1.7.2—
Specialized Structures

Many of the modifications of cell wall structure which occur are related to communication or transport between cells. In cells with primary walls, intercellular communication is facilitated by plasmadesmata. Plasmadesmata are small pores of approximately 40 nm diameter which extend through the wall between two adjacent cells (Ledbetter & Porter, 1970; Fig. 1.7). The plasmalemma

Figure 1.7
Plasmadesmata of plant cell walls.
Plasmadesmata (Pd) in the end walls of collenchymal cells of a wheat
stamen filament. The 'core' material in the plasmadesmata can be seen
as an electron-dense dot in the centre of the plasmadesmatal pores.
(See Ledbetter & Porter, 1970. Reproduced courtesy of the authors.)

of the cell(s) also extends through these pores, so that the cytoplasm of one cell is continuous with that of adjacent cells. There appears to be a core of electron dense material in the center of the plasmadesmatal tubes (see Fig. 1.7). This core may be a specific (though as yet speculative) structure capable

Figure 1.8
Pits in tracheids of  Taxus canadensus.
A full bordered pit (BP) can be seen in cross-section in the upper left portion of the figure.
The faint line in the centre of the pit is the disc. The pits lie between tracheids (Tr), which are
long, cylindrical, cytoplasmless tubes that carry water from the roots to the leaves. The primary
cell walls and middle lamella can be seen as electron dense material running in a thin line between
cells and in the corners where 3 or 4 cells meet. Layering of the thick secondary wall material is
evident. Several half-bordered pits (BP/2) between tracheids and ray cells (RC) can be seen.
(See Ledbetter & Porter, 1970. Reporduced courtesy of the authors.)

Figure 1.9
Collenchymal cells.
The walls of these cells are unusually thick primary walls, heavily
hydrated and unlignified. Cellulose microfibrils can be seen throughout
the walls. These cells elongate very rapidly, and the walls are quite plastic.
(See Ledbetter & Porter, 1970. Reproduced courtesy of the authors.)

of regulating what can and cannot be passed between cells. Regardless of the nature of the core, it is clear that the plasmadesmata are an aspect of plant biology which has no obvious counterpart in animal tissues. Each cell may have thousands of plasmadesmata, either randomly scattered as individual

pores or grouped in distinct fields where the primary wall is often thinner than usual.

In cells with secondary walls, and particularly in tracheids, intercellular communication and transport is facilitated by pits (Ledbetter & Porter, 1970; Fig. 1.8). In the full, bordered type of pit, the primary wall and middle lamella are largely dissolved. The hole thus formed is surrounded and partly overgrown by depositions of secondary wall material which create a raised, overhanging border. The primary wall and middle lamella in the pit is replaced by a relatively impermeable disc or torus of thickened wall material. The disc is suspended by an easily permeable, radial network of cellulose microfibrils. The diameter of the disc is slightly larger than the aperture of the overhanging pit border. Thus the disc can act as a valve to close the pit when pressed against the border by a large pressure differential (Albersheim, 1965). Gas bubbles, which would break the flow of water through the tracheids, can be sealed off by the action of such valves.

Where the tracheids are adjacent to ray cells, the pits are differentiated only on the tracheid side, giving half-bordered pits (Fig. 1.8). The walls of the ray cells in the pit region appear to remain intact or only slightly modified.

A further important example of a cell wall modification which facilitates intercellular communication or transport is the sieve plate of primary phloem. The phloem cells have primary cell walls with many plasmadesmata. Some of the plasmadesmata of the crosswalls between adjacent phloem cells enlarge considerably and become lined with callose (a b -1,3 glucan) on the wall side of the plasmalemma. The pores of the sieve plate thus formed become filled with a fibrous protein (which may be analogous to the core material of plasmadesmata).

Many other specialized cell wall structures or modifications could be described (see also Fig. 1.9). Several examples may be mentioned just to indicate the range of possibilities: the walls of pollen cells, which are so indestructible that they are used by archeologists to characterize the flora of dwelling sites many thousands of years old; the walls of bark cells, made largely of suberin, a wall material similar to cutin that can form a protective coating over wounds so as to prevent water loss or pathogen invasion; and the walls between adjacent endodermal cells, which have very dense Casparian strips which prevent the movement of ions through the walls and into the xylem stream.

The wall structures and modifications described in this section may all be seen with the microscope—there are undoubtedly many other important differentiations of the cell wall which we cannot see.