|
|
PHOTO UNIT 7: THE
FUNDAMENTAL SYSTEM:
COLLENCHYMA-SCLERENCHYMA
OBJECTIVES:
1.
Characterize the three collenchyma types in terms of cell structure and
wall type, both longitudinally
and transversely
2.
Compare
occurrence of collenchyma in dicots and monocots and state why this
difference is not necessarily significant to the welfare of the plant
body.
3.
Analyze
positioning of both wall thickening of collenchyma and the arrangement
of collenchyma tissue in the plant body and explain the functional
significance of such structural arrangements.
4.
Define
sterome to include collenchyma and sclerenchyma and compare and contrast
them.
5.
Differentiate between fibers and sclereids on the basis of origin and
location and relate structure to function of each type.
6.
Classify
sclerenchyma fibers as xylary and extraxylary and characterize each
type. Subdivide xylary fibers into fiber tracheids, libriform fibers
and septate fibers. Distinguish by thickness and pitting and recognize
extraxylary fibers in bundle caps and bundle sheaths (cortical) fibers,
pericyclic (perivascular) fibers and bast (phloem) fibers.
7.
Characterize fiber cells and indicate the origin and development of
fibers.
Photos for this study:
COLLENCHYMA
7-1: Cucurbita
(squash): t.s. stem cortex, collenchyma tissue
7-2: Malva
(common mallow): stem cortex t.s. (5 photos, 3 magnifications)
7-3: Helianthus
(sunflower): stem cortex t.s.
7-4: Helianthus
(sunflower): stem cortex
l.s.
7-5: Apium
(celery): leaf petiole t.s. (3 photos)
SCLERENCHYMA
7-6: Tilia
(basswood): macerated bark of stem (bast fibers)
7-7: Tilia
(basswood): t.s. 2-year stem phloem, bast fibers
7-8: Prunus
(peach): macerated fruit stone sclereids
7-9: Prunus
(peach): sectioned fruit pericarp
7-10: Aristolochia
(Dutchman’s pipe): t.s. stem with
sclereids
PHOTO STUDY 7-1 Cucurbita (squash): t.s. stem cortex, collenchyma tissue
Beneath the epidermis is a tissue, three to several cells in thickness, which is characterized by wall thickenings at the cell angles. This is collenchyma tissue, and since the thickenings are restricted mainly to the angles, it is called angular collenchyma. Do the cells of this tissue have protoplasmic contents, or are they empty and dead? Are there intercellular spaces? Do the thick areas of the cell wall show indications of lignin (red staining) deposits?
PHOTO STUDY 7-2 Malva (common mallow): stem cortex t.s.
Collenchyma as seen in a transverse section of a young stem. In this type of collenchyma, walls are thickened wherever they border on an intercellular air space. Here and there you will see small air spaces with thick deposits of cellulose on all sides. In many instances these wall deposits have encroached upon the intercellular space, even to the point of completely filling it, and therefore obliterating it. Collenchyma of this type is called lacunate collenchyma. Is there any evidence here of lignification of suberization of walls? Do these cells have protoplasmic contests? Three magnifications and two specimens allow for better overall view, then closeup view.
PHOTO STUDY 7-2 Malva (common mallow): stem cortex t.s.
Collenchyma as seen in a transverse section of a young stem. In this type of collenchyma, walls are thickened wherever they border on an intercellular air space. Here and there you will see small air spaces with thick deposits of cellulose on all sides. In many instances these wall deposits have encroached upon the intercellular space, even to the point of completely filling it, and therefore obliterating it. Collenchyma of this type is called lacunate collenchyma. Is there any evidence here of lignification of suberization of walls? Do these cells have protoplasmic contests? Three magnifications and two specimens allow for better overall view, then closeup view. PHOTO STUDY 7-2 Malva (common mallow): stem cortex t.s.
Collenchyma as seen in a transverse section of a young stem. In this type of collenchyma, walls are thickened wherever they border on an intercellular air space. Here and there you will see small air spaces with thick deposits of cellulose on all sides. In many instances these wall deposits have encroached upon the intercellular space, even to the point of completely filling it, and therefore obliterating it. Collenchyma of this type is called lacunate collenchyma. Is there any evidence here of lignification of suberization of walls? Do these cells have protoplasmic contests? Three magnifications and two specimens allow for better overall view, then closeup view.
PHOTO STUDY 7-2 Malva (common mallow): stem cortex t.s.
Collenchyma as seen in a transverse section of a young stem. In this type of collenchyma, walls are thickened wherever they border on an intercellular air space. Here and there you will see small air spaces with thick deposits of cellulose on all sides. In many instances these wall deposits have encroached upon the intercellular space, even to the point of completely filling it, and therefore obliterating it. Collenchyma of this type is called lacunate collenchyma. Is there any evidence here of lignification of suberization of walls? Do these cells have protoplasmic contests? Three magnifications and two specimens allow for better overall view, then closeup view.
PHOTO STUDY 7-2 Malva (common mallow): stem cortex t.s.
Collenchyma as seen in a transverse section of a young stem. In this type of collenchyma, walls are thickened wherever they border on an intercellular air space. Here and there you will see small air spaces with thick deposits of cellulose on all sides. In many instances these wall deposits have encroached upon the intercellular space, even to the point of completely filling it, and therefore obliterating it. Collenchyma of this type is called lacunate collenchyma. Is there any evidence here of lignification of suberization of walls? Do these cells have protoplasmic contests? Three magnifications and two specimens allow for better overall view, then closeup view.
PHOTO STUDY 7-3 Helianthus (sunflower): stem cortex t.s.
Collenchyma tissue and epidermis as seen in a transverse section of a young stem. In this stem there is a third type of collenchyma, the plate type. In the two or three outer cell layers of the tissue, see that the cells have thickened both their inner and their outer tangential walls, leaving the radial walls unthickened. Note the banded (plate-like) appearance that such a growth habit produces. Deeper in the tissue (five or six cells under the epidermis) you can see that the plate-type shades off into the angular type. Do these cells have cytoplasm? Nuclei? Plastids? Are walls lignified? Is cell shape in the banded area the same as that in the angular area? How do epidermal cells compare, in size and shape, with collenchyma cells?
PHOTO STUDY 7-4 Helianthus (sunflower): stem cortex l.s.
Here the stem cortex is shown in longitudinal section. See that the walls are unevenly thickened. Some walls are represented radially, some tangentially. Which are thickened? Hint: View previous photo in transverse section. PHOTO STUDY 7-5 Apium (celery): leaf petiole t.s.
In this lower power view of a ridge, see how the distribution of collenchyma tissue here differs from its distribution in the stems you have studied. This is called strand collenchyma, the reason for which is obvious. It is the leaf petiole of celery that is used for food, and when the collenchyma strands are tough, they make the celery petioles stringy. Is this collenchyma the angular, lacunate, or plate type? The abaxial surface of the petiole is ridged. Where are the collenchyma strands with reference to the ridges? Then view a third photo of a vascular strand in the petiole. Does the bundle have a cap of thickened cells? These are also strand collenchyma. What do you consider the function of the cap collenchyma cells to be?
PHOTO STUDY 7-5 Apium (celery): leaf petiole t.s.
In this HP photo of a celery petiole ridge, the nature of the collenchymatous thickenings are more obvious. PHOTO STUDY 7-5 Apium (celery): leaf petiole t.s.
This photo shows collenchyma restricted to a bundle cap at the outer edge of a vascular bundle. These are also strand collenchyma. Are they annular, plate, or lacunar? What do you consider the function of the cap collenchyma cells to be? PHOTO STUDY 7-6 Tilia (basswood): macerated bark of stem (bast fibers)
Shown in low power, conspicuous among the scattered cells are the deeply stained bast (phloem) fibers: these are sclerenchyma cells. How long are they in proportion to their width or thickness? Are the ends square, or rounded, or oblique, or tapering? Walls of course, are lignified. View the high power photo for better detail. Are walls pitted? Are protoplasts present? Note the very narrow lumen. Notice cross “walls” in one of the fibers. These are not true walls, but septa, for they do not have the compound nature of abutting walls.
PHOTO STUDY 7-6 Tilia (basswood): macerated bark of stem (bast fibers)
View the high power conspicuous among the scattered cells are the deeply stained bast (phloem) fibers: these are sclerenchyma cells. How long are they in proportion to their width or thickness? Are the ends square, or rounded, or oblique, or tapering? Walls of course, are lignified. View the high power photo for better detail. Are walls pitted? Are protoplasts present? Note the very narrow lumen. Notice cross “walls” in one of the fibers. These are not true walls, but septa, for they do not have the compound nature of abutting walls. PHOTO STUDY 7-7 Tilia Section 1 (basswood): t.s. 2 or 5-year stem phloem, bast fibers
Compare fibers close to the cambial zone (bordering the lowest level of lignified fibers) with those out near the cortex (the upper part of the photo). Account for differences in wall thickness and staining reactions. Note that some fibers have the lumen almost completely obliterated by the thickened wall. In the HP photo, notice the pit pair in the center of the photo. This is a simple pit pair.
PHOTO STUDY 7-7 Tilia Section 2 (basswood): t.s. 2 or 5-year stem phloem, bast fibers
In high power, compare fibers close to the cambial zone (bordering the lowest level of lignified fibers) with those out near the cortex (the upper part of the photo). Account for differences in wall thickness and staining reactions. Note that some fibers have the lumen almost completely obliterated by the thickened wall. In the HP photo, notice the pit pair in the center of the photo. This is a simple pit pair.
PHOTO STUDY 7-8 Prunus (peach): macerated fruit stone sclereids
Fragments of the stone have been treated with reagents that have dissolved the cementing substance between cells, allowing the latter to fall apart so they can be studied singly. Compare numerous cells as to size and shape. See that they are alike in the great thickness of the cell wall and canal-like pits. Note the small size of the cell lumen and the absence of protoplasmic contents. These cells were dead, of course, in the mature stone. In the lower right center cell, you should see that the thick wall is stratified (layered). Such cells as these are called stone cells, or sclereids.
PHOTO STUDY 7-9 Prunus (peach): sectioned pericarp
Now study a thin section of peach stone, and see how sclereids appear when they constitute a sclerenchyma tissue. See how pits on opposite sides of a compound middle lamella match one another in relative position. Are pits simple, or branched? Such branched pits are called ramiform pits. Can you describe how a ramiform pit pair is formed during cellular development?
PHOTO STUDY 7-10: Aristolochia (Dutchman’s pipe): t.s. stem with sclereids
Interpret this slide in terms of what you learned about sclereids from Prunus. Note the numerous simple pits in the end walls. Note the nuclei and cytoplasm in some of the cells. Is it possible that such thick-walled cells can be living? How?
PHOTO STUDY 7-1 Cucurbita (squash): t.s. stem cortex, collenchyma tissue
Beneath the epidermis is a tissue, three to several cells in thickness, which is characterized by wall thickenings at the cell angles. This is collenchyma tissue, and since the thickenings are restricted mainly to the angles, it is called angular collenchyma. Do the cells of this tissue have protoplasmic contents, or are they empty and dead? Are there intercellular spaces? Do the thick areas of the cell wall show indications of lignin (red staining) deposits?
|
