Stem Anatomy

Stem Anatomy

The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma cells are the most common plant cells (see the figure below). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.

Micrograph shows a stem about 1.2 millimeters across. The central pith layer is about 800 microns across. Pith cells stain greenish-blue and are about 50 to 100 microns in diameter in the middle, and smaller toward the outside. Surrounding the pith is a ring of xylem cells about 75 microns across and four cells deep. Xylem cells, which are about 15 microns across, radiate out from the center in rows. Rows of green-staining phloem cells radiate out from the xylem cells.  Phloem cells are about half the size of xylem cells. Outside the phloem is a ring of cells that make up the peripheral cortex. Cells in the peripheral cortex are rounded rectangles that lie perpendicular to the phloem. The outermost epidermis is made up of cells similar in shape to the peripheral cortex cells but a bit larger. On opposite faces of the stem the peripheral cortex bulges outward, forming buds about 150 microns across.

The stem of common St John’s Wort (Hypericum perforatum) is shown in cross section in this light micrograph. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. (credit: Rolf-Dieter Mueller)

Collenchyma cells are elongated cells with unevenly thickened walls (see the figure below). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.

 Micrograph shows collenchyma cells, which are irregularly shaped and 25 to 50 microns across. The collenchyma cells are adjacent to a layer of rectangular cells that form the epidermis.

Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures. (credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell)

Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (see the figure below).

Art Connection

 Part A shows a cross section of a flax stem. The pith is white tissue in the center of the stem. Outside the pith is a layer of xylem. The inner xylem cells are large, while ones further out are smaller. The smaller xylem cells radiate out from the center, like spokes on a wheel. Outside the xylem is a ring of phloem cells. The phloem is surrounded by a layer of sclerenchyma cells, then a layer of cortex cells. Outside the cortex is the epidermis. Part B is a painting of women working with linen cloth. One is smoothing the cloth on a table, and the other women are sitting with linen on their laps. Part C is a photo of flax plants, which have long, wide leaves that taper toward narrow tips.

The central pith and outer cortex of the (a) flax stem are made up of parenchyma cells. Inside the cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. The (c) flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the source of linseed oil. (credit a: modification of work by Emmanuel Boutet based on original work by Ryan R. MacKenzie; credit c: modification of work by Brian Dearth; scale-bar data from Matt Russell)

Which layers of the stem are made of parenchyma cells?

  1. cortex and pith
  2. phloem
  3. sclerenchyma
  4. xylem

Answer

A and B. The cortex, pith, and epidermis are made of parenchyma cells.

Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.

Dermal Tissue

The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (see the figure below). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.

 The electron micrograph in part A shows the lumpy, textured of a leaf epidermis. Individual cells look like pillows arranged side by side and fused together. In the center of the image is an oval pore about 10 microns across. Inside the pore, closed guard cells have the appearance of sealed lips. The two light micrographs in part B shows two kidney-shaped guard cells. In the left image, the stoma is open and round. In the right image, the stoma is closed and oval shaped. Part C is an illustration of the leaf epidermis with a oval stomatal pore in the center. Surrounding this pore are two kidney-shaped guard cells. Rectangular epidermal cells surround the guard cells.

Openings called stomata (singular: stoma) allow a plant to take up carbon dioxide and release oxygen and water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked by two guard cells that regulate its (b) opening and closing. The (c) guard cells sit within the layer of epidermal cells (credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth University; credit b: modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell)

Vascular Tissue

The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue (see the figure below).

 Part A is cross section of a dicot stem. In the center of the stem is ground tissue. Symmetrically arranged near the outside of the stem are egg-shaped vascular bundles; the narrow end of the egg points inward. The inner part of the vascular bundle is xylem tissue, and the outer part is sclerenchyma tissue. Sandwiched between the xylem and sclerenchyma is the phloem. Part B is a cross section of a monocot stem. In the monocot stem, the vascular bundles are scattered throughout the ground tissue. The bundles are smaller than in the dicot stem, and distinct layers of xylem, phloem and sclerenchyma cannot be discerned.

In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Sclerenchyma fibers cap the vascular bundles. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue.

Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Ground Tissue

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.

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