Introduction: This is the second of two labs that focus on each of the primary structure of the three higher plant organs (root, stem, leaf). It is assumed that you have already learned about tissue and cell types. If not, you should review Cells and Tissues of the Plant Body. There are two basic objectives that are integrated with the lecture for all three of these labs:
In most plants, the stem serves as a support for the leaves which act as solar collectors to produce food for the plant. Stem and leaf are tightly integrated together into the shoot system. Selective pressure in certain plant groups has resulted in modified stems that serve a number of different functions including food storage and defense. In some plants (cacti are examples), the stem is the primary photosynthetc organ and the leaves are very reduced. These modifications are considered in the final section of your lab exercise. Link to raw teaching images for the primary structure of the stem.
The Actively Growing Shoot Tip:
After the apical meristem of the shoot produces new cells through mitosis and cytokinesis, they become organized into one of the three primary meristematic tissues (protoderm, ground meristem, procambium). Also different regions below the apical meristem become organized into leaf primordia and bud primordia. Because of the tight integration of leaf and stem, stem anatomy is a bit more complex than that of the root. Strands of vascular tissue in the stem diverge into developing leaves. As they do so they leave behind an area of parenchyma tissue called a leaf gap. As presented earlier this is one of the defining characters of megaphylls (remember the leaves we are considering in these anatomy exercises are fundamentally different from those found in the phylum Lycophyta! - microphylls are not even homologous to megaphylls).
- Link to views of longitudinal section of a shoot tip Coleus
Cross Section of Herbaceous Dicot: This is basically a review of what we have already seen. Here, however, we are not simply focusing on the recognition of the tissue types but on how they are organized. Pay particular attention to the vascular cambium consisting of both fascicular and interfascicular cambia. These two meristematic areas are integrated to form one continuous cylinder of dividing cells.
- Link to views of whole Medicago stem cross section
Cross Sections of a Monocot Stem: The Zea (maize) stem cross section, like that of Medicago, consists of ground tissue surrounded by epidermis. Vascular bundles are embedded within the ground tissue. Note however, that the vascular bundles are not organized into a well defined ring, but are scattered through the ground tissue. Hence, the ground tissue is not divided into pith, cortex, and pith rays. In maize we simply use the more general term, ground tissue to refer to these areas of parenchyma. Further, in monocots, the vascular bundles are closed: That is there is no area of cambium between the primary xylem and primary phloem in the mature stem. Hence secondary growth does not occur in Zea. This arrangement of tissues is typical of the monocots.
- Link to view of a cross section of Zea (maize) stem
Syringa (lilac) - an example of a mesomorphic leaf: Lilac is a plant adapted to environments where there is adequate soil moisture. Hence, these leaves are called mesomorphic. First note the morphology of a lilac leaf. It is broad and flat, facilitating the process of gas exchange, and providing maximum surface area for light absorption. We look at two slides: one of a cross section through the leaf; the other a paradermal section. The paradermal section is made in a plane nearly parallel to the epidermis. It progressively cuts through the various layers of the leaf allowing us a face view of those tissues.
Cross-Section: In this view we see the upper and lower epidermis are one layer thick. While it is not obvious from these slides, a waxy cuticle is present on the leaf surfaces.The ground tissue is divided into two distinct regions: 1. the palisade parenchyma and 2. the spongy parenchyma. These are the primary photosynthetic tissues in lilac. The epidermis consists of a single layer of cells that encase the ground tissue. Veins constitute the vascular tissue with the xylem on top and phloem below. This is consistent with the fact that in the stem xylem is inside the phloem.
- Views of Cross Section of Syringa Leaf
Paradermal-Section: In this plane of section we clearly see each tissue layer in face view. First, identify the upper epidermal layer. The upper epidermis can be differentiated from the lower because it has fewer stomata. The layer bordering the upper epidermis then must be the palisade parenchyma, and the one bordering that, the spongy parenchyma. The last layer is then the lower epidermis. These sections together with the cross sectional view, provide a clearer understanding of the three-dimensional make up of the leaf than that provided by the cross section alone.
Views of paradermal section of Syringa Leaf
How does the observed structure of this leaf relate to function?
1. What selective advantage is it to the plant to have the stomata concentrated in the lower epidermis?
2. What selective advantage is it to the plant to have the cells of the palisade parenchyma arranged with their long axis perpendicular to the plane of the leaf?
3. What selective advantage is it to the plant to have the cells of the spongy parenchyma arranged more or less with their long axis parallel to the plane of the leaf, and with large intercelluar spaces?
Nerium (oleander) - an example of a xeromorphic leaf: Oleander is a plant adapted to dry environments, hence the term, xeromorphic (xer- is a prefix that denotes "dry." It is the same prefix used in Xerox, the company that developed the dry copying process). Again note that the leaf is broad and flat, hence, adapted to optimize photosynthesis.
Cross-Section: This leaf is similar to lilac in that its ground tissue consists of areas of both palisade and spongy parenchyma. Unlike those in the lilac leaf, however, both the upper and lower epidermal layers in the oleander leaf are several cell layers thick. Further, the stomata are not only restricted to the lower epidermis, but are also found only inside invaginations of that epidermis called stomatal crypts. Both of these characters are adaptations for conserving water. The veins are bicollateral with phloem both on top and below, with xylem in the middle.
Nymphaea (Water Lily) - an example of a hydromorphic leaf: Water lily is a plant adapted to an aquatic environment, hence the term, hydromorphic (hydro- is a prefix denoting water. It is used in in the word hydroelectric power to convey "water-generated electricity"). Again note that the leaf is broad and flat, hence, adapted to optimize photosynthesis.
Cross-Section: This leaf is similar to lilac in that its ground tissue consists of areas of palisade and spongy parenchyma. However, the spongy parenchyma has a huge amount of intercelluar space. This provides the leaf with buoyancy and a potential increase in the rate of gas diffusion. Parenchyma with this amount of intercelluar space is common in aquatic plants and is called aerenchyma. Note that the lower epidermis lacks stomata, whereas the upper epidermis has numerous stomata. What would be the adaptive significance of this characteristic? Also note that the veins have very little xylem. Why would this be an adaptive advantage?
Zea (maize) - an example of a C-4 leaf: The ground tissue in this leaf is not divided into palisade and spongy parenchyma, and is simply termed mesophyll. As in the other leaves we have seen, there are parenchyma cells surrounding the veins called bundle-sheath cells. However, these are particularly well developed in maize. Maize is a C-4 plant, and a division of labor exists between the bundle-sheath cells and the mesophyll cells. In the mesophyll cells CO2 is complexed with a three carbon compound in a reaction that is catalyzed by PEP-carboxylase. this reaction produces a four-carbon product. This compound is then transported into the bundle-sheath cells where they are broken into a three-carbon compound and CO2. This CO2 is then taken up in the by the Calvin cycle in the bundle-sheath cells. C-4 photosynthesis is an adaptation that limits photorespiration.
Pinus (Pine) - an example of a confer leaf
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