|
|
PHOTO UNIT 2: THE
PLANT CELL
OBJECTIVES:
1.
Recognize the following cellular structures:
a.
Plastids
b.
Nucleus and nuclear envelope
c.
Nucleolus
d.
Cell wall (and middle lamella) and compound middle lamella
e.
Plasmodesmata, primary pit fields, primary pits, and secondary
pits
f.
Vacuole(s)
2.
Interpret the marginal area of two adjacent cells including
development of the middle lamella and the primary and secondary walls of
adjacent cells.
3.
Recognize the function and significance of pit fields in primary
cell walls.
4.
Relate plasmodesmata to wall features and primary pit fields.
5.
Explain the origin of plasmodesmata and their importance to cell
function.
Photos for this study:
2-1: Smilax
(Carrion flower): Transverse
section old root cortex
2-2: Smilax
(Carrion flower): Transverse
section old root cortex (subepidermal)
2-3: Diospyros
(Persimmon): Sectioned seed
endosperm
2-4: Diospyros
(Persimmon): Sectioned seed endosperm (980x)
2-5: Phoenix
(Palm): Sectioned seed endosperm
2-6: Phoenix
(Palm): Sectioned seed endosperm
(980x)
PHOTO STUDY 2-1 Smilax (Carrion flower): transverse section, old root cortex
At several cells beneath the epidermis, identify middle lamella. Although it is a barely distinguishable line between adjacent cells, it becomes more perceptible at a point where two adjoining cells separate to enclose an air space. This is the cementing substance called the middle lamella. On opposite sides are primary walls of adjoining cells. The double nature of the "wall" is seen where adjoining cells separate at an air space. PHOTO STUDY 2-2 Smilax (Carrion flower): transverse section, old root cortex.
Observe cortical cells that abut on the epidermis. Here there is a secondary layer of wall in addition to the primary one. See how much thicker the wall is on the side against the epidermis than on the side facing the deeper cortex. This extra thickness results from a second layer of wall material that was laid against the inner surface of the primary or first wall. Note that the middle lamella here is deeply stained. When primary walls, and the middle lamella between them, are all stained alike so that it is impossible to distinguish among them, the three layers together are referred to as the compound middle lamella. Note in the cortex large cells with crystalline inclusions. These are actually cylindrical clusters of calcium oxalate, seen transversely sectioned (above) and diagonally sectioned (below).
PHOTO STUDY 2-3 Diospyros (Persimmon): Sectioned seed endosperm (430x)
Here you see greatly thickened walls and, extending through them, very slender threads of deeply stained cytoplasm, the plasmodesmata. When these cells are made to swell, plasmodesmata become visible. Cell walls in this tissue are wholly primary, even though they are thick.
PHOTO STUDY 2-4 Diospyros (Persimmon): Sectioned seed endosperm (980x)
Does the middle lamella constitute an interruption of the plasmodesmata, or do they extend right through it? In some cells the middle lamella may be perceptible. All living plant cells, in a multicellular body, are connected with one another by means of these strands of living substance, but in most instances the strands are too short and too slender to be as evident as those of Diopspyros.
PHOTO STUDY 2-5 Phoenix (Palm): Sectioned endosperm of a seed (430x)
In this endosperm tissue, the cells do have both primary and secondary walls. You should be able to distinguish between them. Can you see the middle lamella? Observe the canal-like pits in the cell wall, each pit occupied by protoplasm. Does a pit extend through the secondary wall, or the primary wall, or both?
PHOTO STUDY 2-6 Phoenix (Palm): Sectioned endosperm of a seed (980x)
Directly across the wall from any pit, discover another pit in an adjoining cell, matching this one in position. Two such pits constitute a pit-pair. Between the two pits of a pit pair, plasmodesmata, although numerous, are not visible because of the dark stain. A pit does not extend through the primary wall, but only through the secondary, So, between the two pits of a pair there is a three-layered pit-closing membrane, consisting of two primary walls and the middle lamella.
PHOTO STUDY 2-1 Smilax (Carrion flower): transverse section, old root cortex
At several cells beneath the epidermis, identify middle lamella. Although it is a barely distinguishable line between adjacent cells, it becomes more perceptible at a point where two adjoining cells separate to enclose an air space. This is the cementing substance called the middle lamella. On opposite sides are primary walls of adjoining cells. The double nature of the "wall" is seen where adjoining cells separate at an air space.
|
