Courses in botany and plant science often address organisms that are not, in the strict sense, plants but that nevertheless are appropriately studied in the same context. Hence, although bacteria differ from plants primarily because of their cells, which are prokaryotic (lacking a nucleus and most cytoplasmic organelles), they are often studied in botany courses. Bacteria are early and evolutionarily significant organisms. Some are closely related to the protists known as algae and therefore important in the study of photosynthesis.
Fungi, too, are often studied in the context of plant science. These are mostly multicellular fila mentous eukaryotic organisms lacking chlorophyll. Because they do not make their own food but live in or on the food provided by plant and animal tissues, fungi are heterotrophs (rather than autotrophs, like plants, which make their own food through photosynthesis). In some ways, therefore, fungi are more similar to animals than they are to plants. Nevertheless, they are traditionally studied in the context of plant courses because they were once considered to be plants, given their lack of movement and other gross similarities. Their world significance parallels that of bacteria. They, too, are important as decomposers, returning nutrients and other elements to the environment. The study of fungi is mycology.
Protists are unicellular eukaryotes, forming one of the four kingdoms of Eukarya, the others being fungi, plants, and animals. Included among the protists are slime molds and protozoans which, lacking chlorophyll, are said to be heterotrophic protists, obtaining their food from other sources, generally other organisms. Also included are the algae, which are autotrophic eukaryotes many using photosynthesis, like plants, to generate their own food. For this reason, protists are often studied in the context of plant science, and algae are almost always included in such studies. The study of algae is phycology.
Viruses, the study of which is virology, are non-cellular entities that can reproduce only inside specific host cells. Not generally considered to be living, they are nevertheless important because of the infections they cause. To the extent that they cause infection in plants, they are important in the study of plant science, particularly plant pathology.
See also: Morphology, Anatomy, Cytology, Physiology, Genetics, Molecular Biology, Ecology, Pale botany
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