Seedless Plants

Adaptations Enabling the
Move to Land

In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out

The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO₂, nutrient-rich soil, and few herbivores or pathogens

Land presented challenges: a scarcity of water and lack of structural support

The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants

Systematists are currently debating the boundaries of the plant kingdom

Some biologists think the plant kingdom should be expanded to include some or all green algae

Until this debate is resolved, we will retain the embryophyte definition of kingdom Plantae

Derived Traits of Plants

Four key traits appear in nearly all land plants but are absent in the charophytes:

Alternation of generations (with multicellular, dependent embryos)

Walled spores produced in sporangia

Multicellular gametangia

Apical meristems

Additional derived traits such as a cuticle and secondary compounds evolved in many plant species

Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients

Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations

The gametophyte is haploid and produces haploid gametes by mitosis

Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis

The diploid embryo is retained within the tissue of the female gametophyte

Nutrients are transferred from parent to embryo through placental transfer cells

Land plants are called embryophytes because of the dependency of the embryo on the parent

The sporophyte produces spores in organs called sporangia

Diploid cells called sporocytes undergo meiosis to generate haploid spores

Spore walls contain sporopollenin, which makes them resistant to harsh environments

Gametes are produced within organs called gametangia

Female gametangia, called archegonia, produce eggs and are the site of fertilization

Male gametangia, called antheridia, are the site of sperm production and release

Plants sustain continual growth in their apical meristems

Cells from the apical meristems differentiate into various tissues

The Origin and
Diversification of Plants

Fossil evidence indicates that plants were on land at least 475 million years ago

Fossilized spores and tissues have been extracted from 475-million-year-old rocks

Those ancestral species gave rise to a vast diversity of modern plants

Land plants can be informally grouped based on the presence or absence of vascular tissue

Most plants have vascular tissue; these constitute the vascular plants

Nonvascular plants are commonly called bryophytes

Seedless vascular plants can be divided into 2 clades:

Lycophytes (club mosses and their relatives)

Pterophytes (ferns and their relatives)

Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade

A seed is an embryo and nutrients surrounded by a protective coat

Seed plants form a clade and can be divided into 2 further clades:

Gymnosperms, the “naked seed” plants, including the conifers

Angiosperms, the flowering plants

Bryophyte Gametophytes

In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes

Sporophytes are typically present only part of the time

A spore germinates into a gametophyte composed of a protonema and gamete-producing gametophore

Rhizoids anchor gametophytes to substrate

The height of gametophytes is constrained by lack of vascular tissues

Mature gametophytes produce flagellated sperm in antheridia and an egg in each archegonium

Sperm swim through a film of water to reach and fertilize the egg

Bryophyte Sporophytes

Bryophyte sporophytes grow out of archegonia, and are the smallest and simplest sporophytes of all extant plant groups

A sporophyte consists of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristome

Hornwort and moss sporophytes have stomata for gas exchange

The ecological and economic importance of mosses

Mosses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlands

Some mosses might help retain nitrogen in the soil

Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat

Sphagnum is an important global reservoir of organic carbon

Seedless vascular plants

Living vascular plants are characterized by:

Life cycles with dominant sporophytes

Vascular tissues called xylem and phloem

Well-developed roots and leaves

In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern

The gametophytes are tiny plants that grow on or below the soil surface

Vascular plants have two types of vascular tissue: xylem and phloem

Xylem conducts most of the water and minerals and includes dead cells called tracheids

Phloem consists of living cells and distributes sugars, amino acids, and other organic products

Water-conducting cells are strengthened by lignin and provide structural support

Increased height was an evolutionary advantage

Roots are organs that anchor vascular plants

They enable vascular plants to absorb water and nutrients from the soil

Roots may have evolved from subterranean stems

Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

Leaves are categorized by two types:

Microphylls, leaves with a single vein

Megaphylls, leaves with a highly branched vascular system

According to one model of evolution, microphylls evolved first, as outgrowths of stems

Sporophylls are modified leaves with sporangia

Sori are clusters of sporangia on the undersides of sporophylls

Strobili are cone-like structures formed from groups of sporophylls

Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte

All seed plants and some seedless vascular plants are heterosporous

Heterosporous species produce megaspores that give rise to female gametophytes, and microspores that give rise to male gametophytes

Classification of
Seedless Vascular plants

There are two phyla of seedless vascular plants:

Phylum Lycophyta includes club mosses, spike mosses, and quillworts

Phylum Pterophyta includes ferns, horsetails, and whisk ferns and their relatives

Giant lycophytes thrived for millions of years in moist swamps

Surviving species are small herbaceous plants

Club mosses and spike mosses have vascular tissues and are not true mosses

Ferns are the most diverse seedless vascular plants, with more than 12,000 species

They are most diverse in the tropics but also thrive in temperate forests

Horsetails were diverse during the Carboniferous period, but are now restricted to the genus Equisetum

Increased photosynthesis may have helped produce the global cooling at the end of the Carboniferous period

The decaying plants of these Carboniferous forests eventually became coal