Download BIOCHEMICAL ASPECTS OF BIODIVERSITY

Adaptations Enabling the Move to Land
Many species of charophyte algae inhabit shallow waters around
the edges of ponds and lakes, where they are subject to occasional
drying.
In such environments, natural selection favors individual algae
that can survive periods when they are not submerged in water.
In charophytes, a layer of a durable polymer called
sporopollenin prevents exposed zygotes from drying out. A
similar chemical adaptation is found in the tough sporopollenin
walls that encase the spores of plants.
The accumulation of such traits by at least one population of
charophyte ancestors probably enabled their descendants-the first
land plants-to live permanently above the waterline.
These evolutionary novelties opened a new frontier: a terrestrial
habitat that offered enormous benefits:
The bright sunlight was unfiltered by water and plankton;
the atmosphere offered more plentiful carbon dioxide than did
water;
the soil was rich in mineral nutrients;
and initially there were relatively few herbivores and pathogens.
Challenges of the Move to Land
But these benefits were accompanied by challenges:
A relative scarcity of water
A lack of structural support against gravity.
Derived Traits of Plants
Many of the adaptations that appear to have emerged
after land plants diverged from their algal relatives
facilitated survival and reproduction on dry land.
Alternation of Generations; Multicellular, Dependent
Embryos
Walled Spores Produced in Sporangia
Multicellular Gametangia
Apical Meristems
The Origin and Diversification of Plants
Nonvascular plants are often informally called bryophytes
(from the Greek bryon, moss, and phyton, plant).
Although the term bryophyte is commonly used to refer to
all nonvascular plants, debate continues over the relationships
of Iiverworts, hornworts, and mosses to each other and to
vascular plants.
Whereas some molecular studies have concluded that
bryophytes do not form a monophyletic group (a clade).
Several recent analyses of amino acid sequences in
chloroplasts assert that bryophytes do form a clade.
Whether or not bryophytes are monophyletic, they share
some derived traits with vascular plants, such as multicellular
embryos and apical meristems, while lacking many
innovations of vascular plants, such as roots and true leaves
The Origin and Diversification of Plants
Vascular plants, which form a clade that comprises about
93% of all plant species.
These can be can be categorized finto smaller dades and
larger clade.
Two of these smaller clades are the lycophytes (club mosses
and their relatives) and the pterophytes (ferns and their
relatives).
The plants in each of these clades lack seeds, which is why
collectively the two dades are often informally called seedless
vascular plants.
A third clade of vascular plants consists of seed plants, which
represent the vast majority of living plant species.
A seed is an embryo packaged with a supply of nutrients
inside a protective coat.
Seed plants can be divided into two groups:
 Ggymnosperms and angiosperms, based on the absence or
presence of enclosed chambers in which seeds mature.
Gymnosperms (from the Greek gymnos, naked, and sperm,
seed) are grouped together as "naked seed" plants because their
seeds are not enclosed in chambers.
Living gymnosperm species, the most familiar of which are
the conifers, probably form a clade.
Angiosperms {from the Greek angion, container} are a huge
clade consisting of all flowering plants.
Angiosperm seeds develop inside chambers called ovaries,
which originate within flowers and mature into fruits. Nearly
90% of living plant species are angiosperms.
The Ecological and Economic Importance of Mosses
Wind dispersal of their lightweight spores has distributed
mosses throughout the world. These plants are particularly
common and diverse in moist forests and wetlands
Some mosses colonize bare, sandy soil, where researchers
have found they help retain nitrogen in the soil.
Other mosses inhabit such extreme environments as
mountaintops, tundra, and deserts. Many mosses are able to live
in very cold or dry habitats because they can survive the loss of
most of their body water, then rehydrate when moisture is
available.
Few vascular plants when can survive the same degree of
desiccation.
One wetland moss genus, Sphagnum, or upeat moss; is
especially widespread, forming extensive deposits of partially
decayed organic material known as peat.
Boggy regions dominated by this moss are called peatlands.
Sphagnum does not decay readily, in part because of phenolic
compounds embedded in its cell walls.
The low temperature, pH, and oxygen level of peatlands also
inhibit decay of moss and other organisms.
As a result, some peatlands have preserved corpses for
thousands of years.
Peat has long been a fuel source in Europe and Asia, and it is
still harvested for fuel today, notably in Ireland and Canada.
In addition, peat moss is useful as a soil conditioner and for
packing plant roots during shipment because it has large dead
cells that can absorb roughly 20 times the moss's weight in
water.
Worldwide, an estimated 400 billion tons of organic carbon
are stored in peat. These carbon reservoirs help stabilize
atmospheric CO2 concentrations .
Current overharvesting of Sphagnum may reduce its
beneficial ecological effects and contribute to global warming
by releasing stored CO2.