xercise 3: Variations in leaf structure

IMPORTANT NOTICE: The virtual plant includes a number of images -- THUMBNAILS are normally displayed.- DOUBLE CLICKING the thumbnail will access a high-resolution image. The Virtual plant has an on-line glossary. Clicking on underlined words, will pop up a small reference window, with relevant text, and an image, if this is available. The pop-up boxes are elastic and resizable. Click on the highlighted example to the left to see what happens( click me ). The popup windows will close automatically after 30 seconds. Because the glossary items differ in content, you will have to resize (drag the frame larger with your mouse) so that you can see all the information in the popup.


   Core Objectives                                     



To understand the basic variation in leaf structure

To identify cells and tissues in the leaf.




When studying the anatomy of various organs, it is important to keep in mind, the functions that these organs perform. This is particularly important in the case of the leaf. An understanding of leaf anatomy is impossible unless it is correlated with some knowledge of leaf function. For example, the main functions of the leaf are photosynthesis and transpiration,  which involves gaseous exchange between living cells and the atmosphere. It is necessary therefore to consider such features as:-
  1. The total absorbing surface, both for gas exchange and sunlight utilization,

  2. The permeability of the epidermis to gases,

  3. The extent of the total intercellular space,

  4. The nature of, and distribution of the vascular tissues (structure of the vein network) and

  5. Climate and habitat

Leaves of dicotyledonous, monocotyledonous and gymnospermous plants may differ structurally, but they all have similar functions - and gas exchange and photosynthesis are the prime function.

Click on this link, for more background information about leaf structure


   Core specimens                                     



  Road Map                                                   



Neriuml2.jpg (6445 bytes)



Nerium oleander


zeavb1b.jpg (5948 bytes)

pneedl1.jpg (10106 bytes)


Zea mays

Pine needle

Phormium tenax


   Text Book                                                               



Cutler, Botha and Stevenson, Plant Anatomy, an Applied Approach and Raven, Evert and Eichhorn, Biology of Plants (6th or later edition) are highly recommended for additional background information.

     A. The dicotyledonous foliage leaf                


1. Ligustrum (Privet)






Click on the image to have a look at high-resolution micrograph and more details about the structure of the privet leaf. 


identify the following cell and tissue layers:

(a) The midrib of Ligustrum contains a single, large collateral vein. Here and on the lamina the upper epidermis has a thick cuticle. The large vein is completely surrounded by parenchyma cells, which form a bundle sheath. A well-developed cambium separates the upper (adaxial) xylem from the lower (abaxial) phloem. Most of the xylem and phloem is primary. Some of the parenchyma cells above the lower epidermis are collenchymatous and thickened.

(b) The lamina consisting of:

  1. Upper and Lower Epidermis. Note cuticle and distribution as well as the appearance of guard cells. The guard cells have conspicuous outer ledges composed of cutin. Trichomes occur on both upper and lower epidermis.
  2. Mesophyll. Distinguish between palisade and spongy mesophyll: numbers of layers in each tissue, form of cells and size of intercellular spaces, manner of connection, if any, of cells to each other, content of cells, arrangement of chloroplasts in the two kinds of mesophyll cells, and relation between stomata and intercellular spaces (any sub-stomatal chambers) in the mesophyll.
  3. Vascular Tissue. You have already examined the large vein or bundle of the midrib. The smaller bundles are embedded in the mesophyll, but each is bounded by a bundle sheath. Note the distribution of the chloroplasts in the sheath cells, the position of the xylem and phloem within the bundle. Some of the bundles are seen in TS, some in LS, and some are oblique - why?


   2. Amaranthus                                                        





Click on the image to have a look at high-resolution micrograph and get more detailed information about the aerenchyma



Examine the detail micrograph of this leaf. The following points should be recognized.
  1. Epidermis. Both upper and lower epidermises in Amaranthus have a prominent cuticle and relatively thick outer tangential walls. Stomata are easy to find in both epidermises. Note the prominent inner and outer ledges of the guard cells.
  2. Mesophyll. Not differentiated into palisade and spongy parenchyma, but with small cells with small intercellular spaces among them below either epidermis. The mesophyll cells in between are radiate, larger and contain well-developed, sizeable intercellular spaces. Note the distribution and arrangement of chloroplasts. This is typical of a C4 dicotyledonous plant - it is an example of a C4 photosynthetic specimen (Z. mays is also an example of a C4 plant, but is monocotyledonous).
  3. Vascular bundles. Very uniform in size and arrangement.
  4. A prominent bundle sheath composed mostly of parenchyma cells, some of which contain crystals (raphides) cut in transection. Some sheath cells have thin secondary walls. The bundle sheath cells may contain very prominent chloroplasts. This is typical of a C4 photosynthetic dicotyledon.


   3. Nerium oleander                                                





 Neriuml2.jpg (6445 bytes)

Nerium is an example of a xeromorphic leaf . Examine the prepared slide of a TS of the oleander leaf. Under LP note the many invaginations in the lower epidermis. These are stomatal crypts, which contain many trichomes as well as a large number of stomata. There are no stomata on the upper surface of the leaf at all. Note the distribution and size of the veins.


Click on the image to have a look at high-resolution micrograph and to get more detailed information about the oleander leaf.

 Note the following:

  1. Upper and lower epidermis with thick cuticle. As mentioned, the stomata are restricted to the stomatal crypts of the lower epidermis. These invaginations with their numerous trichomes, probably greatly reduce the loss of water vapour from the leaf .
  2. The Mesophyll. The mesophyll or ground tissue, of the oleander leaf is differentiated as a hypodermis beneath both upper and lower epidermises. This tissue is devoid of chloroplasts. Beneath each hypodermis are 2-3 layers of palisade mesophyll (parenchyma) with spongy mesophyll in the centre of the leaf. This leaf is isolateral. Very large intercellular spaces occur among the spongy parenchyma. Notice the crystals (druses) in some of the mesophyll cells.
  3. Vascular tissue. Examine the large vascular bundle of the midrib. This is a collateral vascular bundle. A vascular cambium has developed on the lower side of the bundle and has produced some secondary xylem, but little if any, secondary phloem. The smaller vascular bundles or veins of the leaf (that is, those found in the lamella) are collateral. Note the conspicuous, well-developed parenchymatous bundle sheaths surrounding the vascular tissues. Some of the bundles contain bundle sheath extensions.


   4. Nymphaea                                                            




The floating leaf of the Waterlilly is an excellent example of a hydromorphic leaf.

Click on the image to have a look at high-resolution micrographs showing more details of this leaf.

Examine the detail image and locate the following



  1. Upper and lower epidermis. Do both lack a cuticle? All of the guard cells occur in the upper epidermis. Why is this so? The lower epidermis contains many small mucilage secreting cells.

  2. Mesophyll. Most of the ground tissue of this leaf is highly lacunose, that is, it has a large volume of intercellular space. The most compact mesophyll tissue is the palisade which consists of 2-3 layers of cells and comprises only about a quarter of the total thickness of the leaf. The spongy tissue is made up mostly of chains of parenchyma cells which form a network enclosing the large intercellular spaces. Note that where the chains meet, the parenchyma cells are branched. Note also that the large branched sclereids (trichosclereids), with arms and processes extending into the air chambers of the mesophyll. Small angular crystals are embedded in the walls of the sclereids.

  3. Vascular tissue. The spongy parenchyma cells form a compact tissue and sheath around the vascular tissues in the region of the veins.

  4. Note the large proportion of phloem to xylem in the veins of this hydromorphic leaf.   Why do you think this is so?























   5. Canna leaf                                                           





Click on the image to have a look at high-resolution micrograph and to get more detail

Click on the image to see the aerenchyma cells in detail.

Canna is an example of a monocotyledonous plant. Canna is a widespread genus, often growing in wetlands where it can choke waterways. It is popular because of its showy flowers, and there are many hybrids. The petiole of Canna contains large conspicuous airspaces, which are demarcated by stellate aerenchyma cells  with thin, finger-like projections. In these examples, we have cut sections through the petiole region of Canna to demonstrate the aerenchyma cells which  make up a lot of the mesophyll associated with the midrib region. These cells are thin-walled and translucent  (colourless). The image to above left was taken with phase contrast to highlight the structure and shape of the aerenchyma cells, the image above right, was taken using transmitted light.


    B. The Monocotyledonous foliage leaf         




   6. Zea mays                                                          




 zeavb1b.jpg (5948 bytes)


Click on the images to have a look at high-resolution micrographs and more details about the structure of the maize leaf. There are several additional images provided, to illustrate details about the vascular bundles and the phloem tissue in monocotyledons.

Look at the image of a Zea mays leaf above. Zea is an example of a C4 plant. Several anatomical features are commonly used to distinguish C4 from C3 plants. The most notable of these is the presence of a bundle-sheath surrounding the veins in the leaf which usually contains large, conspicuous chloroplasts.  Another important feature of C4 plants is the close spacing of the longitudinal vascular bundles.

Note that the mesophyll radiates from the bundle sheath - it looks wreath-like, hence its German-origin name, the Kranz mesophyll. This Kranz mesophyll layer contains numerous plasmodesmata through which intercellular transport from the mesophyll as well as from the mesophyll to the bundle sheath, can take place. These structures are minute -- ranging in size from 50 - 100 nm in total diameter!  Click here to have a look at some aspects of internal leaf structure, as well as plasmodesmatal structure, with the scanning electron microscope.

A diagnostic feature which can be used to separate monocot from dicot foliage leaf, is the fact that when monocot leaves are viewed in transverse section (i.e. such as the section that you can see in the sections presented in this exercise) all the veins are cut in transverse plane, whereas typical transverse sections of dicots will have veins which are mostly cut obliquely. Can you work out the reason why this should be so?

Most monocot leaves have groups of large, swollen epidermal cells, interspersed with smaller epidermal cells. These large cells are called bulliform cells. These bulliform cells have a significant role in the life of the plant , in that in times of water stress, the leaf is able to roll up, due to the rapid loss of water from the cytoplasm of the bulliform cells. This results in a smaller portion of the leaf being exposed to the atmosphere and as a result, a lower rate of water loss due to transpiration or evaporation. You should examine the leaf, and make a LP diagram which shows the essential features of the leaf' anatomy.



   7. Phormium tenax                                               





Phormium  or New Zealand flax, is grown extensively in many countries as a source of high quality fibre. The cross section to the left illustrates several points about this monocotyledonous foliage leaf. Notice the conspicuous red-stained fibre caps above the vascular bundles visible in this transverse section. These are long sclerenchymatous fibres. These fibres impart a great deal of mechanical strength to the leaf


    C. The Gymnosperm leaf                                   



    8. Pinus (Pine)                                                      


pneedl1.jpg (10106 bytes)

Click on the image to see this mature pine needle in more detail.  


This is an example of a Conifer (Gymnosperm) leaf. Conifer leaves vary in form from being needle-like to rather flattened structures. The pine leaf, is of course, an example of a needle-like leaf.

  1. Epidermis. Epidermal cells with thick walls and cuticle. The guard cells are sunken and attached to prominent subsidiary cells.
  2. Mesophyll. The outer cells of the ground tissue are differentiated as a hypodermis and are sclerenchymatous. Many of the remaining cells of the ground tissue have internal ridges projecting into the cell lumina. This is plicate mesophyll. In transverse sections the mesophyll appears compact but longitudinal sections would reveal prominent intercellular spaces. Most pine leaves contain two or more resin ducts, but the leaf you are examining contains several such ducts.
  3. Vascular tissue. This pine leaf contains two vascular bundles, which are surrounded by transfusion tissue. The latter is composed of tracheids and parenchyma cells. transfusion tissue is surrounded, in turn, by a conspicuous endodermis, the innermost layer of ground tissue. The endodermal cells may have somewhat thickened walls, especially the outer tangential wall, which contains conspicuous simple pits. At one time these endodermal cells may contain a well-developed Casparian strip. Each vascular bundle contains a vascular cambium. Some secondary xylem and secondary phloem has been laid down.




Assignment: Find out what the function of the albuminous tissue in Gymnosperms is.




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