horizontally and are often

attached to multiple sporophytes.

Hornworts(Phylum

Anthocerophyta)

Mosses(Phylum Bryophyta)

Moss gametophytes grow vertically and they are the structures that

primarily make up a carpet of moss. Moss sporophytes turn brown when they

are ready to release spores.

A wetland moss genus called Sphagnum is common and it forms

widespread deposits of partially decayed organic material known as peat. It is

estimated that 400 billion tons of organic carbon are stored in peat worldwide.

These reservoirs of carbon help to stabilize global atmospheric CO2

concentrations. In addition, peat is a fuel source, notably in Canada and Ireland.

Peat also serves as a soil conditioner, so it is often used for packing plant roots

during shipping. Unfortunately, current overproduction of Sphagnum may

reduce its ecological benefits.

Ecological importance:

Liverworts

(Phylum

Hepatophyta)

This is a peat bog being harvested.

“Tolland Man” is a bog mummy dating from 405- 100 BC. Sphagnum can preserve a human body for thousands of years due to its acidity and lack of oxygen.

Vascular plants:

Seedless vascular plants:

Vascular plants dominate today’s landscape. Fossils indicate that the

earliest vascular plant ancestors were on earth more than 400 million years ago.

Unlike Bryophytes, these species had sporophytes which were branched and not

dependent on gametophytes for growth. This branching also led to more complex

bodies with multiple sporangia. This evolutionary development increased sperm

production and increased survival despite herbivory. The main traits which

distinguish vascular plants are: life cycles with dominant sporophytes, xylem and

phloem, and roots and leaves. Vascular plants can be divided into two groups:

seedless vascular plants and seed plants.

Pterophytes(Phylum Pterophyta)

There are three types of Pterophytes: whisk ferns and relatives, horsetails,

and ferns. Nearly all seedless plants are homosporous.

Ferns:Ferns sporophytes often have horizontal stems which led to the formation

of fronds (large leaves). Most species of fern have megaphylls, stalked

sporangia with spring-like devices to catapult spores into the air, and are

homosporous. Some species produce more than a trillion spores over their

lifetime.

Seedless vascular plants have a sporophyte- dominant life cycle. The fern

life cycle (on the next page) is representative of all Pterophytes.

Vascular seed plants:

Gymnosperms:

The fern life

cycle:

Seed plants can be divided into two groups: gymnosperms and angiosperms. They have had an enormous impact on human society. The domestication of these plants, especially angiosperms, led to one of the most important cultural changes in human history, the shift from hunter- gatherer societies to permanent farming settlements.

Heterospory:Seed plants evolved from plants that had megasporangia and

microsporangia. Megasporangia produce spores that give rise to female

gametophytes and microsporangia produce microspores that give rise to male

gametophytes.

A seed is a sporophyte embryo, along with its food supply, packaged into

a protective coat. Seeds are more resistant than spores and can be widely

distributed by wind or animals.

Seeds:

Gymnosperms are plants which have “naked” seeds, for their seeds are

not enclosed in ovaries. Their seeds are exposed, usually in the form of cones

(strobili). Of the ten plant phyla of land plants, four are gymnosperms:

Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.

Gymnosperms are plants which have “naked” seeds, for their seeds are

not enclosed in ovaries. Their seeds are exposed, usually in the form of cones

(strobili). Of the ten plant phyla of land plants, four are gymnosperms:

Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.

Phylum Cycadophyta

Ginkgo biloba is the only species

of this phylum. It is the source of

herbal extract that reportedly

improves memory. It also tolerates

high amounts of air pollution. And it

has fan-like leaves that turn gold in

the fall.

Phylum Ginkgophyta

Cycads have large cones and

palmlike leaves. Only 130 species

survive today, but they thrived

during the Mesozoic era, or the “Age

of Cycads”.

Phylum Gnetophyta

Gnetophytes consist of three genera: Gnetum, Ephedra, and

Welwitschia. Although these species are different in appearance, the genera

are grouped based on molecular data.

Welwitschia mirabilis lives only in the deserts of southwestern Africa. Its strap- like leaves are among the largest leaves known.

Gnetum includes about

35 species of tropical trees,

shrubs, and vines.

Ephedra are desert shrubs, commonly called “Mormon tea”. They produce ephedrine which is used medically as a decongestant.

Phylum Coniferophyta

This phylum is by far the largest of the gymnosperm phyla. Most

conifers are evergreens; they retain their leaves throughout the year.

Angiosperms:

Douglas fir provides more timber than any other North

American tree species.

The sequoia is one of the largest living organisms and also among the most ancient.

The “berries” of the common juniper are actually ovule- producing cones consisting of fleshy sporophylls.

Gymnosperm evolution:Gymnosperms appear early in fossil record and dominated Mesozoic

terrestrial. Early gymnosperms lived in Carboniferous ecosystems still

dominated by seedless vascular plants. Flora and fauna changed dramatically

as many groups of organisms disappeared and others became prominent.

Gymnosperms, such as pines and furs, adapted to arid conditions, and though

they do not dominate terrestrial ecosystems, gymnosperms remain an

important part of the Earth’s flora.

The life cycle of a pine:

gymnosperm life cycle is the long

lived sporophyte. The female

gametophyte, the ovule, is

nutritionally dependent on the

sporophyte. Fertilization of the sperm

and egg cell produces a woody naked

seed.

All angiosperms are classified under phylum Anthophyta. The key

adaptations of angiosperms are flowers and fruits. Flowers generally consist of

four whorls of modified leaves: sepals, petals, stamens (which produce pollen),

and carpels (which produce ovules). Ovaries ripen into fruits, which are often

carried by wind, water, or animals to new locations. In the angiosperm lifecycle,

double fertilization occurs when a pollen tube discharges two sperm into the

female gametophyte. One sperm fertilizes the egg, while the other combines

with two nuclei at the center of the female gametophyte and starts the

development of food- storing endosperm tissue. The endosperm nourishes the

developing embryo.

Angiosperms underwent adaptive radiation during the late Mesozoic

era. Evolutionary relationships among angiosperms are still actively debated.

Stamens could have evolved from a leaflike structure; an alternative is that

they came from slender branch systems bearing terminal sporangia. Sepals

were probably derived directly from leaves. Petals possibly

originated from leaves in some groups, probably derived

from stamens in most groups. Many angiosperms co- evolved

with insect species, for pollination of flowers by animals and

transport of seeds by animals are very important factors of

angiosperm evolution.

The evolution and co- evolution of angiosperms:

Monocot

s

DicotsThe name Monocot comes

from the fact that they have one

cotyledon, or seed leaf, in the

embryo.

The name Dicot comes from

the fact that they have two

cotyledons, or seed leaves, in the

embryo.

Products made from seed plants:

Most of the food humans depend on come from angiosperms. Wheat,

rice, maize, cassava, and sweet potatoes make up 80% of the calories

humans consume. We also depend on angiosperms to feed livestock. In

addition to basic crops, flowering also provides two of the world’s most

popular beverages: tea and coffee. Spices and cocoa beans are also derived

from flowering plants. Seed plants are also sources of wood. Wood is a

primary source of fuel and used to make paper, like the paper in this book! In

addition, humans have depended on seed plants to create medicines for

centuries. In conclusion, humans depend on seed plants for food, wood, and

many medicines.

Threats to plant diversity:

The ever- expanding human population is causing plant species to near

extinction at an alarming rate. The most common destruction is slash- and-

burn clearing of rainforests for agricultural use, and as these forests

disappear so do thousands of plant species. Once these species disappear

they will never come back. There are many ethical and practical reasons for

concern about the alarming rate at which this loss of plant diversity is

occurring. For example, most of our food is based on the cultivation of only

about two dozen species of seed plants, and fewer than 5,000 have been

studied as potential sources of medicines, this leaves many people

wondering if we are not only destroying a species, but a future cure.

Preserving plant diversity is an important issue that raises many questions

about how we treat our current environment.