Summaries
TISSUES:
- a group of cells performing a common function is called a tissue.
- Apical meristems are found n the vicinity of the tips of roots and stems; the vascular cambium and the cork cambium occur as lengthwise cylinders within roots and stems; intercalary meristems occur in the vicinity of nodes of grasses and related plants.
- Nonmeristematic tissues are produced by meristems, and each consists of one to several kinds of cells. They include parenchyma, collenchyma, sclerenchyma, secretory tissues, epidermis, xylem, phloem, and periderm.
- Parenchyma cells are thing-walled, while collenchyma cells have unevenly thickened walls that provide flexible support for various plant organs.
- Two types of sclerenchyma--fibers (which are longer and tapering) and sclereids (which are short in length)--occur; both types have thick walls and are usually dead at maturity.
- Secretory tissues occur in various places in plants; they secrete substances, such as nectar, oils, mucilage, latex, and resins.
- The epidermis is usually one cell thick, with fatty cutin (forming the cuticle) within and on the surface of the outer walls. The epidermis may include guard cells that border pores called stomata; roots hairs, which are tubular extensions of single cells; other hairs that consist of one to several cells; and glands that secrete protective substances.
- Complex tissues have more than one kind of cell. The principal types are xylem, phloem, and periderm.
- Xylem conducts water and dissolved substances throughout the plant. It consists of a combination of parenchyma, fibers, vessels (tubular channels), tracheids (cells with tapering end walls that overlap) , and ray cells (involved in lateral conduction).
- Phloem conducts dissolved food materials throughout the plant. It is composed of sieve tubes (made up of cells called sieve-tube elements), companion cells (which apparently regulate adjacent sieve-tube elements), parenchyma, ray cells, and fibers. Sieve cells, which have overlapping end walls, and adjacent albuminous cells take the place of sieve-tube elements and companion cells in ferns and cone-bearing trees.
- Periderm, which consists of cork cells and loosely arranged groups of cells comprising lenticels involved in gas exchange, constitutes the outer bark of woody plants.
ROOTS:
- Roots function primarily in anchorage and in the absorption of water and minerals in solution. The radicle of a germinating seed becomes the first root, from which develops either a taproot with branch roots, or adventitious roots that give rise to a fibrous root system in which the roots are of similar diameter. Many plants have combinations of both systems, with some also developing adventitious roots as they mature.
- In developing roots, four zones or regions are traditionally recognized:(1) a thimble-shaped root cap at the tip, which protects the root as it pushes thru the soil and also aids in the perception of gravity. (2) A region of cell division, which is surrounded by the root cap. It is composed of an apical meristem that subdivides into a protoderm, which produces the epidermis; a ground meristem, which produces the cortex; and a procambium in the center, which produces the primary xylem and primary phloem. (3) A region of elongation in which the cells produced by the apical meristem become considerably longer and slightly wider (4) A region of maturation in which the cells mature into the distinctive cells types of primary tissues.
- Some of the epidermal cells in the region of maturation develop protuberances called root hairs; the root hairs greatly increase the absorptive surface of the root. The tissues that mature in the region of maturation are similar to those of stem tips, except that pith is absent in most dicot roots, and of a different origin (the procambium) in monocot roots.
- The cortex has an endodermis at its inner boundary. This consists of a cylinder, one cell wide, of cells that have suberin bands called Casparian strips around the radial and transverse walls.
- Immediately adjacent to the endodermis toward the center of the root are parenchyma cells constituting the pericycle. Branch roots and the vascular cambium arise in the pericycle.
- The primary xylem initially frequently forms a solid core in the center of the root; this core usually develops with two to several (usually four) arms, at least in dicot roots. A pith may be present in monocot roots.
- At first, primary phloem is produced in discrete patches between the arms of the primary xylem, but as secondary tissues are added, the tissues eventually appear as concentric cylinders. In woody plants, a cork cambium usually arises in the pericycle and produces cork tissues similar to those of stems. Roots may graft together naturally. There are no nodes or internodes in roots.
STEMS:
- (completely unrelated to your course)
- The shoot system of plants, with branches and leaves, is usually erect but some stems may be horizontal or have modifications that permit climbing or storage of food or water.
- Woody twigs have leaves arranged alternately, oppositely, or in a whorl. Nodes are stem regions where leaves are attached; internodes occur between nodes. Most leave have petioles and blades. Axillary buds occur in leaf axils. Most buds are protected by bud scales. Terminal buds occur at twig tips. When terminal bud scales fall off, they leave groups of bud scale scars that can be used in determining the age of a twig.
- Stipules are paired appendages present at the base of some leaves; when they fall off, they leave small scars on the twig. When whole leaves fall, they cause leaf scars on the twig, with tiny bundle scars within the leaf-scar surfaces.
- Each stem has an apical meristem at its tip that produces tissues resulting in increase in length. Leaf primordia develop into mature leaves when growth begins. Three primary meristems develop from an apical meristem: the protoderm gives rise to the epidermis, which becomes coated with a cuticle; the procambium produces primary xylem and primary phloem; and the ground meristem produces pith and cork.
- As each leaf and each bud develops from a primordium, a trace of xylem and phloem branches off from the main cylinder, leaving a leaf gap or a bud gap.
- A vascular cambium, producing secondary tissues, may arise between primary xylem and phloem. Secondary xylem cells include tracheids, vessel elements, and fibers. Secondary phloem cells include sieve-tube elements and companion cells.
- In many plants, a cork cambium producing cork and phelloderm cells develops near the surface of the stem. Cork cells, which are part of the outer bark (periderm), have suberin in their walls. Suberin is impervious to moisture and the outer bark therefore aids in protection. Lenticels in the bark permit gas exchange.
- Primary vascular tissues and the pith, if present, constitute the stele. Protosteles have a solid core of xylem, usually surrounded by phloem; siphonsteles are tubular, with pith in the center; eusteles have the vascular tissues in discrete bundles.