Which vascular tissue is outermost in a stem

This is a preview of subscription content, log in to check access. Brebner G On the anatomy of Danaea and other Marattiaceae. Blackwell Publishing, Malden Google Scholar. DeMason DA The primary thickening meristem: definition and function in monocotyledons. Observations from histochemistry and autoradiography. Dickinson WC Integrative plant anatomy.

Hardcourt, Inc. Esau K Plant Anatomy. Wiley, New York Google Scholar. Esau K Anatomy of seed plants. Fahn A Plant Anatomy, 4th edn. Geldner N The endodermis. 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 do the sieve-tube cells, which lack some cellular organelles.

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.

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem.

Herbaceous plants mostly undergo primary growth, with little secondary growth or increase in thickness. Primary and secondary growth : In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer.

The bark of a tree extends from the vascular cambium to the epidermis. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size. Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly-dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth.

The growth of shoots and roots during primary growth enables plants to continuously seek water roots or sunlight shoots. The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems.

Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape. The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants.

Lateral meristems include the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem tracheids and vessel elements to the inside and secondary phloem sieve elements and companion cells to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem.

The cells of the secondary xylem contain lignin, which provides hardiness and strength. In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells bark containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss.

The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere.

This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem. Example of lenticels : Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened.

This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids.

It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem. An examination of the number of annual rings and their nature such as their size and cell wall thickness can reveal the age of the tree and the prevailing climatic conditions during each season. Annual growth rings : The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth.

Seasonal changes in weather patterns can also affect the growth rate. Note how the rings vary in thickness. Stem modifications, either aboveground, underground, or aerial, enable plants to survive in particular habitats and environments. Some plant species have modified stems that are especially suited to a particular habitat and environment. A rhizome is a modified stem that grows horizontally underground; it has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns.

Corms are similar to rhizomes, except they are more rounded and fleshy such as in gladiolus. Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example.

Tubers are modified stems that may store starch, as seen in the potato. A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Each cell in this layer is suberized in a Casparian strip around the radial walls initially and later over its whole surface.

Inside the endodermis are a few layers of cells that remain meristematic, then the phloem and finally the xylem which is often arranged in a band or cross. The root apical meristem does not branch. Roots continue their exploration of soil space by developing new apical meristems from the pericycle. The new roots break through the cortex as they grow. Ranunculus root, transverse section. Close up of stele with endodermis, etc. Although they are not as diverse as shoots, roots are sometimes modified for special purposes, particularly food storage.

Some of our "root crops" are really stems but carrots, parsnips and sweet potatoes are roots. Aerial roots of an epiphytic orchid can absorb moisture from the air. Storage roots Hibiscus moscheutos used to be the source of marshmallow. Stem anatomy Shoots are more complex than roots in external structure so it is no surprise that their internal anatomy is more complex. Usually roots only branch to produce more roots, whereas various structures may arise from a shoot leaves, flowers, shoots and roots so that the apical meristem gives rise to various kinds of meristem or primordia there is no equivalent of these structures in roots which rarely give rise to other kinds of structure - it is usually difficult to propagate plants from true roots.

Leaf primordia arise from the stem apical meristem in a pattern and sequence that determines the arrangement of leaves singly or in pairs and in one or more planes. In the axil of each leaf primordium is another meristematic area, the bud primordium The node remains a site of meristematic potential, and not just for shoots; roots are also more readily induced to form at nodes than elsewhere.