Rainforests appear chaotic, but are quite highly organized into vertical strata to a degree unknown in temperate forests. (It should be noted that some ecologists do not recognize the presence of strata in rainforests). There are the tallest trees, or “emergents,” sometimes more than 50 meters in height, which appear at irregular intervals above the almost continuous mid-layer. These largest trees, comprising less than 10% of forest trees, contain approximately half of the above-ground biomass of the forest. Emergent species require much light and can tolerate high temperatures and the drying effects of direct sunlight. The trees in the understory layers are very numerous and competitive and have less expansive crowns and more slender trunks than do emergents. Still closer to the forest floor are small trees and shrubs, and, on the forest floor, there are a variety of non-woody plants, seedlings, and herbs. The forest is a mosaic composed of multitudes of small groupings, based on their histories. For example, wherever a gap has formed, either by treefalls or some other mechanism, a new grouping arises, unique within the forest. The composition of these small units depends upon the size of the gap, the availability of light, the temperature and humidity, soil type, and the kinds of seedlings or seeds available. The forest structure reflects the necessities of life for plants: the need to support themselves, the requirement for sufficient light for photosynthesis, and their pattern of continuous growth. Young trees will have slender trunks and grow rapidly toward the light, allocating most of their resources to growth rather than to reproduction. Only when a tree has reached a height where it has sufficient light can it afford to expend energy on flowering and fruiting and to reduce its growth rate. At this point tall canopy trees make large crowns to support their more massive root and trunk structures, whereas in the understory there is no space in the crowd for a large crown.