Plant Tissues and Primary and Secondary Growth

Core Idea

A vascular plant body is built from three tissue systems: dermal tissue (the protective outer covering), vascular tissue (xylem for water and minerals, phloem for sugars), and ground tissue (everything else, used for photosynthesis, storage and support). New cells come from meristems, small zones of dividing cells. Apical meristems at root and shoot tips cause primary growth, which makes the plant longer. Lateral meristems (the vascular cambium and cork cambium) cause secondary growth, which makes stems and roots of woody plants thicker.

How It's Best Learned

Compare a cross-section of a young herbaceous stem with a section of a woody branch, and label the three tissue systems in each. Then look at tree rings and ask where each ring came from. A root-tip slide that shows the root cap, the zone of cell division, the zone of elongation and the zone of maturation makes the idea of tip growth concrete.

Common Misconceptions

• A tree grows taller by stretching its whole trunk. (Height is added only at the shoot tips by apical meristems, so a mark on a trunk stays at the same height for the life of the tree.)
• All the cells that carry water are living. (The water-conducting cells of xylem, tracheids and vessel elements, are dead at maturity and work as hollow tubes.)
• Wood is a kind of bark. (Wood is secondary xylem made toward the inside by the vascular cambium; bark is everything outside the vascular cambium, including secondary phloem and the cork layers.)

Explainer

Animals grow in all parts of the body at once while they are young, and then most growth stops. Plants grow differently. They keep growing for their whole lives, but only in particular places called meristems. A meristem is a group of undifferentiated cells that keep dividing. Some daughter cells stay in the meristem, and others go on to become specialized tissue.

Every organ of a vascular plant (root, stem and leaf) is made of the same three tissue systems. Dermal tissue is the outer layer. In young plants it is the epidermis, often covered by a waxy cuticle that slows water loss. In woody plants a tougher layer called periderm replaces it. Vascular tissue is the plumbing. Xylem carries water and dissolved minerals up from the roots. Phloem carries sugars from where they are made or stored to where they are used. Ground tissue fills the space between. Its main cell types are parenchyma (thin-walled cells that do photosynthesis and storage), collenchyma (flexible support in growing parts) and sclerenchyma (rigid support, with thick lignified walls).

Primary growth comes from apical meristems at the tips of roots and shoots. A root tip shows this clearly. A root cap protects the tip as it pushes through soil. Just behind it is a zone where cells divide, then a zone where they elongate and push the tip forward, and then a zone where they mature into specialized tissues and grow root hairs. Because new length is added only at the tips, the parts of a stem that already exist do not move upward.

Secondary growth adds thickness. It occurs in woody eudicots and in gymnosperms such as pines, and most monocots, such as grasses, lack it. Two lateral meristems are involved. The vascular cambium forms a cylinder between xylem and phloem. It adds secondary xylem to the inside and secondary phloem to the outside. Secondary xylem builds up year after year as wood. The cork cambium, nearer the surface, makes cork cells that protect the stem. Together, everything outside the vascular cambium is called bark.

This plan of growth fits a life spent in one place. A plant cannot walk to new resources, so it keeps adding roots and shoots toward water, minerals and light, and it thickens its stems to carry the extra weight.