Lab: Plant Structure: the Plant Body: Part Two
STEMS:
The stem supports the aerial portion of the plant and transports materials (water, minerals and food) to and from the various organs of the plant. Some plants (cacti and Palo Verde trees are two Arizona examples) carry out photosynthesis in their stems. When we look at the oldest plant fossils, the only organ they have is the stem – no leaves, flowers, roots, etc!
- Locate the terminal or apical bud on the bean plant picture earlier in this lab. Notice the numerous tiny leaves protecting this tender area of the stem. Growth in the length of the stem is a result of continued cell division in this area and subsequent elongation and maturation of the resulting daughter cells.
Click the circles to reveal the name of the different parts of the diagram.
- Examine the photo below of a prepared slide of the Coleus stem tip. This slide is cut along the length of the stem tip. You should be able to locate the apical meristem, one of the regions of the plant body that continues to divide by mitosis. Notice how tightly packed the cells are in this region and their small size and prominent nuclei. The apical meristem is surrounded and protected by embryonic leaves or leaf primordia. In the axil (where the petiole attaches to the stem) of each leaf you should see an axillary bud whose structure is reminiscent of the apical meristem; cells in both the apical meristem and the axillary bud retain the capability to divide by mitosis.
Perform this drag and drop activity to identify the various parts indicated.
- Examine the prepared slide of a cross section of the Zea (corn) stem shown below. The section is circular in outline since it was cut across the entire stem. Notice the presence of numerous vascular bundles; this is typical of a stem. Vascular bundles of monocots are scattered randomly throughout the stem.
- Identify the epidermis covering the outer surface of the stem. Within each vascular bundle identify the xylem (typically cells that are stained red and occur towards the center of each vascular bundle) and the phloem.
Epidermis
Roll over the circles to identify the various parts indicated.
Perform this drag and drop activity to identify the various parts indicated.
- What is transported in the xylem?
- What is transported in the phloem?
A sheath of supportive sclerenchyma (fibers) tissue, usually stained red-purple surrounds each vascular bundle. The tissue between the vascular bundles is parenchyma.
- Examine the image of a prepared slide of a cross section of Medicago stem and examine it under the compound microscope. Notice that the vascular bundles are arranged in a circular arrangement. Is this the stem of a monocot or dicot?
Perform this drag and drop activity to identify the various parts indicated.
- Identify the epidermis and parenchyma in the stem cross section of Medicago.
The parenchyma can be divided into two regions: the pith inside the ring of vascular bundles and the cortex outside the vascular bundles.
The xylem and phloem of each vascular bundle is separated by a layer of vascular cambium that, like the apical meristem, retains the capability of mitotic division. In woody plants, the vascular cambium produces new layers of xylem and phloem each year resulting in annual growth rings and an increase in the girth of the stem.
 
- Examine the prepared slide of a cross section of the woody dicot, Tilia . You can readily recognize this as the stem of a woody plant rather than a herbaceous stem (like Medicago) because of the concentric annual growth rings.
Perform this drag and drop activity to identify the various parts indicated.
As a plant becomes woody, the vascular bundles fuse laterally to create a continuous circle. Continued division of the vascular cambium produces a new layer of xylem to the inside and a new layer of phloem to the outside of the stem.
- Label the oldest and the youngest xylem in the picture of a woody stem cross section (x.s.) above.
The oldest rings of xylem are generally filled with resins and metabolic wastes and are consequently no longer important in transport within the plant. This heartwood is often the most aromatic and beautiful wood of a tree.
- What is the age of the stem you are examining above?
- The original pith is still located in the center of the stem (inside the vascular bundles). What type of cells makes up the pith?
The epidermis and oldest phloem cannot keep up with the increasing girth of a woody stem; these layers generally split and degenerate as they are forced outward.
Roll over the circles to reveal the name of the different parts of the diagram.
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