Trees are an evolutionary adaptation to competition for space, by growing taller trees are able to compete better for sunlight.[24] They have modified structures that allow them to grow much taller and spread out their foliage, such as thicker stems that are composed of specialized cells that add structural strength and durability. They are long-lived perennial plants that can increase their size each year by producing woody stems. They differ from shrubs, which are also woody plants, by usually growing larger and having a single main stem;[11] but the distinction between a small tree and a large shrub is not always clear,[25] made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimated 100,000 species, the number of trees worldwide might total twenty-five percent of all living plant species.[26] Their greatest number grow in tropical regions and many of these areas have not yet been fully surveyed by botanists, making tree diversity and ranges poorly known.[27] Trees exist in two different groups of vascular or higher plants, the gymnosperms and the angiosperms. Both groups are seed plants. The gymnosperm trees include conifers, cycads, ginkgophytes and gnetales. Angiosperm trees are also known as broad-leaved trees. Most angiosperm trees are eudicots, the "true dicotyledons", so named because the seeds contain two cotyledons or seed leaves. A relatively smaller number of other angiosperm trees are paleodicots; these include Amborella, Magnolia, nutmeg, avocado, and others. Wood gives structural strength to a tree stem which is used to support the plant as it grows larger. The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to g ow as large as they do. The three main parts of trees include the root, stem, and leaves; they are integral parts of the vascular system which interconnects all the living cells. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth. Trees are either evergreen, having foliage that persists and remains green throughout the year,[28] or deciduous, shedding their leaves at the end of the growing season and then having a dormant period without foliage.[29] Most conifers are evergreens but larches (Larix and Pseudolarix) are deciduous, dropping their needles each autumn, and some species of cypress (Glyptostrobus, Metasequoia and Taxodium) shed small leafy shoots annually in a process known as cladoptosis.[11] The crown is a name for the upper part[clarification needed] of a tree including the branches and leaves [30] and the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy.[31] A sapling is a young tree.[32] Tree-like plants include some palms which are not trees but herbaceous[19][20] monocots that do not undergo secondary growth and never produce wood,[21][22][22][23] and hence do not meet the definition of tree used in this article. In many tree-like palms, the terminal bud on the main stem is the only one to develop so they have tall, unbranched trunks with spirally arranged large leaves. Some of the tree ferns, order Cyatheales, have tree-like growth forms, growing up to 20 metres (66 ft) but they are structurally very different from other trees: their trunks are composed of rhizomes which grow vertically and which are covered by numerous adventitious roots.