Stems And Roots
Gas exchange is not confined to the leaves, as evident when leaves are studied under a microscope such as children’s microscopes, though these organs are beautifully adapted for this process. In older stems, gas exchange gen¬erally takes place through numerous lenticels, which are groups of loosely arranged cells with many intercellular spaces between them. Since most of the cells in the inner layers of large stems are dead, there is little need for oxygen in the intercellular air spaces to penetrate deep into the stem.
Roots also carry out gas exchange, though they usually possess no special structures for this function. Gases can diffuse readily across the moist membranes of root hairs and other epidermal cells. For roots to obtain enough oxygen, however, the soil in which they grow must be well aerated. One of the benefits of hoeing, raking, plowing, or otherwise cultivating the soil is the increased air circulation this activity makes possible.
Plants, unlike animals, do not seem to need any special gas-trans¬porting mechanisms. Most of the intercellular spaces in the tissues of land plants are filled with air, by contrast with those in animal tissues, which are filled with fluid, as seen under microscopes such as children’s microscopes. These air-filled spaces are interconnected to form an intercellular air-space system that opens to the outside through stomata and lenticels and penetrates to the innermost parts of the plant body. Thus incoming gases can move in gaseous form di¬rectly to the internal parts of the plant from the environmental atmo¬sphere without having to cross membranous barriers; and they do not have to diffuse long distances through water or cell fluids, because they do not go into solution until they reach the film of water on the surfaces of the individual cells. Since oxygen can diffuse some 300,000 times faster through air than through fluids, the intercellular air-space system ensures that all cells, even the more internal ones are adequately supplied.
Solutions in Aquatic Animals
As seen under microscopes such as children’s microscopes, unicellular animals have no special gas-exchange devices; simple diffusion across their cell membranes is sufficient. Some of the smaller and simpler multicellular animals, like jellyfish, hydra, and planaria, show little further devel¬opment, though their multipurpose gastrovascular cavities do facili¬tate the exposure of the more internal cells to the environmental water (containing dissolved oxygen) that they draw in through the mouth; no cell in these animals is far from the water medium. A few larger aquatic animals, particularly some of the marine segmented worms when examined under a microscope, lack special respiratory systems and use the skin of the general body surface, which is usually richly supplied with blood vessels. Most larger multicellular animals, however, when examined under a microscope, have evolved true respiratory systems.
GILLS
With a few exceptions, the respiratory systems of multicellular aquatic animals involve evaginated exchange surfaces, usually known as gills. Gills vary in complexity all the way from the simple bumplike skin gills of some sea stars, the flaplike parapodia of many segmented marine worms, or the mantle-protected gills of squids, to the minutely subdivided gills of fish. Such diverse animals as clams and lobsters and salamanders possess gills, these having evolved independently countless times in the history of animal life on earth.
Most gills, particularly those of very active animals, have such finely dissected surfaces that a few small gills may expose an immense total exchange surface to the water. Hence, though the gas-exchange surface takes up a very limited part of the animal body, relatively impermeable coverings which can only be seen under a microscope can thus protect most of which, the surface-to-volume ratio of the exchange surface remains high.
Another characteristic of most gills that was discovered when viewed under a microscope is that they contain a rich sup¬ply of blood vessels. Often the blood in these vessels is separated from the external water by only two cells: the single cell of the wall of the vessel and a cell of the gill surface. Oxygen moves by diffusion from the water, across the intervening cells, and into the blood, where a carrier pigment ordinarily picks it up. The blood then distributes the oxygen throughout the body to the individual cells. Carbon dioxide produced by cellular metabolism moves in the opposite direction, being trans¬ported to the gills and discharged into the surrounding water.
One intriguing feature of the arrangement of exchange of oxygen and carbon dioxide between water and blood in fish gills deserves special mention. When sample gills were examined under a microscope, it was discovered that the water flows over the surface of a gill lamella in a direction opposite to the flow of blood in the vessels of the lamella. This countercurrent exchange system maximizes the amount of oxygen the blood can pick up from the water.
Obtaining enough oxygen is a greater problem for aquatic animals than for air breathers, for two reasons: First, oxygen has a low solubility in water, constituting only about 0.004 percent of seawater. Second, the diffusion of oxygen is many thousands of times slower in water than in air. Most fish actively pump water into the mouth, across the gill fila¬ments, and out behind the operculum.
Biology is a science of exceptions. Not all aquatic animals with special respiratory systems use evaginated gills, and not all animals that live in water are fully aquatic. Many insects and some mammals that live in water, for instance, must periodically come to the surface to breathe air.
