Cells in plants are arranged in tissues such as epidermis, cortex, and pith. Several tissues also make up a tissue system such as the vascular system.
Meristems
Throughout the life of a plant, new cells are continuously being formed at sites called meristems. Meristems consist of undifferentiated cells that are found at
shoot tips, at root tips, in the vascular cambium, and in the cork cambium. Meristems produce cells that differentiate into specialized tissues of three systems: dermal, ground, and vascular. Only primary growth (left column), resulting from the activity of the shoot and root apical meristems, is found in some herbaceous dicots, most monocots, and lower vascular plants. Conifers and woody dicot shrubs and trees exhibit secondary growth
(right column).
Shoot Tissue Systems
The shoot is the portion of the plant above the roots and is composed of stems, leaves, flowers and fruits. The stem apical meristem (a) forms leaf primordia (b) on its flanks and young stem tissues below during the vegetative phase, and in flowering plants, flower primordia during the reproductive phase.
Dermal Tissue System.
The outer layer of the apical meristem gives rise to the epidermis (c) of the primary plant body. In the stems of plants with secondary growth, the epidermis is replaced by periderm (d), commonly called the “outer bark.” Periderm consists of the
meristematic cork cambium producing cork (phellem) outward and the pelloderm inward.
Ground Tissue System.
The cells and tissues of the ground tissue system are derived from the apical meristem. In the diagram, ground tissue is manifest as cortex (e), located between epidermis and vascular bundles, and pith (f) in the center of the stem.
Vascular Tissue System.
Also derived from the apical meristem, the procambium (g) initiates the vascular system with cells that differentiate inside into primary xylem (h), the water- and nutrient-conducting tissue, and outside into primary phloem (i), the food conducting tissue.
In plants with secondary growth, a persistent cambium, the vascular cambium (j), makes possible added layers of secondary xylem (k) and phloem (l), resulting in an increase in stem diameter.
Root Tissue Systems
The root apical meristem (m) produces new root cap (n) cells ahead (below) of the root apex, as well as cells of the protoderm (young epidermis), ground meristem, and procambium back of the root apex.
Dermal Tissue System.
With primary growth, roots are covered with epidermis (o). With secondary growth, the epidermis and root hairs (derived from epidermal cells) are sloughed off because of formation of periderm (p) on the outside.
Ground Tissue System.
Cortex (q) lies between the epidermis and vascular system in portions of roots undergoing primary growth. The innermost layer of cortex, the endodermis (r), bounds the pericycle tissue (s), which surrounds the primary vascular system. In portions of roots undergoing secondary growth, the cortex, like the epidermis, is shed as the periderm develops cork cells.
Vascular Tissue System.
In the root, unlike the stem, the primary vascular system is in a central cylinder (t) with xylem and phloem arranged in an alternate radial manner. Primary xylem “arms” (u) radiate out from the center with alternate poles of primary phloem (v) in the outer portion of the cylinder. Between them is procambium (w). In older portions of roots, where secondary growth is evident, the vascular cambium (x) produces successive layers of secondary xylem (y) and phloem (z)
