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| Green algae
Land plants (embryophytes)
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Plants are one of the five kingdoms used to classify all life. The plant kingdom, known as Plantae is divided into progressively smaller subgroups, usually based on their family tree. The primary division is between vascular and non-vascular plants - with the vascular plants being of most interest to gardeners.
Non-vascular versus vascular
Vascular plants - including both those that flower and those that don't have great diversity, since their adaptable root and shoot systems have given them the ability to grow in many habitats. Some, like ferns, reproduce by means of spores, which is how non-vascular plants reproduce as well, but the vast majority (over 250,000 species) reproduce by seed. Non-vascular plants are primitive types such as liverworts and mosses, which lack conductive tissue to circulate water and nutrients. This limits them to moist environments. Widespread in the wild, but of limited value in gardens due to their smaller size and somewhat dull appearance.
Plants that bear seed
Vascular plants that do bear seed are divided into gymnosperms ("naked seed") and angiosperms (meaning "covered seed"). Gymnosperms produce seed which are only partly enclosed by tissues from the parent plant. Conifers normally bear seed on the scales of the cones they produce, and form the largest family, which includes some 550 species. Many, like the pines (Pinus) and spruces (Picea) are very tolerant of heat, cold, or drought, which makes them very important, horticulturally. Other gymnosperms under cultivation include cycads and ginkgos.
Lifespan
Flowering plants are divided by life span into the following categories: annuals, biennials and perennials. Annuals live for one season of growth. Biennials live for two seasons, usually using the first to produce foliage and gather food reserves, and the second season usually comprises flowering and fruiting before dying. Perennials live for several seasons, usually flowering each year once established. Some perennials have been known to live thousands of years.