Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section One:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Biology: Plant Organs, Tissues, and Cells: Part Seven Monocots and Dicots Angiosperms, the flowering plants, are often divided into two groups: the monocotyledons and the dicotyledons. The group names are usually shortened to “monocots” and “dicots.” Monocot seeds have one cotyledon or “seed leaf.” Dicots have two cotyledons. Both groups, however, have the same basic structure of nodes, internodes, and other features. Comparison of monocot (left, oat) and dicot (right, bean) gross anatomy. Vascular bundles are groupings of transport tissues within a plant. Monocot stems have scattered vascular bundles, most of which are near the outside edge of the stem. Large parenchyma cells in the cortex region surround the bundles. There is no pith region in monocots, as there is in dicots. Dicot stems have their vascular bundles in a ring arrangement. The ring of vascular bundles surrounds very large parenchyma cells in a pith region. Between the bundles and the epidermis are smaller (as compared to the pith) parenchyma cells, which make up the cortex region. a b Arrangement of vascular bundles in the stems of a monocot (Zea mays, corn, figure a) and a dicot (Medicago, b). Interestingly, monocot roots have their vascular bundles arranged in a ring. Dicot roots have their xylem confined to the center of the root, with the phloem to the outside of the xylem area. A carrot is a root vegetable that is a dicot. If you cut across the short dimension of a carrot near the greens, you will produce a cross section that should show the carrot’s pith and cortex areas under a small power magnifying glass. In the slide images below, corn serves as our example of a monocot root and a buttercup is our example of a dicot root. Shown here is a scanning electron microscope image of a Nicotiana alata stem cross section. The image shows an outer epidermal layer, followed by the cortex and then by large vascular bundles. The vascular bundles contain the phloem and xylem. The phloem is nearest the cortex. Diagram illustrating the tissue layers and their organization within monocot and dicot roots. Cross sectional views of a corn root (left) and buttercup root (center), with an enlargement (right). Note the ringed array of vascular bundles in the corn root cross-section and the X-shaped xylem in the center of the buttercup root cross-section. The purple stained structures inside the parenchyma cells of the cortex are starch grains. This illustrates the storage role of roots. In monocot leaves, leaf veins are arranged parallel to each other and to the long axis of the leaf. Botanists call this arrangement, “parallel venation.” A common example of this is a blade of grass or the husk of corn (corn is a member of the grass family, which is a group of monocots). Dicot leaves have an interconnecting network of veins arising from a mid-vein. Botanists call this pattern, “net venation.” Examples of dicot leaves include those of maples, oaks, geraniums, and dandelions. Monocot (corn) and dicot (bean) leaves. Monocots have their flower parts (such as the showy, colored petals) in threes or multiples of three. Tulips, irises, and lilies all show this petal arrangement. Dicots have their flower parts in multiples of four or five. Examples of some common dicot flowers include those of the garden pea (remember Gregor Mendel?), geraniums, snapdragons, and citrus plants.   Left: flowers of the monocot, Lilium, the lily. Right: flowers of the dicot Pyrus malus, the apple tree. Note the typical monocot arrangement of flower parts in multiples of three and the dicot array of flower parts in multiples of four. There are other differences between monocots and dicots. Generally, monocots have only a single germination aperture (or opening) on their pollen. In contrast, dicots may have many of such openings. Some dicots, such as oak, maple, and walnut, can produce large amounts of wood by the process of secondary growth. Only a few monocots (palms and bamboo) carry out secondary growth, and these use a vastly different process to make wood. Using what you've learned so far about monocots and dicots, see if you can remember the characteristics of each before clicking to see the answer.   Monocots Dicots Veins in Leaves Usually parallel Usually netlike Number of Cotyledons One Two Flower Parts Usually in multiples of three Usually in fours or fives Arrangement of primary vascular bundles in stem Scattered In a ring Pith region None Present Present Number of Germination openings One From one to many Lab: Plant Structure 100 points The goal of this lab is to learn about and be able to recognize the various types of cells that occur in a flowering plant. Submit your completed lab to the Lab: Plant Structure assignment link for grading. For information on how this assignment will be graded, please visit the Course Information section. Now go on to the next part.

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