Difference Between Algae and Plants
For many centuries, scientists lumped algae into the Plant Kingdom. Even today, most people think of them as “water plants.” But in recent years, powerful microscopes and chemical tests confirm what scientists had suspected: despite many similarities, algae are not true plants. But why not?
First, algae are built differently than are plants. Algae lack specialized plant tissues such as roots, leaves, and stems. Nor are their cell walls always made of cellulose, as are those of all plants. Algae also possess a far greater variety of photosynthetic pigments than do plants. Many of these are designed to catch the wavelengths of light that penetrate the water in which algae live.
Second, algae reproduce quite differently than do plants. Most algae are equipped with animal-like sex cells that actively swim through the water with whiplike tails. Algae also lack the special egg- and sperm-producing structures found in all true plants.
For these and other reasons, scientists today place algae in the Kingdom Protista, along with other simple organisms such as slime molds, diatoms, and protozoa. Like algae, many of these organisms have both animal-like and plantlike traits. This is not to say that algae and plants are not closely related. In fact, the first true plants most likely evolved from some type or types of green algae—much as the first land animals evolved from some type of fish. True plants then evolved the many special adaptations (roots, stems, leaves, and the like) needed to conquer solid land.
Size and Distribution
In size, algae range from the tiny, one-celled swimmer Micromonas (0.0004 inch—0.010 millimeter in diameter) to giant ropes of sea kelp more than 200 feet (60 meters) long. Like Micromonas, many algae consist of just one cell. Others have two or three cells, and the largest have millions. The larger algae have simple tissues designed for specialized functions such as photosynthesis or anchorage. Unlike plant roots, algae anchors do not absorb water or nutrients from the soil.
In general, algae found on land are limited by their need for constant wetness and warmth. In contrast, algae that live immersed in water have boundless moisture and even temperatures. Their growth can be limited by lack of sunlight, which is filtered, or absorbed, by water, and by a scarcity of nutrients. This is why certain types of pollution—which supply nutrients—cause sudden overgrowths, or algal “blooms,” which we see as green “scum” or “red tides.”