96 Margins may be toothed, lobed or entire. The leaf tip is called apex while the part near the petiole is called the leaf base. There are various types of margins, leaf apices and leaf bases. There are two types of leaves: simple and compound. A simple leaf has a single blade or lamina and may have any of the above characteristics. They may be deeply lobed. A compound leaf is composed of two or more separate leaflets. There are two basic kinds of compound leaves: pinnately compound and palmately compound. In a pinnately compound leaf, the leaflets occur in a linear sequence lined up along both sides of a central axis, called the rachis. A compound leaf that is palmate is one in which three, five, seven or more leaflets are all attached at one point near the tip of the petiole and they radiate out from this tip. How do you tell if a leaf is compound? Look at the base of the leaf or petiole where it is attached to the stem. If there is a bud in the angle formed by the petiole and stem, like an axillary bud, then the entire structure supported by the petiole is a leaf, regardless of the number of leaflets present. All the leaflets of a compound leaf occur in the same plane. Also, some very large pinnately compound leaves sometimes resemble an entire branch. The branch will have a terminal bud, but a compound leaf won't have a terminal branch. Leaves are attached to the stem in different ways. Leaf arrangement is referred to as phyllotaxy. In the alternate or spiral arrangement, one leaf occurs at each node. A variation of alternate is distichous in which the leaves occur only on two rows. The opposite arrangement is with two leaves at a node facing each other. A variation of opposite is decussate in which two opposite leaves are perpendicular to the two opposite leaves below or above them. The whorled arragement has three or more leaves at one node. Record the characteristics of leaves provided in the laboratory and fill up the table on your worksheet. II. Internal structure of the leaf The outermost layer of cells is called the epidermis. These epidermal cells secrete a waxy substance which forms a covering over the outer dermal cells rendering these cells impermeable to water. The waxy covering is called cuticle. There is an upper epidermis and a lower epidermis in a leaf with dorsiventral orientation. The openings in the epidermis are called stomates which are formed between specialized epidermal cells called guard cells. In many plants, trichomes (hairs) are found on the undersurface of the leaves and herbaceous stems. These help reduce evaporation of water from plants or protect young shoots from herbivores. The mesophyll tissue, with the exception of the vascular bundles, comprises all of the cells between the upper and lower epidermis and is usually divided into two parts. The cells toward the upper epidermis, which are elongated, make up the palisade layer. The zone below the palisade layer, composed of irregularly shaped cells is called the spongy layer. Inter-cellular air spaces are found between mesophyll cells. In some monocots, there is no distinct palisade and spongy layers because the leaves are isobilateral. Thus all the parenchyma cells with chloroplast between the epidermis comprise the mesophyll layer. The vascular bundles function both as supporting and conducting tissue. They are composed of two fundamental tissues: xylem and phloem. These conducting tissues are surrounded by vascular bundle 97 sheath cells and are collectively called veins. B.1. The leaf epidermis Make a fresh mount of Rhoeo discolor leaf epidermis. Using a scalpel or blade, strip off pieces of the lower epidermis of Rhoeo discolor. Examine the epidermis and note the compact arrangement of the cells. Focus on the stomates, slit like openings surrounded by guard cells. Draw and label the leaf epidermis including the stomates on your worksheet. B. 2. The dicot leaf Study the cross section of Ixora leaf. Identify the parts listed above. Look at the drawing on your worksheet and label parts. B.3. The monocot leaf Study the cross section of a Zea mays (corn) leaf. Identify the parts listed above. Look at the drawing on your worksheet and label parts. III. Modified leaves Just like in root and stems, some leaves have modification. These changes in their shapes, structures and functions are usually adaptations to the plants' habitats. Fill up the table on the worksheet. 98 References 1. Balbach, Margaret and Lawrence Bliss. 1991. A Laboratory Manual for Botany. Saunders College Publishing. Orlando, Florida. pp. 109-115; pp 121- 127. 2. Botany 11 Laboratory Manual for Biology. 1989. University of North Carolina. pp. v1-v6. 99 Student?s Name ___________________________Date Performed __________ Course/Year/Section _______________________Date Submitted __________ Professor?s Name___________________ WORKSHEET # 11 THE LEAF I. Leaf diversity Specimen Type Phyllotaxy Margin Apex Venation Narra Adelfa Mayana Gumamela San Francisco Bamboo Kamias Petchay Sibuyas (Leaf) Kangkong Gabi (leaf) Ampalaya Malunggay Repolyo Santol 100 1. From the leaf samples above, give the name of a plant with a. a leaf with palmate venation b. a leaf with pinnate venation c. a leaf with stipules d. a pinnately compound leaf e. a palmately compound leaf f. a sessile leaf g. a section of a stem with opposite phyllotaxy h. a section of a stem with distichous phyllotaxy i. a section of a stem with whorled phyllotaxy j. a section of a stem with alternate phyllotaxy 2. Why are the blades of leaves flat? 3. What is the counterpart of the petiole of the dicot leaf in monocots? II. Internal structures of the leaves A. The Leaf epidermis B. Ixora cross section Biology LBYBIOA MANUAL Part 2B1.pdf
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