Plants are descended from aquatic green algae. As they moved onto the land, they developed physical and physiological adaptations that enabled them to cope with the challenges of a terrestrial lifestyle. These include structures for support against gravity and for internal transport, water conservation measures, and methods for protecting and dispersing gametes.

The majority of plants possess roots and mychorrhizae for the absorption of water and nutrients from the soil, leaves to serve as photosynthetic centers, and stems to connect the roots to the leaves. Running through these structures is a framework of specialized tissue that acts as a plumbing system. This vascular tissue carries water and dissolved nutrients up from the roots, and food down from the leaves.

There are two types of vascular tissue in plants, xylem and phloem. Xylem consists of elongated, hollow cells that are arranged end to end to form a tube. Acting like pipes, these cells transport water and minerals from the roots to the leaves. The walls of xylem are thickened and reinforced with lignin and cellulose, and act to support the stems and leaves. Xylem cells are dead at maturity.

Phloem is made up of long, thin-walled cells that move the sugar produced by photosynthesis to parts of the plant where it will be used or stored. The phloem is made of living cells, and is usually located close by and parallel to the xylem

Seeds
In order for sexual reproduction to occur in most terrestrial environments, plants need ways to protect gametes from drying out and to disperse them without water. These problems are solved by the production of pollen and seeds. Every pollen grain is a tiny male gametophyte that can be swept to a female by animals, insects, or the wind. Two sperm are produced within each pollen grain. The female gametophyte is also small, made of little more than an egg and a bit of other tissue tucked within a sac-like ovule inside a flower or cone.

When sperm and egg fuse, a diploid embryo is produced. This tiny plant, along with the ovule that holds it, is called a seed. Most seeds have three parts – an embryo, some surrounding tissue, and a coat. This structure provides the young plant with several advantages:

Flowers and Fruit
Flowers and fruit are specialized structures developed from modified leaves that function in sexual reproduction. Flowers produce the male and female gametophytes and contain all the reproductive tissues. Pollen is produced by multiple stamens, while a single pistil holds the ovary and egg cells. A swirl of petals usually surrounds the pistil and stamens.

Flowers also act as attracting devices for the purposes of pollination. Different flowers are designed to attract different pollinators (insects, birds, bats), so a particular pollinator carries pollen from one individual flower to another of the same species.

A fruit is a swollen flower ovary that contains fertilized seeds, and is designed entirely for enhanced seed dispersal. Large, fragrant, tissue-rich fruits exist to tempt animals into eating them. The seeds pass through the animal’s digestive tract, and receive a jolt of fecal fertilizer as a bonus. Some fruits aren’t swallowed, but instead hook or stick onto passing animals. Others have wings or fluff that help them drift on the wind.