Evolution and Adaptations of Land Plants

Protection and Gas Exchange

On land, water loss is a constant threat. Plants evolved a cuticle, a waxy coating on their surfaces that retards water loss. However, because a cuticle also blocks the entry of $CO_2$, plants developed stomata—small openings in stems and leaves that can open and close to regulate gas exchange and water loss.

Growth and Reproduction

Unlike many algae, land plants exhibit apical meristems, which are regions of active cell division at the tips of roots and shoots that allow for directed growth. For reproduction, land plants utilize gametangia, which are multicellular organs that enclose and protect the gametes.

Alternation of Generations

A fundamental characteristic of all land plants is the alternation of generations. This is a life cycle that alternates between two distinct multicellular stages:

1. Gametophyte: The haploid ($n$) generation that produces gametes (sperm and eggs) via mitosis.
2. Sporophyte: The diploid ($2n$) generation that produces spores via meiosis within sporangia.

Evolutionary Trend Tip: In the evolutionary history of plants, there is a clear trend: the gametophyte generation becomes increasingly reduced in size and becomes more dependent on the sporophyte for nutrition.

The Divergence: Nonvascular vs. Vascular Plants

The colonization of land led to two major structural groups: nonvascular and vascular plants.

Nonvascular Plants

Nonvascular plants, such as liverworts, mosses, and hornworts, lack specialized tissues for transporting water and nutrients. Because they rely on diffusion for water transport, they are generally small and must live in moist habitats.

Vascular Plants

The evolution of vascular tissues was a revolutionary turning point. These plants, also called tracheophytes, possess specialized conducting cells:

• Xylem: Conducts water and minerals from the soil upward. Some xylem cells contain lignin, a compound that provides the rigid structural support necessary for plants to grow tall.
• Phloem: Conducts the products of photosynthesis throughout the plant body.

The ability to grow tall allowed vascular plants to compete more effectively for sunlight and aided in the dispersal of spores.

The Rise of Seed Plants

While early vascular plants (like ferns) still rely on water for sperm to swim to the egg, the evolution of pollen and seeds allowed plants to colonize much drier environments.

Pollen and Seeds

Pollen grains contain the male gametophyte and are protected by sporopollenin, one of the most resistant biological compounds known. This allows sperm to be transferred via wind or animals without the need for a film of water.

Seeds provide a protected, dormant stage for the embryo. A seed is composed of three generations of tissue: the seed coat (from the parent sporophyte), the food supply (from the parent gametophyte), and the embryo itself (the new sporophyte).

Gymnosperms and Angiosperms

Seed plants are divided into two major groups:

1. Gymnosperms: "Naked-seeded" plants, such as conifers, where ovules are not enclosed in an ovary.
2. Angiosperms: Flowering plants, which represent the most diverse group.

Angiosperms introduced the flower, which facilitates complex mutualistic relationships with pollinators like insects, birds, and bats. They also feature double fertilization, a process where one sperm fertilizes the egg to form a zygote, while another fuses with two nuclei to create the endosperm, a nutrient-rich tissue that feeds the developing embryo.

Ecological Importance

Plants are the foundation of nearly all terrestrial ecosystems. As primary producers, they convert solar energy into chemical energy, supporting entire food webs. Beyond food, they regulate the atmosphere by producing oxygen and sequestering $CO_2$, contribute to soil formation, and provide the basis for many modern medicines, such as taxol for cancer treatment.