OBJECTIVES : 1. Learn the basic anatomy of plant dermal tissue. 2. Develop knowledge of the associated terminology. 3. Appreciate the diversity of types of epidermis tissues on plant species. 4. View epidermal structures. 5. Make leaf surface impressions for viewing surface features and to identify epidermal features. 6. Review terminology and structures introduced in week one of the lab. OVERVIEW: In this lab you will learn about dermal tissue. This tissue, also known as the plant epidermis, forms the outermost layer of cells and is usually only one cell layer thick. Epidermal cells typically are flattened and rectangular in shape. Epidermal cells can have various functions depending on the type of plant and where they are in the plant body.
The epidermis of the above ground part of the primary plant body, particularly leaves, have stomata. Stomatal pores allow CO2 to enter the leaf for photosynthesis. Stomatal pores are surrounded by guard cells. It is the action of these guard cells that control the opening and closing of the pores.
Trichomes are outgrowths of the epidermis. They can be one celled or multicelled, dead or alive. Some of them contain chemicals that are of economic and/or medicinal value. For example, menthol is derived from trichomes on the peppermint plant. Trichomes in Cannabis sativa produce chemicals responsible for all the effects of marijuana. Other trichomes on plants such as nettles sting any creature unfortunate enough to brush against them, thus deterring herbivores. And trichomes of carnivorous plants secrete enzymes that digest trapped insect prey.
Root hairs also are outgrowths of the epidermis of roots. These structures are specialized epidermal cells for taking up minerals and water from the environment. Typically found near the apex of roots and on the youngest root tissue, these “hairs” increase the surface are for absorption by order of magnitude. The plant epidermis has several functions germane to plant survival. Depending on which organ the epidermal tissue covers, it may be involved in absorption and retention of water and minerals, protection against herbivores, and control of gas exchange (CO2 uptake, transpiration). These functions are influenced by epidermal features such as trichomes, intercellular spaces, and cuticle made up of waxes. Most epidermal cells are flat and packed together. Typically, these cells are mostly transparent and lack chloroplasts (what is the advantage of this).
TERMINOLOGY: LOOK UP ALL OF THE TERMS USED IN THIS LAB USING YOUR BOOK OR OTHER REFERENCE. KNOW THEIR DEFINITIONS AND (IF THEY ARE STRUCTURES) WHAT THEY LOOK LIKE
Caution : Broken pipettes, coverslips and slides, and razor blades are very sharp. Dispose of them in the “sharps” container and NOT the regular trash can.
PROCEDURE : Examine the fresh material and the slides provided in the lab. Before you leave, make sure you can recognize the tissues and cells listed, and that you know the meaning of the terminology.
A. Gross morphology of the epidermis:
1. Look at the fresh plant material available at the “Whole plant epidermis” station. View the plant under the dissecting microscope at its highest power. How does the epidermis look on the root versus stem versus leaf of each of the two plants?
2. Now look at the bromeliad plant. How does its surface compare with the two plants you just viewed? This plant grows as an epiphyte on other plants. Do you think the microclimate around this plant would be wet, moist or dry? What features of its epidermis lead you to this judgment?
B. Prepared slides of epidermal tissue:
1. Look at the prepared root slide in longitudinal section. Sketch the root, noting the epidermis and root hairs. How do the root hair cells differ from the other epidermal cells?
2. Now look at a cross section of a root – also noting the epidermis and the root hairs. Again, can you see additional differences between the root hairs and other epidermal cells?
3. Look at a prepared stem slide in longitudinal section. Note the cuticle, epidermis, and any other features of the stem epidermis.
4. Now look at a cross section of the stem. Do you see stomatal pores and guard cells?
Why or why not?
5. Slides from 4 different types of leaves are available in the lab. Sketch a small section of each, labeling the parts of the epidermis. Could you find the cuticle, epidermis, guard cells, stomatal pore, trichomes, and anything else? Now that you have looked at these materials closely, predict which specimens came from wet versus moist versus dry environments. What morphological evidence did you use to make your determination. Compare your predictions with those of other students in the class.
6. Now you will become an expert at making epidermal peels. Refer to the supplementary handout on how to use this technique. Choose five plants from around the lab (hairless is better for this part of the lab).
a. Make one slide from each plant. Use the grease pencil to write T on one edge of T B
the slide and B on the other edge
of the slide.
b. Place the peel from the top of the leaf on the slide near the T and the peel from the bottom of the leaf next to the B.
c. Observe your slide under the light microscope and make a quick sketch of what you see. In general, on which side of leave do you find stomata? Were there exceptions to this “general rule”? What other generalizations can you make about the appearance of epidermal cells from the different plants?
d. Use the videoscope to make images from your 2 most interesting peels. Label according to the course protocol. Print out the images and tape them into your notebook as you have done in previous labs.
7. Now choose 3 hairy plants from around the lab. Look at the surface of each under a dissecting microscope. For each sketch, speculate on whether the hairs are living or dead. How did you reach this judgment? How do the trichomes of these three plants compare with the bromeliad you saw earlier
8. Look at the trichomes of the carnivorous plant? What is unique about its trichomes? Sketch representative trichomes of each plant using high power.
C. Review of Materials from Weeks 1 and 2.
As time permits, complete your overview of materials used during weeks 1 and 2 to introduce you to plant anatomy and morphology. Representative materials are on the side bench. You can also use this time to make additional photo-micrographs using the video microscope of new material you have seen today, or to finish any assignments.
DISPOSAL - CLEAN UP
Wash and dry all slides when you are done and place them in the separate glass containers near the sink! Turn off your microscope, cover it and put it back into the microscope cabinet. Clean and dry your workspace before leaving lab.
Include the following information in your lab notebook: Drawings, sketches, printouts of your best images, and list of file names made according to the course format.
Brewer, C. A. 1992. Responses by stomata on leaves to microenvironmental conditions. Pages 67-75, in Tested studies for laboratory teaching. Volume 13. (C. A. Goldman, Editor). Proceedings of the 13th Workshop/Conference of the Association for Biology Laboratory Education ( ABLE ), 191 pages.
Crang, R. and A. Vassilyev. Plant Anatomy CD. ISBN 0-07-251084-6.
Dickison, W.G. 2000. Integrative Plant Anatomy. Academic Press, San Diego.
Uno, G., R. Storey, and R. Moore. 2001. Principles of Botany. McGraw Hill, NY.
Plants and animals both have a layer of tissue called the epidermal layer . Plants have special pores called stomata to allow passage of material. The stomata pores are surrounded on both sides by jellybean shaped cells called guard cells. Unlike other plant epidermal cells, the guard cells contain chlorophyll to do photosynthesis. This allows the cells to expand/ contract to open or close the stomata. Guard cells also close when dehydrated. This keeps water in the plant from escaping. The opening or closing of guard cells can be viewed in a microscope by adding different water concentration to the leaf tissue. Most stomata are on the lower epidermis of the leaves on plants (bottom of the leaf). The number of stomata on the epidermal surface can tell you a lot about a plant. Usually, a high concentration of stomata indicates fast growth and wet climate. Lower concentrations of stomata indicate lower rates of photosynthesis and growth or adaptations for dry weather Purpose: To view and compare the stomata from the leaves of several species of plant STUDY OF STOMA Introduction
Materials: 3 leaves (1 from 3 different species), compound light microscope, 3 microscope slides, clear nail polish, transparent tape
Obtain three leaves from different types of plants.
Paint a thick patch (at least one square centimeter) of clear nail polish on the underside of the leaf surface being studied.
Allow the nail polish to dry completely.
Tape a piece of clear cellophane tape to the dried nail polish patch.
Gently peel the nail polish patch from the leaf by pulling on a corner of the tape and "peeling" the fingernail polish off the leaf. This is the leaf impression you will examine.
Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any excess tape. Label the slide with plant name.
Examine the leaf impression under a light microscope at 400X.
Search for areas where there are numerous stomata, and where there are no dirt, thumb prints, damaged areas, or large leaf veins. Draw the leaf surface with stomata.
Count all the stomata in one microscopic field. Record the number on your data table.
Repeat counts for at least three other distinct microscopic fields. Record all the counts. Determine an average number per microscopic field.
From the average number/400X microscopic field, calculate the stomata per mm2 by multiplying by 8.
Follow procedures 2 - 11 with the other leaves.
Data: Leaf 1 Leaf 2 Leaf 3 Name of Leaf Drawing in 400x (with several stomata) Stomata in field 1 Stomata in field 2 Stomata in field 3 Average Stomata in field Stomata/ mm2
Fossils tell the story About four hundred and fifty million years ago, at the end of the Ordovician and in the beginning of the Silurian, the land was desolate and empty. Barren, hardly wethered rockgrounds, empty sand-, gravel- and clayplains, no green. Maybe some lichens. And at wet spots some algae with a couple of spider-like little creatures creeping around. In the neighbourhood of the mouth of rivers, where the water regularly flooded the land, it was probably green with algae. At such places, e.g. in Australia, traces of big seascorpions have been found. There was not much happening on the land. Life enacted itself nearly completely in the water. The oldest indications for the existence of real land plants have been found in cores from boreholes in Oman. They contained fours of mutually connected spores (tetrads) enveloped by remains of the spore sac in which they had been formed. Research on the spore walls point to a relationship with the liverworts. The fossils have been found in the Middle Ordovician and are about 475 million years old . How plants conquered the land
Cooksonia The first fossils of macroscopic land plants have been found in the Middle Silurian of Ireland. They are about 425 million years old. They consist of small bifurcations some centimeters in size. Only in the very last part of the Silurian fossils of land-plants become more common and also more complete. The best known plant from that time is called Cooksonia . It is named after Isabel Cookson, who occupied herself with intensive collecting and describing plantfossils. The little plant looked very simple: a stem which bifurcated a couple of times topped with small spheres in which the spores were formed. Thus sporangia. No leaves, no flowers, no seeds. And roots? Probably horizontal growing stems, connected with the soil by root hairs, took the function of roots. But this is not sure for fossils proving this have not been found yet. During many millions of years it was mainly this kind of plants that grew on humid places on the land. The evolution from algae to land plants must have been a lengthy process. Many conditions had to be fulfilled before plants were able to maintain themselves on the land. There is at first the everlasting danger of desiccation .
The remedy which developed is a thin waxy layer at the surface of the plant: the cuticle. Generally algae don't have a cuticle, nearly all land plants do. But a land plant also has to breath and it needs the possibility to assimilate carbondioxide from the air to generate its nutriments. Thus the isolation by the cuticle can not be absolute. That's why the stomata have evolved, which can be opened and closed if necessary.Another problem for land plants is that they miss the upward force of the water. To be able to keep upright supporting tissue is needed. Already in Cooksonia xylem vessels have been recorded. These are vessels with annular or spiral shaped thickenings at the walls, which give them solidity. Through these vessels water is transported from the soil to the plant cells. Another important adaptation to landlife is the very tough wall that evolved around the spores. This provided the spores with an excellent protection against desiccation, fungi, and so on. In fact they became nearly invulnerable, for they have been fossilised very often. Species of Cooksonia are found at several places on earth, e.g. in Wales, Scotland, England, Czechia and Canada. The finding of a fairly complete plant is a rare occurrence. I was already very glad with the bifurcated little stem with two sporangia between Cooksonia -chaff on the photo. Cooksonia has become extinct in the Early Devonian.
The vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem (inwards, towards the pith) and the secondary phloem (outwards), and is located between these tissues in the stem and root. A few leaf types also have a vascular cambium.  The vascular cambium usually consists of two types of cells: Fusiform initials (tall cells, axially oriented) Ray initials (almost isodiametric cells - smaller and round to angular in shape) The vascular cambium is a type of meristem - tissue consisting of embryonic (incompletely differentiated) cells from which other (more differentiated) plant tissues originate. Primary meristems are the apical meristems on root tips and shoot tips. Another lateral meristem is the cork cambium , which produces cork, part of the bark. Vascular cambia are found in dicots and gymnosperms but not monocots , which usually lack secondary growth. For successful grafting , the vascular cambia of the stock and scion must be aligned so they can grow together. Structure of Vascular Cambium
The function of the vascular cambium is to produce secondary growth, thus the vascular cambium must be formed before secondary growth can occur Function Of Vascular Cambium
1. Root-stem transition was investigated in Solanum tuberosum, Lycopersicum esculentum, Datura tatula, Physalis virginiana, Petunia acuminata, and Solanum pseudo-capsicum. 2. The method of transition in each of the plants investigated is the same and agrees with the method in Solanum melongena. 3. Illustrations of transition of the potato were used as representative of the other members of the group. The first change from the diarch, radial protostele of the root is a breaking up of the xylem plate and a division of each of the two primary phloem groups into three distinct parts. 4. This is followed by a bifurcation of the metaxylem of the two primary xylem units. These units were not observed to swing outward, one following a left and the other a right curve, as reported by Artschwager. 5. The central groups of phloem cells from each side differentiate toward the center of the axis and become the internal phloem. Each of the four remaining groups is gradually inclined in a tangential direction toward the original position of the protoxylem points. This development continues until the bicollateral condition is established. ROOT STEM TRANSITION
6. One of the primary xylem units, formed by the breaking of the original diarch xylem plate and consisting of one protoxylem point, its metaxylem, and internal and outer phloem groups, becomes the vascular trace of one cotyledon; the second unit, that of the other. 7. In the cotyledonary petiole and midrib the protoxylem differentiates adaxially and finally is nearer to the upper epidermis than is the metaxylem. Simultaneously the metaxylem differentiates abaxially until the endarch condition is established