The Cut That Tells You Which Kind of Plant You're Holding

Every organ in a flowering plant is built from tissues organised, based purely on their structure and location, into three tissue systems: the epidermal tissue system, the ground or fundamental tissue system, and the vascular or conducting tissue system. The epidermal tissue system forms the outermost covering of the entire plant body, made of compactly arranged, usually single-layered, parenchymatous cells, with a small amount of cytoplasm lining the wall around a large central vacuole. Its outer surface is typically covered by a waxy cuticle that prevents water loss, absent only in roots, and it carries two kinds of specialised structures: stomata, mainly in leaves, and epidermal hairs. This threefold division matters because it reflects a genuine functional split: the epidermal system protects, the ground system fills and supports, and the vascular system transports, and nearly everything else about a plant's internal anatomy follows from how these three systems are specifically arranged within a given organ.

Stomata, present in the epidermis of leaves, are each composed of two bean-shaped guard cells, dumb-bell shaped instead in grasses, enclosing a stomatal pore between them. The guard cells' outer walls, away from the pore, stay thin, while their inner walls, towards the pore, are heavily thickened, an asymmetry that lets the cells bow outward and open the pore when they take up water. Guard cells uniquely possess chloroplasts among epidermal cells and actively regulate the opening and closing of the stomatal pore, which controls both transpiration and gas exchange for the whole leaf; a few surrounding epidermal cells often become specialised in shape and size as subsidiary cells, and the pore, guard cells and subsidiary cells together are called the stomatal apparatus. The epidermis also bears hairs, unicellular elongations called root hairs on roots that absorb water and minerals, and usually multicellular, sometimes branched, sometimes secretory structures called trichomes on the shoot system, which help reduce water loss through transpiration.

All tissue except the epidermis and the vascular bundles makes up the ground tissue, consisting of the three simple tissue types: parenchyma, present mainly in cortex, pericycle, pith and medullary rays in primary stems and roots, collenchyma and sclerenchyma. In leaves specifically, the ground tissue instead consists of thin-walled, chloroplast-containing cells and is given its own name, mesophyll. The vascular tissue system consists of two complex tissues, xylem and phloem, which together constitute a vascular bundle. In dicotyledonous stems, a cambium sits between the xylem and phloem, giving these vascular bundles the ability to later form secondary xylem and secondary phloem, and for that reason they are called open vascular bundles; in monocotyledons, no cambium is present at all, so no secondary tissue ever forms, and these bundles are called closed. When xylem and phloem sit on alternating radii within an organ, the arrangement is called radial, typical of roots; when they instead sit together along the same radius, usually with phloem positioned only on the outer side of the xylem, the arrangement is called conjoint, typical of stems and leaves.

A transverse section of a dicot root, sunflower for instance, shows a clear sequence from outside to inside. The outermost layer, the epiblema, has cells that protrude as unicellular root hairs; beneath it, the cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces, and the innermost cortical layer, the endodermis, comprises a single layer of barrel-shaped cells packed with no intercellular spaces at all, its tangential and radial walls deposited with a waterproof, waxy material called suberin in the form of casparian strips. Just inside the endodermis lies the pericycle, a few layers of thick-walled parenchymatous cells where lateral roots and, later, the vascular cambium of secondary growth both originate. The pith is small or inconspicuous, and there are usually only two to four xylem and phloem patches, with a cambium ring developing between them later; everything inside the endodermis, pericycle, vascular bundles and pith together, is called the stele. The monocot root is broadly similar in having epidermis, cortex, endodermis, pericycle, vascular bundles and pith, but differs in two clear ways: it usually has more than six xylem bundles, called polyarch, compared to the dicot's few, and its pith is large and well developed rather than small; monocot roots, crucially, never undergo any secondary growth at all.

A young dicot stem's transverse section shows the epidermis as the outermost protective layer, covered by a thin cuticle and sometimes bearing trichomes and a few stomata. Between epidermis and pericycle lies the cortex, itself divided into three sub-zones: an outer hypodermis of collenchymatous cells just below the epidermis, providing mechanical strength to the young stem; parenchymatous cortical layers below that, with conspicuous intercellular spaces; and an innermost endodermis rich in starch grains, also called the starch sheath for that reason. The pericycle sits just inside the endodermis and above the phloem as semi-lunar patches of sclerenchyma, with radially placed parenchymatous medullary rays lying between the vascular bundles. Crucially, a dicot stem's vascular bundles are arranged in a ring, each individually conjoint, open and with endarch protoxylem, and this ring arrangement is itself a defining characteristic of dicot stems; a large parenchymatous pith with generous intercellular spaces fills the central portion. The monocot stem differs sharply: its hypodermis is sclerenchymatous rather than collenchymatous, its vascular bundles are scattered throughout a large, conspicuous parenchymatous ground tissue rather than arranged in a ring, and each bundle is conjoint and closed, surrounded by its own sclerenchymatous bundle sheath, with peripheral bundles generally smaller than centrally located ones; monocot stems also lack phloem parenchyma entirely and instead have water-containing cavities within their vascular bundles.

A vertical section of a typical dicot leaf through the lamina, called dorsiventral because its two surfaces differ, shows three main parts: epidermis, mesophyll and vascular system. The epidermis covers both the adaxial, upper, and abaxial, lower, surfaces, with a conspicuous cuticle; the abaxial epidermis generally carries more stomata than the adaxial, which may even lack them entirely. Between the two epidermal layers lies the mesophyll, made of chloroplast-containing parenchyma differentiated into two distinct layers: an adaxially placed palisade parenchyma of elongated cells arranged vertically and parallel to each other, and an oval or round, loosely arranged spongy parenchyma below it, extending to the lower epidermis with numerous large air spaces and cavities between its cells. Vascular bundles run through the veins and midrib, their size scaling with vein size, and are surrounded by a protective layer of thick-walled bundle sheath cells. A monocot leaf's anatomy, called isobilateral because both surfaces resemble each other, is similar in many ways but differs in two clear respects: stomata are present on both surfaces of the epidermis rather than mostly one, and the mesophyll is not differentiated into palisade and spongy layers at all. In grasses specifically, certain adaxial epidermal cells along the veins modify into large, empty, colourless bulliform cells: when turgid with absorbed water they expose the leaf surface normally, but when flaccid under water stress they cause the leaf to curl inward, minimising further water loss. The parallel venation of monocot leaves is reflected internally too, in the near-uniform size of vascular bundles across the leaf, except at the main veins.