Live Plant Specimens
Exploring the diversity and evolution of the plant kingdom.
Overview
Plants are a major group of living organisms that include familiar forms such as trees, flowers, herbs, and ferns. It is estimated that approximately 350,000 plant species exist worldwide. As of 2004, roughly 287,655 species had been identified, with flowering plants accounting for the majority of known species.
Historical Classification
Early attempts to classify living organisms divided life into plants and animals, a distinction famously proposed by Aristotle. Later, Linnaeus formalized this division into the kingdoms Vegetabilia (later Plantae) and Animalia. Modern research has shown that the original concept of Plantae included several unrelated groups. Fungi and multiple algal lineages were later reassigned to separate kingdoms, though they are still often referred to as plants in informal contexts.
Defining Characteristics
Most plants are multicellular land organisms known as embryophytes. This group includes vascular plants, which possess true leaves, stems, and roots, as well as closely related non-vascular plants commonly referred to as bryophytes, such as mosses and liverworts.
Plant cells are eukaryotic and have cell walls composed primarily of cellulose. Most plants produce their own food through photosynthesis, using light energy and carbon dioxide. A small number of plant species are non-photosynthetic and instead act as parasites on other plants. Plants are distinguished from green algae by specialized reproductive structures that protect developing reproductive cells.
Bryophytes
Bryophytes first appeared during the early Paleozoic era and typically require moist environments to survive. These plants remain relatively small throughout their life cycle and exhibit an alternation of generations between a dominant haploid gametophyte and a short-lived diploid sporophyte that depends on the gametophyte for nutrients.
Vascular Plants
Vascular plants emerged during the Silurian period and diversified rapidly by the Devonian period, spreading into a wide range of terrestrial environments. Adaptations such as vascular tissues for internal water transport and a protective cuticle allowed these plants to overcome the limitations faced by bryophytes. In vascular plants, the sporophyte typically functions as an independent organism, while the gametophyte stage is greatly reduced.
Educational Resources
For classroom use and further study, the PLANTS Database maintained by the United States Department of Agriculture provides standardized information on vascular plants, mosses, liverworts, hornworts, and lichens found throughout the United States and its territories.