This activity replaces Parts A through B3 of the Cells and Tissues lab
Introduction: In this exercise you will make observations of various tissues and cells found in the stem of herbaceous dicots: specifically of a free-hand section of Coleus stem, and of the stained cross section of Medicago (Alfalfa) stem on a prepared slide. There are labelled images of these in our computer bank which your TA can use to assist you. Further, these same images are available on the course web page for review.
Getting Started: Work in groups of four. If someone has the knack for making good sections have that person share with others in the group. Further, members should assist one another to identify the various tissues and cells described below.
Making free-hand sections of Coleus: Coleus plants will be available on each student bench. Cut pieces of stem should be placed in the bowls of water at the front of each student bench for others to use. Before cutting into the stem, roll the stem between your fingers. Notice the square shape of the stem. This is a characteristic of all members of the Lamiaceae (the mint family). Do not use the razor blades in your drawer to make these sections. Use the razor blades provided in the petri dish at the front of each bench. These are particularly sharp. They should be used up one at a time, and everyone at a bench should use the same blade until it becomes dull, at which point it should be thrown away and replaced by a fresh blade. Please be careful not to mix these blades with the ones in your drawers!
Preliminary Observations: After preparing a wet mount observe the cross section with your 4x objective. The stem is made up of three different tissue systems. Identify these in turn:
The dermal tissue system consists of the outer layer of cells of the stem. These cells make up the epidermis.
The vascular tissue system is found in distinct vascular bundles. These are arranged in a circular pattern inside the stem.
The ground tissue system consists of everything enclosed by the epidermis except the vascular bundles. It is divided into three regions: pith, pith rays, and cortex. The pith is the tissue encircled by the vascular bundles; the cortex is the tissue outside this ring and inside the epidermis; the pith rays are the ground tissue found between the vascular bundles.
After identifying these tissue systems and regions with your Coleus slide do the same with the prepared slide of Medicago.
I. The Epidermis: the epidermis functions to control water loss from the plant.This is accomplished primarily by the excretion of a waxy layer called the cuticle. Like a sheet of wax paper, the cuticle limits the passage of water. In doing this it also blocks the passage of gases between the plant and its environment. To keep the tissues from suffocating, the epidermis must have openings. These openings are called stomata (singular = stoma).
a. Stomata: Stomata are not easily seen in a free-hand section, hence, look for these using the Medicago cross section. First identify large intercellular spaces directly below an area of epidermis using low power. These are substomatal chambers and they are positioned directly below each stoma. Switch to 400x and observe the stoma and its two guard cells. The level of turgor of these two cells serve to open and close the stoma, controlling the movement of gases and water into and out of the stem.
b. Trichomes: Another way the epidermis limits the plant's loss of water is through the growth of tricomes. Switch back to your section of Coleus and observe these hairs projecting from the epidermis. Note that there are different types of trichomes present.
Can you think of any other possible adaptive function of these structures?
II. Ground Tissue:
a. The Pith: Using your Coleus section observe the pith (the region of the cortex inside of the vascular bundles). In this stem, this region is composed entirely of parenchyma tissue consisting of parenchyma cells. These are the same type of cell studied in the eukaryotic cell lab. Now Stain your section with I2KI.
Based on your observation of the staining, what is one obvious function of the pith?
b. The Cortex: The cortex mostly consists of parenchyma tissue, however, collenchyma tissue consisting of collenchyma cells with unevenly thickened primary cell walls will be found in the four corners of the stem. This is the same tissue you peel from celery. Now switch to your Medicago slide. This stem has ribs also and collenchyma is positioned at those points. Note, however, that in this preparation the cells are dehydrated and their walls are uniformly thick. When living, these cells looked the same as those in Coleus.
c.Pith Rays: The pith rays primarily consist of parenchyma, but note the interfascicular cambium forming a continuous cylinder of secondary tissue with the fascicular cambium.
III. Vascular Tissue: Vascular tissue is localized in about eight distinct vascular bundles. Observe the model of the stem of Coleus at the front. Vascular tissue includes xylem and phloem, both of which are complex tissues: that is, they each consist of more than one cell type. The xylem will be in the innermost area of each vascular bundle facing the pith. The most prominent cells here are vessel members. These are large dead cells with thick secondary walls. The vessel members function as pipes through which water moves by mass flow. Note that these are surrounded by parenchyma cells that are also considered part of the xylem tissue. The phloem is towards the outside of each vascular bundle. It consists of sieve-tube members, companion cells, phloem parenchyma cells, and fibers. These slides are not good enough to distinguish between the four cell types. This will be explored using our Cucurbita slides later. Again switch to your Medicago slide and observe the effects of staining on the vascular bundles. Secondary walls in particular stand out because their lignin picks up the red stain. Hence, vessel members in the xylem and fiber cells in the phloem are particularly easy to see.
IV. Vascular Cambium: Now using your Coleus slide observe the vascular cambium connecting the vascular bundles. A cambium is a lateral meristem that produces secondary tissues by cell division. The cambium consists of fascicular cambium (found between xylem and phloem in the vascular bundles) and interfascicular cambium (found bridging the pith rays). Fascicular cambium forms from procambium, interfascicular from ground meristem, yet together they form a continuous cylinder. The cells resulting from the cell division of this cambium layer becomes mostly sclerenchyma (to be studied next). Note, that depending on where on the plant you made your section, your stem will vary. If the section was made close to the apical meristem there may be no evidence of a vascular cambium. On the other hand, if a section was made four or five nodes down, you will see not only a continuous vascular cambium, but also considerable seconday growth.