Department Handouts

Plant Classification

Thesis: According to their method of obtaining food, plants are classified into three basic groups—autotrophic, heterotrophic, and mutualistic.

I.   Methods of Classification

 II.   Classification of Autotrophic Plants
       A.   Phototrophs
        B.   Chemotrophs
III.   Classification of Heterotrophic Plants
        A.   Parasites
        B.   Saprophytes
        C.   Phagotrophs
IV.   Classification of Mutualistic Plants
        A.   Plants
        B.   Animals
V.   Purpose of Classification

The classification of plants goes back to antiquity. As man became more advanced in his scientific knowledge, the grouping of plants into various classes became a necessity. At first plants were only classified according to a few convenient morphological characteristics. But later, following the invention of the microscope, botanists began to categorize plants according to their physiological activities also. Today one division of physiological classification is concerned with the way in which an organism acquires food. According to their method of obtaining food, plants are classified into three basic groups—autotrophic, heterotrophic, and mutualistic.

Autotrophic plants are able to manufacture all of their own food, and they use one of two energy sources in their food-manufacturing process. Phototrophic organisms, ranging in size from one-celled aquatic plants to giant forest trees, use light energy in their food production. Each of these organisms uses carbon for cell synthesis, and the transformation of energy from light into chemical bond energy. This chemical bond energy is used in photosynthesis to produce food for the plant. Phototrophs include green plants such as corn, oak, sugar beet, and a few kinds of bacteria. Another classification of autotrophic plants is the chemotrophic plants. Chemotrophs use chemical energy for food manufacture by the oxidation of inorganic compounds. This group consists of nitrifying bacteria, sulfur bacteria, and iron bacteria. Nitrifying bacteria oxidize ammonia to nitrite, then to nitrate in a process known as nitrification.

The energy obtained from nitrification is used in building carbon dioxide into organic compounds. The phototrophs and chemotrophs comprise the autotrophic plant classification.

Another plant classification, according to the plant's method of obtaining food, is the heterotrophic class. Heterotrophs are unable to manufacture their own food, thus requiring an external source of complex organic compound. The heterotrophs are divided into three basic groups—the parasites, the saprophytes, and the phagotrophs. Parasites get their food in dissolved form from the tissues of plants or animals at the expense of the host. However, the degree of parasitism varies greatly. Parasitic mistletoe obtains very little food from the host. At the other extreme of parasitism, some plants such as broomsape get all of their nourishment from the host. The parasites also include dooder, squaroot, and many bacteria and fungi. A second division of the heterotrophic group is the saprophytes which get their food in dissolved form from the non-living organic remains of plants and animals. The saprophytes include puffballs, mushrooms, many molds and most bacteria. Bacteria secrete extra cellular enzymes that act upon insoluble food materials. The soluble materials then move into the cell by diffusion or by active transport. The third division of heterotrophs is the phagotrophs. Plants in this category ingest their food in solid or particulate form. The phagotrophic group is comprised of animal-like flagellates and slime molds. The Carteria flagellate obtains its food in particulate form through a mouthpart and gullet. The heterotrophic group of plants is thus comprised of parasites, saprophytes and phagotrophs.

Some plants procure food through autotrophic channels, and some plants obtain food through heterotrophic means. However, plants may also employ mutualistic means of obtaining food. Mutualistic plants live in more or less intimate association with another organism to their mutual benefit. Some plants live in a mutualistic relationship with other plants. A classic example of this relationship is exhibited by flowering plants and their insect pollinators. Bees and other insects visit flowers and gather pollen or nectar for food. As a result of their visit some of the pollen may stick to the body of the insect. Later when this same insect visits another flower, some of the pollen may adhere to the stigma, causing cross-pollination. Plants may enter into mutualistic association with other plants or with animals.

Depending upon their method of obtaining food, plants may be classified as autotrophic, heterotrophic, or mutualistic. The grouping of plants into these three classes is beneficial in the systematic classification process employed by botanists in research projects. The knowledge of a particular plant's method of obtaining food is helpful in studying the effects of outside influences. For example, in studying a phototrophic plant a botanist could test to find out which type of light is best for the plant. The theoretical research that is done by the botanist can be employed by the farmer to help yield a bigger and better food production for the world's expanding population.