The plant body is composed of cells and their products. All plant cells are
surrounded by a rigid cell wall that is produced by the cell. Inside the wall
is the cell membrane, which selectively regulates the movement of materials
into and out of the cells. Plant cells are compartmentalized into organelles
that are structurally organized to support their function. The nucleus, chloroplast,
ribosomes, and vacuole are examples of plant cell organelles.
Molecules flow into and out of the cells by simple diffusion, facilitated diffusion
via passive transport, and active transport. Osmosis is the diffusion of water
across a membrane. Osmosis occurs in response to a water potential gradient
produced by differences in solute concentration on each side of a membrane.
Several adaptations to land were required for the evolution of terrestrial plants.
These adaptations include the four kinds of tissue systems in plants: meristems,
ground tissue, dermal tissue, and vascular tissue. Meristems are localized regions
of cell division that form cells. Ground tissue, which usually constitutes most
of the plant body, may perform photosynthesis in a leaf, store starch in a root,
or add strength to a stem. Dermal tissue forms the epidermis, which surrounds
the plant body and serves to absorb nutrients and protect a plant from desiccation.
Vascular tissues, the xylem and phloem, provide support and move water and solutes
throughout the plant. The activity associated with apical meristems, at the
tips of shoots and roots, results in growth in length and forms the primary
body of the plant-roots, stems, and leaves.