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 MossesWind dispersal of their lightweight spores has distributed mosses throughout the world. These plants are particularly common and diverse in moist forests and wetlandsSome 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.