Tissue Systems

Independent of what kind of cells they are built from, plant tissues also organise themselves into larger functional systems based purely on their position and structure within the organ. Three tissue systems are recognised in this way — the epidermal tissue system, the ground tissue system and the vascular tissue system — and every root, stem or leaf described later in this c …

The epidermal tissue system forms the outermost protective covering of the whole plant body and develops from the protoderm. It has two structural components: the epidermis proper, and epidermal appendages growing out of it.

The epidermis is a single continuous protective layer of compactly arranged cells with no intercellular spaces between them; each cell has a large central vacuole, a thin peripheral layer of cytoplasm and a nucleus. On its outer face the epidermis is usually coated with a thick, waxy layer called the cuticle, whose main job is cutting down water loss from the surface. The epidermis also bears hair-like outgrowths: in the root these are unicellular, elongated root hairs, whose job is absorbing water and dissolved minerals from the soil, while in the stem the equivalent outgrowths are called trichomes. Trichomes are generally multicellular and may be branched or unbranched, stiff or soft, and some are even secretory; functionally they help cut down water loss through transpiration. …

The ground tissue system is essentially a system defined by exclusion — it is everything in the plant body that is not part of the epidermal system or the vascular system. It is composed mainly of simple permanent tissue, above all parenchyma. In the primary root and stem, ground tissue makes up the cortex, the pericycle, the pith and the medullary rays. Where mechanical support is also needed close to the surface, collenchyma and sclerenchyma contribute to the ground tissue system as the hypodermis. In leaves, the chloroplast-rich mesophyll — the ma …

The vascular tissue system is made of the distinct patches of complex tissue — xylem and phloem — running through the plant body, and the different ways these patches can be arranged relative to each other give rise to several named types of vascular bundle.

When xylem and phloem sit on completely separate radii, each as its own separate bundle, the arrangement is called radial — this is the arrangement typically seen in roots. In the stem, by contrast, xylem and phloem sit together on the same radius, side by side, with xylem toward the inside and phloem toward the outside; this is called conjoint, collateral. Collateral bundles are further divided into open bundles, which retain a strip of cambium between the xylem and the phloem and can therefore undergo secondary growth, and closed bundles, which have no cambium and so cannot. When phloem is present on both the inner and the outer side of the xylem, with cambium strips intervening, the bundle is called bicollateral — a distinctive feature of the family Cucurbitaceae. …