5.2 Major Plant Groups

As plants began to colonize land, they evolved additional adaptations that helped them survive and reproduce in terrestrial environments. These adaptations vary among different plant groups and help define their unique characteristics. In this section, we will focus on four major groups of land plants: nonvascular plants (bryophytes), seedless vascular plants, seed plants (gymnosperms), and flowering plants (angiosperms).

Nonvascular Plants (Bryophytes)

Nonvascular plants, also known informally as Bryophytes, appeared around 470 million years ago and are the closest living relatives of early terrestrial plants. Their major adaptations to life on land include a waxy cuticle and root-like structures called rhizoids. Rhizoids anchor the plant but they lack vascular tissue and do not actively absorb water. As a result, bryophytes must live in very moist environments. They are also heavily dependent on water because their sperm are flagellated so require water for mating (sperm must swim in water to find the egg). Bryophytes are very short because they have no mechanism for transporting water up against gravity.

The bryophytes are divided into three divisions liverworts, hornworts, and mosses. There are about 18,000 species of bryophytes but the majority are mosses so they will be the focus of this section.

Mosses are commonly found in moist, shaded environments such as forest floors where they often carpet the soil, bases of trees, rocks, and fallen logs. They are also abundant in wetlands where they grow in dense mats that act like sponges – trapping water, slowing its flow and filtering out sediment. In the tundra, their shallow rhizoids allow them to anchor to surfaces without digging into the frozen soil. They help reduce erosion, retain moisture and nutrients, and provide shelter for small animals as well as food for larger herbivores. Their small size and dense growth help insulate them from the cold. Mosses can also dry out and go dormant, then reactivate when conditions improve.

Seedless Vascular Plants

Seedless vascular plants appeared around 430 million years ago and represent a major evolutionary step in plant adaptation to life on land. Compared to nonvascular plants like bryophytes, seedless vascular plants have three key adaptations:

1. True roots grow deeper into the soil than the rhizoids of bryophytes. This allows seedless vascular plants to absorb more water and nutrients. Many also form partnerships with mycorrhizal fungi (more information in section 5.3), which help them extract even more resources from the soil.
2. Vascular tissue consists of two types of specialized cells: xylem, which transports water from the roots to the leaves, and phloem, which distributes sugars made during photosynthesis throughout the plant. This internal transport system allows seedless vascular plants to grow much taller than bryophytes and thrive in more diverse environments.
3. Lignin is a tough, rigid substance that hardens cell walls. It provides structural support to allows plants to stand straight up.

These features allowed seedless vascular plants to grow larger and colonize drier environments. That being said, like bryophytes, seedless vascular plants still rely on water for reproduction. Their sperm are flagellated and must swim through water to reach the egg, so these plants are still most common in moist environments.

The seedless vascular plants include club mosses, horsetails, and ferns (Fig X). Seedless vascular plants are commonly found in shaded forests, wetlands, and other damp habitats. With their large fronds, ferns are the most readily recognizable seedless vascular plants. About 12,000 species of ferns live in environments ranging from tropics to temperate forests.

Gymnosperms

Gymnosperms, or “naked seed” plants, first appeared around 350 million years ago and represent a major evolutionary advancement in plant reproduction. They possess all the adaptations of seedless vascular plants but also have two additional adaptations that allowed them to thrive in drier environments:

1. Pollen is a protective structure that carries sperm cells. Unlike the flagellated sperm of mosses and ferns, gymnosperm sperm do not need to swim through water. Instead, pollen is carried by the wind to reach the female part of the plant. This adaptation allows gymnosperms to reproduce in dry conditions.
2. Seeds protect the developing embryo. A seed contains a food supply and a protective outer coat that protects the embryo against desiccation (drying out). Seeds can also remain dormant until the environment is right for growth.

There are only around 1000 living species of gymnosperms and the majority of them are conifers. Conifers are tall trees with needle-like leaves. Water evaporation from leaves is reduced by their narrow shape and a thick cuticle. Snow easily slides off needle-shaped leaves, keeping the snow load light and reducing broken branches. Such adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species, like tamaracks, are deciduous and lose their leaves in fall. Many coniferous trees are harvested for paper and timber.

Angiosperms

Angiosperms, or flowering plants, are the most diverse and widespread group of plants on Earth, with over 300,000 known species. They first appeared around 130 million years ago and now dominate most terrestrial ecosystems. Angiosperms include nearly all the plants we eat (e.g. grains, fruits, vegetables, nuts) as well as most ornamental plants. Like gymnosperms, angiosperms have seeds and pollen, but they also have two additional adaptations that make them especially successful:

1. Flowers are specialized structures that help with reproduction. They attract pollinators like insects, birds, and bats, which carry pollen from one flower to another. This is more efficient than relying on wind and increases the chances of successful fertilization.

2. Fruits are any structure that develops from a flower ovary and helps in seed dispersal, such as something sweet that is eaten by an animal so that the seed is deposited somewhere new in the feces (and with its own personal supply of fertilizer!). Fruits thus provide a mechanism for seeds to colonize new territories away from the parent plant.

Flowers

Flowers use a variety of strategies to spread pollen. Some rely on the wind by releasing lots of lightweight pollen into the air. Others have evolved to attract pollinators. Bright colors like red, yellow, and purple catch the attention of bees, butterflies, and birds. Many flowers produce nectar, a sugary liquid, to provide pollinators with a reward to encourage visits. Others use mimicry, imitating the look or scent of something else, like a female insect, to trick pollinators into stopping by. Some flowers even produce strong, unpleasant odors that smell like rotting meat to attract carrion flies or beetles. These diverse adaptations increase the chances of successful pollination.

Although they come in many shapes and colors, most flowers share the same basic parts:

• Sepals are the outermost parts of the flower. They are usually green and protect the flower bud before it opens.
• Petals are located just inside the sepals. They are often brightly colored to attract pollinators.
• Stamens are the male reproductive parts. Each stamen has a thin stalk called a filament and a top part called an anther, where pollen is made.
• Carpels (also called pistils) are the female reproductive parts. A carpel includes:
  • Stigma – a sticky surface where pollen lands
  • Style – a tube that connects the stigma to the ovary,
  • Ovary – which contains one or more ovules. Each ovule holds an egg and will become a seed after fertilization.

Fruit

After fertilization, ovule develops into a seed and the surrounding ovary develops into the fruit. That means that a fruit is a ripened ovary that contains one or more seeds. Many foods we call vegetables are actually fruits because they come from the ovary and contain seeds (e.g. eggplants, zucchini, string beans, and bell peppers). Even acorns and maple keys (called samaras) are considered fruits.

Fruits come in two main types:

• Fleshy fruits are soft and juicy (e.g. berries, peaches, tomatoes).
• Dry fruits are hard or papery when mature (e.g. nuts, rice, wheat).

Some fruits form from a single ovary (e.g. apple) while others form from multiple ovaries in one flower (e.g. raspberries). Still others develop from clusters of flowers (e.g. pineapples).

Fruits play an important role in seed dispersal. Their shapes and features reflect how they spread:

• • Wind dispersal: Lightweight or winged fruits that are carried by the wind (e.g. dandelions and maple keys)
  • Animal ingestion: Colorful, sweet, and juicy fruits are eaten by animals then the seeds are later dropped in their feces (e.g. most fleshy fruits)
  • Animal transportation: Some fruits have hooks or burs that stick to fur or clothing and get carried away (e.g. burdock)
  • Water dispersal: Floating fruits travel across water to new locations (e.g. coconuts)

“14.2 Seedless Plants” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“Organismal Biology” by Georgia Tech Biological Sciences is licensed under a CC BY-NC-SA, except where otherwise noted.