Monocot Leaf Anatomy & Morphology

Refer to No. 15 of the study guide. The next section is that of monocot leaf anatomy and morphology.

Figure 16

This figure represents a cross section of a typical monocot leaf. Proceeding from the top to the lower portion of the leaf we see that the upper portion of the leaf is covered by a cuticle. The cuticle is nothing more than a waxy type material that covers the entire leaf surface. The primary purpose of the cuticle is in moisture conservation in that it acts as a barrier against the diffusion of water through the epidermis and into the atmosphere. The upper layer of cells is referred to as the upper epidermis and is one layer of cells thick and functions primarily as an outer covering for protection of the enclosed tissue. The bulk of the cells between the upper and lower epidermis is referred to as the mesophyll cells. These cells are relatively irregular in shape and is the site of most of the photosynthetic activity of the leaf. Also in the central portion of the leaf cross section is what is referred to as the vascular bundles. The vascular bundles are surrounded by a layer of cells called the sheath cells. The sheath cells primarily function to give some rigidity and protection to the enclosed vascular tissue. The very large elements located in the vascular bundle are referred to as the xylem vessels. The xylem vessels function in the translocation up the plant of both water and mineral nutrients taken up from the soil. The smaller cells located in the vascular bundles are the phloem cells. The phloem cells function largely to translocate photosynthates from their point of origin to a point of utilization or storage within the plant. Located to the exterior of the vascular bundle at the top and the bottom are several layers of cells called the sclerenchyma fibers. The sclerenchyma fibers function primarily in providing support to the vascular bundle and the leaf in that these cells are very rigid. The stomates are small pores located in the upper and lower epidermal layers of the leaf. They allow for the passage of gases into and out of the air space just to the interior of the stomate. These will be discussed in more detail under dicot leaf anatomy. At this time, label the figure under No. 16 of the study guide.

Figure 17

Refer to No. 17 of the study guide. This figure shows the external morphology of a monocot or grass-type plant. Notice that the main stem of the plant is referred to as the culm. Coming off the culm or main stem and originating from a node on the stem is the leaf. The leaf is divided into two primary sections - the leaf blade and the leaf sheath. The leaf sheath is that part of the leaf originating from the node and running parallel to the culm or stem itself. At the point where it turns away from the stem then it is referred to as the leaf blade. At the junction of the leaf blade and the leaf sheath are several structures that need to be noted. The first of these is the auricles. The auricles are clasping appendages that are located at the junction of a sheath and the blade and hook the sheath to the stem itself. The next appendage located at the junction of the blade and the sheath is that of the ligule. The ligule is nothing more than a membrane that is located at this junction and runs between the culm and the blade itself. It should be pointed out here that not all grass species have these structures and if present may vary in size and shape. For example, some species may not have auricles, but if present they may be large or small. Also, other species may or may not have the ligule but if present may again vary in size and shape. Label these structures on the figure under No. 18 in the study guide.

Figure 18

This is an actual photograph of plants having both the ligule and the auricles as well as various other structures that need to be noted. The area noted as No. 1 is the node on the main stem or culm. The area noted as No. 2 is the internodal area of the stem. On the photograph at the right, the area Numbered 3 is the leaf blade while that labeled No. 4 on the left photograph is the leaf sheath. The structure labeled No. 5 is the ligule or the membrane located at the junction of the blade and the sheath while the structures numbered 6 would refer to the auricles - also located at the junction of the blade and sheath.

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Figure 19

Refer to No. 19 in the study guide. This figure shows a split photograph with an actual wheat plant on the right and a drawing of such on the left. Notice that wheat has both the clasping auricles, which are short, slender and hairy, and a long and prominent ligule located at the junction of the blade and sheath. Again, this is highly characteristic of wheat plants.

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Figure 20

This is a depiction of a corn plant. It may be noted that the corn plant has no auricles but has a very prominent ligule.

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Figure 21

This figure depicts the drawing and actual photograph of an oat culm and leaf. Note that as with corn, the plant has no clasping auricles yet has a very prominent short ligule.

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Figure 22

Characteristic of the barley plant is very prominent and long clasping auricles and a very short and inconspicuous ligule.

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Figure 23

Characteristic of the rye plant is very small and sharp pointed clasping auricles and a very small and inconspicuous ligule, again located at the junction of the leaf blade and leaf sheath.

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Dicot Leaf Anatomy & Morphology

Figure 24

Refer to No. 20 in the study guide. Note first of all the portion of the figure in the central and upper portion of the photograph. This represents a cross section of a dicot leaf. Notice again the upper layer of cells is referred to as the upper epidermis. Although not shown on this particular slide, a waxy cuticle covers the entire outer surface of both the upper and lower epidermis. The cuticle, as mentioned previously, functions in moisture conservation. Immediately below the cuticle is the upper epidermis which, as we saw with the monocot leaf, functions primarily in protection of the enclosed leaf tissue. The entire area of cells between the upper and lower epidermis, with the exception of the vascular tissue, is referred to as the mesophyll layer of cells. It is within the mesophyll layer of cells that the bulk of the photosynthetic activity occurs. The upper portion of the mesophyll layer of cells is referred to as the palisade cells. It can be noted that these cells are relatively long and oblong in shape. Immediately below the palisade layer of cells is the other type of mesophyll cells referred to as the spongy cell layer. These cells are relatively oval and again are irregularly shaped. Over in the left portion of the cross sectional area is the vascular bundle. The vascular bundle is comprised of the xylem which, as was true with the monocots, functions in the translocation of water and nutrients up the plant. Also the phloem is located within the vascular bundle and functions primarily to transport photosynthate from the site of production to the site of either storage or utilization. Again the vascular tissue is enclosed in a bundle sheath which gives rigidity, not only to the vascular bundle itself, but also to the leaf. Embedded in the upper and lower epidermis are small pores referred to as the stomates. The stomates are nothing more than small passages which allow for the movement of gas in and out of the leaf. In the lower left portion of the slide is a blown up figure of a stomate. Notice that two specialized cells referred to as the guard cells surround the pore or the stoma itself. The guard cells contain chloroplasts and as a result of such, do carry on a limited amount of photosynthesis. The primary gases that are exchanged through the stomates are CO₂, water vapor, and 0₂.   CO₂ diffuses into the leaf and is used in the process of photosynthesis. Water vapor and 0₂ diffuse out of the leaf and into the atmosphere. At this time label the figure under No. 21 in the study guide Also answer the questions under Nos. 22, 23, and 24 of your study guide.

Figure 25

Refer to No. 25 of your study guide This figure refers to or depicts the leaf morphology of a dicot plant - very similar to cotton and soybeans. Notice that the leaf itself is primarily composed of what is referred to as the blade. The blade of the leaf is attached to a stem like appendage referred to as the petiole and then the petiole is in turn attached to the main portion of the stem. In this particular example, located at the lower portion of the petiole are two small appendages referred to as stipules. Not all plants will have stipules. This is a characteristic that is very species dependent. This completes the first section of crop plant anatomy - that primarily dealing with the vegetative structures of crop plants. The next section will deal with the reproductive anatomy of our crop plants.

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