Classification
and Evolution of the Land Plants (Continued).
The vascular plants with seeds.
Seed plants can be divided into plants with naked seeds
and plants with seeds enclosed in a fruit.
The naked seed vascular plants are collectively called the Gymnosperms.
This group includes several phyla. The living phyla are:
Coniferophyta (conifers)
Cycads
Ginkgoes
Gtenophytes
There are also some ancient extinct groups in the fossil record that have fern-like characteristics. These are called "progymnosperms".
The
conifers are a large successful group. Many have reproductive
structures called cones.
Separate male and female cones often produced on same plant at different times
of year.
Dominant sporophyte and a microscopic gametophyte. The gametophyte is reduced
to a few cells in the cone that give rise to pollen and ova (egg).
The
female gameteophyte is derived from a megaspore, the male gameteophytes develop
from microspores.
Pollen
is carried from male cone to female cone by air currents. The microscopic pollen
grains have two tiny wings to help them get blown by the wind.
Conifers include the worlds oldest trees (brislecone pine, 4900 years), the tallest tree (coastal redwood, 385 feet) and the most massive tree (giant sequoia).
Resin accumulates in special resin ducts found in leaves and stems.
Economically important as source of lumber and chemicals from resin.
Examples include
Ginkgoes
are a very ancient group with fossil forms dating back hundreds of millions
of years. Ancient forms almost the same as modern species. Only one surviving
species. Once almost extinct, centuries ago it only grew in only two locations
in China.
Rescued and propagated by humans, now used all over the world as hardy urban
ornamentals.
Sometimes called madenhair tree because its leaves resemble those of the unrelated
maidenhair
fern.
Separate male and female plants.
One of each can be found growing in the back of the B&Z building. Female
produces rather smelly seeds.
Female plants banned in some cities.
Deciduous - leaves turn gold and drop off at end of growing season.
Ciliated sperm (vestigeal - Ginkgo
produces airborn pollen)
Description of Ginkgo's role in culture and medicine here.
Cycads
are palm-like
in appearance and most are tropical. Very common group during the time of the
dinosaurs (248 million years ago) but less common today.
Gnetophytes. Unusual gynnosperms with certain reproductive characteristics in common. Three genera: Ephedra, Gnetum and Welwitschia
The vascular plants with seeds enclosed by a fruit are called Angiosperms.
Only one phylum:
Angiospermophyta (Anthophyta)
Appeared 200 million years ago but were not dominant until about 60 million
years ago.
Dominant sporophyte and a microscopic gametophyte. The gametophyte is reduced to a few cells that give rise to pollen and ova (egg).
The female gameteophyte is derived from a megaspore, the male gameteophytes develop from microspores.
Have special reproductive structures called flowers.
May be pollinated by wind or by animal pollen carriers such as insects, beetles
or bats.
Animal pollinated species often have brightly colored or fragrant flowers
to attract pollinators. Coevolution to meet the needs of a single pollinator
species is common. It is thought that this helps to ensure "loyal customers"
who seek out that particular flower species. Can act as a mechanism for speciation.
Flowers may contain nectar to attract animal pollinators.
Wind pollinated species have drab flowers that produce large quantities of pollen.
After fertilization of the eggs, ovary wall may become a fleshy fruit used to
protect or nourish the seed or to attract animals that will consume the seed
and disperse it to new locations.
Great variation in pattern of seed and fruit development.
The
ovary wall is made up of three layers (pericarp, endocarp and mesocarp) the
differential development of these layers gives rise to the many
kinds of fruit.
Students should know basic flower anatomy as presented in week one lab.
Angiosperms are divided into two classes.
Dicotyledones (dicots)
Monocotyledones (monocots)
Monocots include wheat, corn, grasses, banana, orchids, some palms.
Monocot seedling have a single leaf. Storage in the seed is mainly accomplished
by triploid endosperm, which is a special type of starch, oil, protein and DNA
rich tissue.
Dicots are the broadleaf plants. Their seedlings have two leaves. These may
sometimes be modified into storage organs, while the endosperm is reduced compared
to the monocots.
There are 65,000 species of monocots and 165,000 species of dicots.
Some important and identifiable families include:
Monocots: Liliaceae. Flower parts in threes or
multiples. Onions, Easter lily, chives, garlic.
Monocots: Grasses. Poaceae (or Gramineae). Wind-pollinated,
flowers inconspicuous, highly reduced, and specialized in structure. Extremely
important as crops and in biosphere.
Monocots: Orchidaceae. Orchids are also highly
advanced with specialized floral parts. The largest family of angiosperms.
Monocots:
Agavaceae. Mainly succulents with stiff, pointed
leaves arranged in a rosette. Examples include the century
plant.
Dicots: Lamiaceae. Mint family including basil,
oregano, catnip and Coleus. Square stems, usually with opposite leaves, and
bilaterally symmetric flowers.
Dicots: Fabaceae. Leguminosae (legumes). One subgroup
has bilaterally symmetric flowers (bean, pea), another has radially symmetric
flowers (Albizzia, Mimosa). Very important family economically (foods) and ecologically
(nodules supply combined nitrogen to soil).
Dicots: Asteraceae. Sunflower family. A huge family
with over 20,000 species including lettuce, marigolds and ragweeds. A key character
is that the flowers are grouped into a head which is an entire inflorescence.
Some of the flowers are tubular and others are ray- or strap-shaped.
The evolutionary progression of the land plants:
Nonvascular plants > seedless vascular plants > Gymnosperms
> Angiosperms
(Know the approximate dates associated with this progression. See first lecture
for timeline)
This progression is marked by:
Trend towards larger diploid sporophyte and smaller haploid gameteophyte.
Loss of requirement for water as a means of transport for the male gametes (motile
sperm replaced by pollen)
Appearance of seeds.
Appearance of fruit
surrounding the seed.
