Solutions in Terrestrial Plants
Most primitive aquatic plants (notably the algae), when examined under a microscope such as teaching microscopes, carry out gas exchange across almost the entire body surface; but in response to the problems of desiccation and large three-dimensional size, most terrestrial plants have evolved more elaborate mechanisms.
LEAVES
Gas exchange associated with both photosynthesis and cellular respi¬ration takes place at a particularly high rate in green leaves, organs strikingly well adapted for this process as seen under teaching microscopes.
Most of the visible outer surface of a leaf, covered as it is by a waxy cuticle, is rather impermeable, and hence ill suited for diffusion of gases. Exchange must therefore take place elsewhere. The mesophyll parenchyma in a leaf, when studied using teaching microscopes, contains large intercellular spaces. A high percentage of the total surface of each mesophyll cell is exposed to the air in these spaces, which are interconnected and are continuous with the exter¬nal atmosphere by way of openings in the epidermis, the stomata (this can be viewed through a microscope). Gases can thus move easily between the surrounding atmosphere and the internal spaces of the leaf. The actual gas exchange manifested by the diffusion of gases into and out of living cells-takes place across the thin moist membranes of the cells inside the leaf.
How the structures of the leaf help it meet the four previously stated requirements for respiratory systems should be considered:
1. The surface area available for gas exchange in the leaf is very large. By comparison with the outer area of the leaf, the total area of cell membrane exposed to the intercellular spaces is enormous. The principle involved is a very elementary one: A chamber irregularly shaped and greatly subdivided by partial partitions will have far more wall space than a round or square one of equal volume.
2. Internal transport of gases occurs in leaves without any special adaptations. Gases can reach each individual cell directly via the in¬tercellular spaces.
3. The danger of mechanical injury is relatively minor for an in¬ternal exchange surface. The epidermis, with its various hairs or spines, functions as a protective covering for the entire leaf.
4. The exchange surfaces remain moist because they are exposed to air only in intercellular spaces. With the humidity within those spaces nearly 100 percent, the membranes of the mesophyll cells always re¬tain a thin film of water on their surfaces. Gases dissolve in this water before moving into the cells. The protective epidermal tissues and the layers of waxy cuticle on their outer surfaces act as barriers between the dry outside air and the moist inside air.
But these barriers are not complete; if they were, movement of gases between the outside and the inside could not take place. So although openings are essential, the stomata in a sense constitute weak links in the protective armor of the leaf. Here we see the sort of compromise that has been a constant feature of evolutionary adaptation. Few characters, however beneficial, are without possible deleterious ef¬fects. What determines the evolutionary fate of a character is not whether it is exclusively beneficial or harmful, but whether or not the beneficial effects outweigh the harmful ones. In this case, stomata present advantages that outweigh the danger of desiccation; moreover, other adaptations minimize that danger.
The two highly specialized epidermal cells called guard cells can be seen under a microscope. Unlike most other epidermal cells, these guard cells contain chloroplasts, bound each opening, or stoma, in the epidermis. These bean-shaped cells have walls of unequal thickness; the walls next to the stoma are considerably thicker than those on the side away from the stoma. When the guard cells take up water and are turgid (usually in the light), the thin outer wall of each cell buckles outward, pulling the rest of the cell with it and opening the stoma. In the dark the reverse usually occurs: The cells lose water, and as they become flaccid, their thick inner walls close the stoma.
When the stomata are open, gas exchange can take place, but at the same time the plant loses water by evaporation, in the process called transpiration, as seen when specimens are examined under a microscope. Thus the requirements of water conservation conflict with those of photosynthesis. But imperfect as the regulation of the stomata may be from the point of view of water conservation, it is in fact remarkably good; the rate of water loss through the stomata is much lower than the rate of carbon dioxide uptake. And though most plants lose large quantities of water by transpiration every day, the humidity’ in the intercellular spaces of the leaf probably does not often drop ap¬preciably, because the lost water is steadily replaced by water drawn up through the stem and distributed throughout the leaf in the many small veins, as seen when these are studied under a microscope.
