PLB 105 Developmental Plant Biology

CONTENTS:

• Study Objective List 8
• Laboratory 16 - Leaf Development, Venation types, and Abscission
• Laboratory 17 - Leaf Anatomy and Specialized Leaves
• Laboratory 18 - Leaf environmental types

Leaf Anatomy Read:

Fahn, Chapter 12

To Review Read:

Fahn, Chapters 1-18 and part of 19

Student will be able to:

1. Discuss the concept of the shoot system, and describe the role of leaves in determining the anatomy of the stem.

2. All of the terms listed below concern leaves and leaf form; define these terms and recognize their structure in the plant body.
   1. Dorsiventral symmetry
   2. Simple leaf
   3. Compound leaf
   4. Stipules
   5. Sessile
   6. Lamina
   7. Petiole
   8. Cataphyll
   9. Phyllode
   10. Cladode
   11. Unifacial leaf
   12. Bifacial leaf

3. According to the traditional view, during leaf development five leaf meristems are activated sequentially or simultaneously. Describe and diagram the development of a "typical" dicot leaf and discuss the function of each meristem.

4. Draw a graph to show the relationship between leaf development and the timing of cell division.

5. Discuss the arguments proposed by S. Poethig and others that the marginal meristem plays a minimal role in leaf development.

6. Describe the basic difference anatomically and developmentally between a phyllode and a dorsiventral leaf.

7. . Describe and recognize the mature anatomy and anatomical variations in the epidermis of leaves.

8. Describe and recognize the mature anatomy, and anatomical variations, in the mesophyll of leaves.

9. Define:
   1. Spongy mesophyll
   2. Palisade mesophyll
   3. Bundle sheath
   4. Paraveinal mesophyll
   5. Bundle sheath extension

10. Describe the characteristic leaf and chloroplast anatomy of tropical grasses and dicots that are capable of carrying out photosynthetic carbon fixation by the C4 or Hatch-Slack pathway.

11. Describe, diagram and give an example of the following terms and venation types:
    1. Open dichotomous
    2. Reticulate
    3. Areole
    4. Parallel
    5. Commissural veins
    6. Transfusion tissue and accessory transfusion tissue (gymnosperms)

12. Describe the direction of differentiation of procambium, phloem and xylem in leaves.

13. Describe and recognize transfer cells in plant tissues, and discuss the anatomical characteristics and function of these cells. (Review Unit 3 - Study Objectives)

14. Recognize and describe the abscission zone in petioles.

15. Compare in general terms the structure of hydrophytic, mesophytic and xerophytic leaves.

16. Define the term heterophylly and give an example. Define the term heteroblastic development, and give an example of a plant with juvenile and adult leaves.

17. . Haber performed some interesting experiments on the effect of irradiation on plant development. He had some definite ideas on the role of cell division on leaf shape. Discuss his ideas and comment on whether you think his interpretation is correct. What are gamma plantlets? (Bring this up in lab if we don't get to it in lecture.)

1. Leaf Anatomy
   1. Dicot leaves
      1. Phaseolus vulgaris (bean-dicot) T.S. and para-dermal. This leaf is used to demonstrate the "typical" anatomy of a dicot leaf. Review its structure with reference to the distribution of cells in the mesophyll, epidermis and vascular tissues.

      2. Cut T.S. of Nerium oleander leaf. Note the multiple epidermis, stomatal crypts, laticifers and midrib with phloem on both sides of the xylem. Wash your hands immediately after handling this material. Also examine the prepared slide of this material.

   2. Monocot leaves
      1. Poa annua (bluegrass) leaf T.S. and P.D. Compare the mesophyll, venation and epidermal characters with the other leaf types. Note the two bundle sheath layers. The outer layer looks like parenchyma and the inner layer has U-shaped thickenings and looks like an endodermis.

      2. Zea mays (monocot) leaf T.S. Use this as a "typical" monocot leaf. How is this different from a dicot leaf? Note especially the bundle sheath cells. Review the text (pp. 244-245) which discusses the anatomical differences between C4 and C3 plants. C4 plants, like sugar cane and corn, are considered to be tropical grasses and have an enlarged bundle sheath of parenchyma cells. The chloroplasts in these bundle sheath cells are different from those of the leaf mesophyll.

   3. Gymnosperm leaves

      Gymnosperms, particularly confers, have certain leaf structural characteristics that differ from the angiosperm leaves you examined previously. Transfusion tissue is the most notable feature.

      1. Pinus T.S. Refer to the text figure for help in interpretation. Note the seeming lack of intercellular air spaces.

      2. Pinus L.S. Compare to the T.S. view you just looked at. Note the regular air chambers which were not apparent in T.S. view.

      3. Sequoia (gymnosperm) leaf T.S. and L.S. Refer to the text for help in interpretation. Note the resin ducts, vascular tissue and mesophyll.

2. Specialized Leaves
   1. Phyllodes

      Some species (Acacia) have leaves (phyllodes) which are oriented parallel to the stem axis. In old literature phyllodes used to be interpreted as flattened petioles.

      1. Cut T.S. of Acacia phyllode, and then compare your sections to the prepared slide. Based on developmental analysis, phyllodes are actually modified leaves.

   2. Cladodes

      A cladode is a leaf-like structure which is actually a flattened stem.

      1. Cut T.S. of Ruscus cladode to determine its internal structure.

   3. Carnivorous leaves

      Each of the following are leaves modified to trap and consume insects. Examine each, analyze the method by which this is done, and determine the specialized leaf structures involved. Whole plants of some of them will also be available.

      1. Drosera (sundew)

      2. Dionaea (Venus flytrap)

      3. Darlingtonia (Pitcher-plant)

      4. Pinguicula (herbaceous bog plant)

3. Review epidermis laboratory and study objective list.

   Recall the terminology for stomatal types, etc.

1. Venation Patterns

   Make a leaf clearing of the following leaves. Determine their venation pattern and examine their epidermal characteristics.

   1. Ginkgo (gymnosperm)-open dichotomous

   2. Taxus (gymnosperm)-single vein

   3. Bambusa (monocot)-parallel venation

   4. Salix (dicot)-reticulate venation

   5. Acer (dicot)-reticulate venation

2. Leaf Abscission
   1. Coleus blumei leaf abscission zone L.S.-Note the early abscission zone and arrangement of early cell divisions.

   2. Look at the process of leaf abscission in Juglans nigra (black walnut). look at each stage and identify the layers of the ab-scission zone-a) the suberized protective layer, and b) the distal separation layer.
      • Initiation of abscission zone. What environmental factors may affect abscission, and what hormones control it?

      • Abscission zone. This slide shows the complete zone with both the protective and separation layers.

      • This year's leaf scar. What materials have impregnated the cells of the scar?

      • Old leaf scar. Compare this to the scar immediately after leaf drop.

   3. Acer sp. leaf abscission zone L.S.-Showing good differentia-tion of the protective and separation zones.

3. Leaf Development

   Before examining various examples of leaf development, you should carefully review the material on formation, growth and development of leaves in the text. Note especially the position and function of the five leaf meristems (apical, intercalary, adaxial, marginal and plate). While examining the samples provided, think in terms of these meristems. Mature leaves of each are provided as demonstrations.

   1. Phaseolus vulgaris (bean-dicot) T.S. and Paradermal-This leaf is used to demonstrate the "typical anatomy" of a dicot leaf. Are any of the meristems more active than the others in the devel-opment of this leaf (i.e., intercalary vs. plate)?

   2. Ficus sp. (fig)-Each slide contains four leaf sections in vari-ous stages of development. Draw a portion of the lamina including a small minor vein from a region from each leaf. Label your drawings and note the subsequent changes in each tissue. Also examine the paradermal section on demonstration.

   3. Zea mays (corn-monocot) shoot apex (L.S. and T.S.)-Look first at the youngest leaf primordia and than at successively older leaves. Are the changes which occur in the mesophyll tissue as striking in this monocot as they were in the dicot Ficus?

   4. Leaf development of the simple leaf of Coleus -Your TA will lead an examination of the 2x2 slide set of early stages of Coleus leaf development on the caramate in the laboratory. Leaf development of Coleus is generally similar to that of other simple dicot leaves. The leaf primordia originate by periclinal divisions in the second layer of the apical meristem. The leaf primordium then grows vertically upwards and finally expands laterally. The petiole is intercalated between the base and the blade and thickens by adaxial growth.
      • Slide 1 - Two new leaf primordia have just been initiated by periclinal divisions in the second layer of the apical meristem.

      • Slide 2 - Further apical and intercalary meristematic activity cause elevation of the leaf primordium above the surface of the apical meristem.

      • Slide 3 - When the young leaf primordium attains a certain height, small bulges or outgrowths appear laterally on either side. These constitute the marginal meristem, and it forms the lamina and sets up the bifacial nature of the leaf blade.

      • Slide 4 - Continued marginal meristematic activity along with plate meristem activity causes continued increase in lamina formation.

      • Slide 5 - Here the blade region is well developed, and areas of the midrib and major veins are evident.

      • Slide 6 - In addition, a group of cells below the adaxial epidermis divides periclinally and contributes to the thickness of the region, which will become the future petiole and midrib. You can see the results of this adaxial meristem in the region of the fu-ture petiole of this young leaf.

1. Environmental Types

   There are three plant environmental types - Mesophyte, Xerophyte and Hydrophyte. Plants in each of these categories grow in extreme climates. The leaves produced by these plants are modified in certain ways to make them best fitted for their environment. In each example below you should make a list of the characteristics of the leaves which make them suited for their environment. At the end of the lab. the TAs will give each of you one of three leaf examples. You should section it and decide, based on their anatomy if it is a mesophyte, xerophyte or hydrophyte.

   1. Mesophyte:
      1. Helianthus (sunflower) leaf TS -The mesophyll cells are mostly aligned in vertical rows with large air spaces between. Note the abundant trichomes.

      2. Syringa (lilac) leaf TS and PD -This is a good example of a typical mesophytic leaf. All of the "standard" features are present. Be sure to examine the paradermal sections. Can you see all of the layers in both views?

   2. Hydrophyte:
      1. Nymphaea sp. (water-lily)-Two slides are provided of this aquatic leaf, the first shows a young leaf with curled margins, including the dramatic astrosclereids. Look at the developmental stage of the cells of the mesophyll. Can you predict the anatomy of the mature leaf? What leaf meristem will be responsible for the un-curling of the leaves? The second slide shows T.S. of mature and immature leaf blade. Describe the cell layers present. How many layers comprise the mesophyll, the epidermis, and the vascular tissue? Notice the densely stained cells in the lower epidermis. These are called hydropotes, and they are apparently involved in water and salt secretion.

      2. Make sections of fresh Nymphaea leaves. Note the anatomical characteristics which make these leaves able to live floating on water.

   3. Xerophyte:
      1. Hakea sp. (evergreen shrub)-Each slide contains three developmental stages. Hakea leaves are cylindrical and, therefore, quite complex anatomically. The palisade mesophyll forms a 2-layered cylinder around the entire leaf. The internal mesophyll fills the central portion of the leaf, with vascular bundles formed somewhat in a ring. Follow the development of this leaf carefully, note also the position of stomata, and be able to name all cell types and tissues present.

      2. Rolled leaves of the grass Elymus are an example of a "protective" mechanism developed for dry climates. The leaf curls to form a moist chamber. All the stomates are inside crypts which contain simple trichomes. What other characters are apparent? Note the prominent bundle sheath extensions of sclerified cells.

      3. Piper orratum leaf TS - This leaf is rather thick and succulent with a thick waxy cuticle and multiple epidermis on both surfaces.

      4. Atriplex butiformis leaf TS - This small desert plant shows collapsed trichomes on both surfaces which act as water storage and heat filter structures. These leaves have C4 photosynthesis and have large bundle sheath cells. The mesophyll is reduced with small air spaces. Note the large crystal cells.

      5. Helleborus niger (Christmas rose) leaf TS - This is a small succulent leaf with a thick cuticle. It probably would be characterized as a xerophyte.

2. Heterophylly:

   Heterophylly is a term which means that leaf form changes with its environment. For example, leaves which form on branches in the shade tend to have different form than those which form in direct sunlight. This characteristic of plants implies that the environmental signal somehow induces the shoot apex to initiate leave with a different form. We will look at two examples.

   1. The sun- and shade-treated leaves on this slide are an example. Which of these leaves had the sun treatment? Why? Which had the shade? How do you know?

   2. The air- and water-grown leaves in this slide are another example. On the basis of their anatomy, which leaf was grown where, and why do you think so?

   3. Heteroblastic leaf development refers to a special type of heterophylly where changes in leaf shape which are associated with age of plant and reproductive maturity, i.e., juvenile and adult leaves. Ivy (Hedera helix) and Eucalyptus are examples of heteroblasty.

3. Practical exercise:

   The TA will give each of you a fresh leaf representative of an environmental type. Make a free hand TS section and determine based on leaf anatomy if the example is a mesophyte, xerophyte or an hydrophyte. Make a drawing and a list of characters for your leaf. When you have finished, the TA will pair you up with another student who did a different leaf. You will then show each other your leaf, drawing and character list and explain how you came to your conclusion.

1. Some of the performances and/or projects from the previous lecture quiz will be shown during this laboratory session.

2. The TA will hand out a lab. evaluation form.

3. Clean up your area.

4. Discussion of study objectives, and other experiments started earlier in the quarter.

PLANT ANATOMY: A STUDY GUIDE AND LABORATORY MANUAL

Thomas L. Rost
Section of Plant Biology
University of California
Davis, California 95616-8537

23rd Edition (1998) Copyright (C); 1975 by Thomas L. Rost
Revised: August, 1998
URL: http://trc.ucdavis.edu/CoursePages/PLB105/HTML/Lab8.html