The growing tips of stems are complex structures responsible for the production of new stem tissue, as well as for the formation of leaves and of stem branches. Each stem tip has a complex mound of apical meristem tissue present. Divisions of cells within the apical meristem produce the protoderm, ground meristem, and procambium which form the region just beneath the stem tip. These primary meristems, in turn, will produce the dermal, ground, and vascular tissues of the mature stem. Also visible at the stem tip are the leaf primordia, and, in the axils of the leaf primordia, the bud primordia. The leaf primordia will expand to produce the mature leaves; the bud primordia will form axillary buds which, under some conditions, may develop into branches.
The stems of mature angiosperms vary with respect to the arrangement of the vascular tissue, the presence or absence of secondary growth, and the amount of secondary growth when present. In all species, the primary tissues include a layer of epidermis, and vascular tissue surrounded by ground tissue. In a typical dicot, the vascular tissue occurs in vascular bundles arranged in a ring passing through the ground tissue. Each vascular bundle contains phloem and xylem, with the phloem located nearest the stem surface and the xylem located nearest the center of the stem. In plants which do not undergo secondary growth, each vascular bundle is likely to be surrounded by a ring of fibers forming a supportive bundle sheath. The region of ground tissue between the epidermis and the vascular bundles is the stem cortex; the region of ground tissue in the center of the stem is the pith; and the ground tissue between adjacent vascular bundles is the interfascicular parenchyma, or pith rays.
In a typical monocot, the vascular bundles are arranged in two or more concentric rings within the ground tissue, or are scattered randomly through the ground tissue. In these stems there is no distinct cortex or pith regions and the ground tissue is collectively called the ground parenchyma. Monocots do not carry out any secondary growth, thus there is no vascular cambium present. In the majority of these plants, the vascular bundles do have bundle sheaths of sclerenchyma cells enclosing them.
In those dicots which do undergo secondary growth, older stems differ significantly from younger ones. As the stem matures the vascular cambium forms and its cells begin to divide. New cells formed to the outside of the ring of vascular cambium become a part of the secondary phloem. New cells formed to the inside of the vascular cambium become a part of the secondary xylem. In most species, the amount of secondary xylem produced greatly exceeds the amount of secondary phloem produced. The secondary xylem consists primarily of tracheids and vessel members, with a small amount of parenchyma included. It is secondary xylem which is commonly known of as wood, and it is the strength and rigidity of the cell walls of tracheids and vessel members that gives wood its strength.
At the same time that the vascular cambium begins to produce secondary vascular tissues, another lateral meristem, the cork cambium, also develops. The original formation of cork cambium occurs within the cortex, although additional layers of cork cambium may develop later from cells within the secondary phloem. The cells of the cork cambium divide, producing several layers of cork cells to its exterior. The cork cells, as they mature, have suberin and lignin deposited in their cell walls. The presence of these substances enables the cork to serve as an effective barrier to water loss from the stem. The cork cambium may also produce a few layers of phelloderm to the inside.' The cork, cork cambium, and phelloderm are known collectively as the periderm, which is a secondary dermal tissue, replacing the epidermis in these stems. The substance commonly called bark is all of the tissues outside of the vascular cambium, including the periderm, the cortex (if any of it remains), and the secondary phloem.
1. Obtain a slide of a l.s. of a Coleus stem tip, 2. Draw the stem tip and label the apical meristem, protoderm, ground meristem, procambium, leaf primordia, and bud primordia.
2. Examine a slide of a c.s. of a Ranunculus stem, 2. Ranunculus is a dicot with a typical arrangement of mature tissues. It is somewhat unusual however, in that, at maturity, the pith partially disintegrates, leaving a lysigenous space in the center of the stem. The cortex of this stem is composed of photosynthetic parenchyma, which is more loosely arranged than the cortex of some other stems. Draw a wedge-shaped portion of this stem and label the epidermis; the cortex; a vascular bundle including xylem, phloem, and a bundle sheath; and the pith.
3. A slide of a Zea stem in c.s, 2.will provide a good example of the arrangement of tissues in a monocot. Draw a wedge-shaped portion of this stem, labeling the epidermis; the ground parenchyma; and a vascular bundle including xylem, phloem, and a bundle sheath, which includes a number of fibers located next to the phloem.
4. Tilia is a dicot which undergoes a considerable amount of secondary growth. A slide of an older Tilia stem, 2 should exhibit some of the secondary tissues. Draw a wedge-shaped portion of this stem, labeling the periderm, cortex, secondary phloem, vascular cambium, secondary xylem, and pith.
5. Examine a slice taken from a woody dicot stem. Sketch this, labeling the bark and the wood. Note the annual rings present within the wood. These result from differences in the size of xylem cells produced at different times in the growing season.
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