Plant Tissues, Organs and Systems

A maple tree is a skyscraper with plumbing

A sugar maple can grow over 30 m tall. Every summer day it lifts water from the soil all the way to its highest leaves, with no pump. Every spring, sweet sap flows in its trunk, which is why maple trees are tapped every spring. Many First Nations, including Anishinaabe and Haudenosaunee communities, have made maple syrup and sugar since long before Europeans arrived, and continue to today. To do all this, the tree needs tissues that protect, support, make food and transport materials, organized into organs and systems, just like you.

The four plant tissue types

1. Meristematic tissue (you met it in the stem cell lesson): unspecialized cells that divide to make new cells. Found at root and shoot tips (apical meristems) and in rings in stems and roots (lateral meristems, like the cambium).

2. Dermal tissue covers and protects the plant.

• The epidermis is usually one layer of tightly packed cells. On leaves and stems it makes a waxy, waterproof cuticle that reduces water loss.
• Special epidermal cells include guard cells (which open and close pores called stomata in leaves) and root hair cells.
• In woody plants, the epidermis of older stems is replaced by bark.

3. Ground tissue makes up most of a young plant.

• Parenchyma cells have thin walls. They do photosynthesis (in leaves) and store food and water (the white part of a potato is mostly parenchyma packed with starch).
• Collenchyma cells have unevenly thickened walls and give flexible support to growing stems and leaf stalks.
• Sclerenchyma cells have very thick, hard walls and are often dead when mature. They give rigid support: the gritty bits in a pear and hard nut shells contain them.

4. Vascular tissue transports materials.

• Xylem carries water and dissolved minerals up from the roots to the leaves. Its main cells (vessel elements and tracheids) are dead when mature: their contents disappear, leaving hollow tubes with thick walls strengthened by lignin. In vessels, the end walls break down, forming long continuous pipes. Xylem also supports the plant: wood is xylem.
• Phloem carries sugars (made in leaves) and other dissolved food to where they are needed or stored: down to the roots, or up to growing shoots, flowers and fruits. Its main cells, sieve tube elements, are alive but have no nucleus; their end walls (sieve plates) are full of holes. Each one has a companion cell beside it that keeps it alive and helps load sugar.

Tissues build organs

Plants have just a few organs, each made of all three main tissue systems (dermal, ground, vascular):

• Roots anchor the plant and absorb water and minerals through root hairs. Many roots store food (carrots, beets, sweet potatoes). Inside, a central core of xylem and phloem carries water up and sugars down.
• Stems hold leaves up to the light, carry water and food between roots and leaves through vascular bundles, and sometimes store food. (A potato is actually an underground stem called a tuber; its "eyes" are buds.)
• Leaves are the main site of photosynthesis and gas exchange. You'll study their tissues in detail in the next lesson.
• Flowers, fruits and seeds are organs of reproduction.

The organs form two systems: the root system (below ground) and the shoot system (stems, leaves, flowers above ground). Xylem and phloem run continuously through both, connecting every part of the plant.

Inside a stem

Cut a stem crosswise and you'll see vascular bundles: strands of xylem and phloem running lengthwise. In a bundle, phloem is toward the outside and xylem is toward the inside.

• In dicots (such as sunflowers, beans, maples and oaks), the bundles form a ring. In woody dicots, a cambium between the xylem and phloem adds a new ring of wood every year.
• In monocots (such as corn, grasses, lilies and onions), the bundles are scattered through the stem. Most monocots don't make wood.