October 2017

Morphology includes the study of the general structure of plants. Morphologists study the parts of a plant and how they are arranged and function. For example, when a seed of an angiosperm (flowering plant) germinates, the radicle of the seed embryo develops downward to form a root system. Growth in the length of the root occurs within the meristem (region of cell division). Branch roots form due to the activity of pericycle cells within the root. Some epidermal cells develop root hairs as extensions of the cells. The shoot system, which includes the stem and leaves, develops from the epicotyl of the seed embryo. Stems are often branched, allowing for the attachment of leaves in such a manner as to permit their maximum exposure to sunlight.

Also included in the study of morphology are the reproductive parts of plants. The pollination of flowers causes the ovary of the flower to mature into a fruit. At the same time, the one or more ovules inside the ovary become seeds.

See also: Anatomy, Cytology, Physiology

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The ground tissues, the second of the primary developmental tissues, differentiate in the zone of maturation to form tissues called parenchyma, collenchyma, orsclerenchyma. The parenchymous tissues are the primary site of cellular metabolism. The organelles of parenchyma cells in different parts of the plant vary so that they can accommodate differences in metabolic functions. Cells of leaf parenchyma and some stem parenchyma have large numbers of chloroplasts to carry out photosynthesis. Stem and root parenchyma cells have amyloplasts, organelles that store starch. Chromoplasts in the parenchyma of flower petals contribute to the color of the flower petals. Parenchyma cells producing large quantities of protein have more ribosome’s than those specialized for starch storage. Reproductive parenchyma cells may have unusual nuclear characteristics that prevent these tissues from competing with the developing embryos for nutrients or space.

Parenchyma fills the inner parts of leaves, stems, and roots. These cells have large, water-filled vacuoles. The water pressure from these vacuoles provides much of the rigidity of the body of nonwoody plants. When a leaf is limp, its parenchyma cells are usually depleted of water. Many of the chemicals that give plants their unique tastes or pharmaceutical characteristics are produced and stored in parenchyma. For example, the bulk of a carrot root (especially outside the central core), the mass of a potato tuber (which is actually a unique form of stem), and much of a lettuce leaf are all made of parenchyma.

Collenchymas cells are similar to parenchyma cells in many ways. They use water pressure to provide support. However, they are normally found near the surface of stems and leaves. Collenchymas cells have a unique pattern of cell-wall thickening that allows expansion in diameter but not in length. This makes collenchymas especially suited to providing support for softbodied plant parts that have completed much of their longitudinal growth. Collenchyma cells rarely provide bulk to plant structures. Instead, they form thin sheets just below the epidermis and outside much of the parenchyma. Because collenchyma is thin, it has a smaller volume than parenchyma and contributes less to the metabolism of the plant organs. It may nevertheless support some of the photosynthesis of the plant, and it provides textures to the organs as well.

Sclerenchyma cells occur throughout the body of the plant and include three types of cells: elongated fibers; branched sclereids, resembling a three-dimensional jigsaw puzzle piece; and globular stone cells. All three cell types have heavy, secondary cell walls and have lost many organelles. Sclerenchyma is a type of differentiated tissue that functions when its cells are dead.

Fibers support plant organs in the same way as does collenchyma, but because the secondary cell walls of sclerenchyma cells resist longitudinal and latitudinal expansion, they are not common in growing tissues. Their rigidity helps to supply support even when tissues are water-stressed, but it also limits the potential for the organs to expand in girth or length. Fibers, sclereids, and stone cells all provide protection against predation. The gritty texture of a ripe pear, the shell of a nut, and the strings of a coconut husk are all composed of sclerenchyma cells and promote the wear and breakage of predators’ teeth and other chewing structures.

See also: Procambium

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The procambium, the third of the primary developmental tissues, differentiates to form primary xylem and primary phloem as well as the vascular cambium. The vascular cambium produces cells that differentiate into secondary xylem and secondary phloem. It also regenerates the supply of cells in the vascular cambium.

An example of xylem is the woody tissue at the center of most trees. (Palm trees are a notable exception.) Smaller bundles of xylem are found in the roots, stems, and leaves of most plants, even when they are not woody. Xylem tissues are made of four cell types: fibers and parenchyma cells (which also occur in sclerenchyma and parenchyma) and xylem vessel elements and tracheids (which are found only in the xylem). These cells work in concert to move water upward through the plant. The xylem vessel elements and tracheids provide the actual channels for the movement, and the fibers serve largely as physical supporting structures.

The parenchyma cells are responsible for some lateral movement in the xylem tissues. These parenchyma cells also have the ability to revert to a meristematic condition, providing a mechanism for the xylem to replace damaged cells. Tracheids and vessels have unusual, patterned secondary cell walls that resist the physical stresses involved in moving xylem sap. The cell organelles are lost before the vessels and tracheids are functional. The sap moves through a channel where the body of the cell had been before it was lost. Xylem is another tissue that contains cell types that function when they are dead.

An example of phloem is the tissue on the inside of the bark of most trees (again, palm trees are an exception). Smaller bundles of phloem are found in the roots, stems, and leaves of most plants, even when they are not woody. Phloem tissues are made of fibers, parenchyma cells, sieve tube elements, and companion cells. These four cell types work in concert to move sugars, other organic molecules, and some ions throughout the body of the plant.

The sieve rube elements (or sieve cells, in some plants) provide the actual channels for the movement. The fibers serve largely as physical supporting structures. The parenchyma cells are responsible for some lateral movement and also provide a mechanism to replace damaged cells. Sieve tube elements and sieve cells have unusual, perforated cell walls whose appearance indeed resembles a sieve. Many of the cell organelles are lost before the sieve rube elements and sieve cells are functional. The phloem sap moves through living cells, but they resemble no other cells in the plant.

The companion cells (which in some plants are called albuminous cells, to indicate a different developmental origin) are similar to parenchyma cells, but they provide substantial metabolic support to the sieve tube elements and sieve cells. These cells function together. The companion cells could live independently, while the sieve tube element could not, but the important function is carried out by the sieve rube element.

Most species that grow in girth are woody, and the wood of woody plants is composed almost entirely of secondary xylem. The bark of woody plants is made of phloem and corky layers. There are two principal cambia, the vascular cambium and the cork cambium. Both contribute to the increase in girth. The vascular cambial tissues produce the cells that will differentiate to form the sec ondary xylem and phloem of woody species. The cork cambium produces the corky cells on the outside of the bark.

See also: Ground Tissues

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