Types of Trees

There are approximately 65,000 to 70,000 different species of trees on our planet.

The 2 main factors to take into account when sorting trees types is the duration of the leaves and the type of flowers the tree produces.

When it comes to leaf duration, trees can be classified one of 2 ways.

EVERGREEN TREE TYPES

In botany, an evergreen tree type or plant is a plant that has leaves in all seasons. This contrasts with deciduous tree type or plants, which completely lose their foliage during the winter or dry season.

There are many different kinds of evergreen plants, both trees and shrubs. Evergreens include: most species of conifers (e.g., hemlock trees, blue spruce tree , red cedar, and white/scots/jack pine trees) live oak, holly, and "ancient" gymnosperms such as cycads
most angiosperms from frost-free climates, such as eucalypts and rainforest trees
An additional special case exists in Welwitschia, an African gymnosperm plant that produces only two leaves, which grow continuously throughout the plant's life but gradually wear away at the apex, giving 20 to 40 years' persistence of leaf tissue.

Leaf persistence in evergreen plants varies from a few months (with new leaves constantly being grown as old ones are shed) to several decades (over thirty years in the Great Basin Bristlecone Pine).

Deciduous trees shed their leaves usually as an adaptation to a cold or dry season. Most tropical rainforest plants are evergreens, replacing their leaves gradually throughout the year as the leaves age and fall, whereas species growing in seasonally arid climates may be either evergreen or deciduous. Most warm temperate climate plants are also evergreen. In cool temperate climates, fewer plants are evergreen, with a predominance of conifers, as few evergreen broadleaf plants can tolerate severe cold below about minus 30 degrees celsius.

In areas where there is a reason for being deciduous (e.g. a cold season or dry season), being evergreen is usually an adaptation to low nutrient levels. Deciduous trees lose nutrients whenever they lose their leaves. In warmer areas, species such as some pines and cypresses grow on poor soils and disturbed ground. In Rhododendron, a genus with many broadleaf evergreens, several species grow in mature forests but are usually found on highly acidic soil where the nutrients are less available to plants. In taiga or boreal forests, it is too cold for the organic matter in the soil to decay rapidly, so the nutrients in the soil are less easily available to plants, thus favouring evergreens.

In temperate climates, evergreens can reinforce their own survival; evergreen leaf and needle litter has a higher carbon-nitrogen ratio than deciduous leaf litter, contributing to a higher soil acidity and lower soil nitrogen content. These conditions favour the growth of more evergreens and make it more difficult for deciduous plants to persist. In addition, the shelter provided by existing evergreen plants can make it easier for younger evergreen plants to survive cold and/or drought.

Evergreen plants and deciduous plants have almost all the same diseases and pests, but long-term air pollution, ash and toxic substances in the air are more injurious for evergreen plants than deciduous plants (for example spruce Picea abies in European cities).

Owing to the botanical meaning, the term "evergreen" can refer metaphorically to something that is continuously renewed or is self-renewing.

DECIDUOUS TREE TYPES

Deciduous means falling off at maturity or tending to fall off and is typically used in reference to trees or shrubs that lose their leaves seasonally and to the shedding of other plant structures such as petals after flowering or fruit when ripe. In a more specific sense deciduous means the dropping of a part that is no longer needed, or falling away after its purpose is finished. In plants it is the result of natural processes. Deciduous has a similar meaning when referring to animal parts such as deciduous antlers in deer, or deciduous teeth, also known as baby teeth, in some mammals (including human children).

In botany and horticulture, deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year. This process is called abscission. In some cases leaf loss coincides with winter - namely in temperate or polar climates. While in other areas of the world, including tropical, subtropical and arid regions of the world, plants lose their leaves during the dry season or during other seasons depending on variations in rainfall.

The converse of deciduous is evergreen, where green foliage is persistent year round. Plants that are intermediate, may be called semi-deciduous; they lose old foliage as new growth begins. Other plants are semi-evergreen and lose their leaves before the next growing season, but retain some during winter or during dry periods. Some trees, including a few species of oak have desiccated leaves that remain on the tree through winter; these dry persistent leaves are called marcescent leaves and are dropped in the spring as new growth begins.

Many deciduous plants flower during the period when they are leafless as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of pollen for wind-pollinated plants, and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Nevertheless, there is much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during the cold winter days.

the deciduous characteristic has developed repeatedly among woody plants. Trees include Maple, many Oaks, Elm, Aspen, and Birch, among others, as well as a number of coniferous genera, such as Larch and Metasequoia. Deciduous shrubs include honeysuckle, viburnum, and many others. Most temperate woody vines are also deciduous, including grapes, poison ivy, virginia creeper, wisteria, etc. The characteristic is useful in plant identification; for instance in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side.

Periods of leaf fall often coincide with seasons: winter in the case of cool-climate plants or the dry-season in the case of tropical plants, however there are no deciduous species among tree-like monocotyledonous plants, e.g. palms, yuccas, and dracenas.

Forests where a majority of the trees lose their foliage at the end of the typical growing season are called deciduous forests. These forests are found in many areas worldwide and have distinctive ecosystems, understory growth, and soil dynamics.

Two distinctive types of deciduous forest are found growing around the world.

Temperate deciduous forest biomes are plant communities distributed in North and South America, Asia and Europe. They have formed under climatic conditions which have great seasonable temperature variability with growth occurring during warm summers and leaf drop in autumn and dormancy during cold winters. These seasonally distinctive communities have diverse life forms that are impacted greatly by the seasonality of their climate, mainly temperature and precipitation rates. These varying and regionally different ecological conditions produce distinctive forest plant communities in different regions.

Dry-season deciduous tropical forestTropical and subtropical deciduous forest biomes have developed in response not to seasonal temperature variations but to seasonal rainfall patterns. During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. Leaf drop is not seasonally dependent as it is in temperate climates, and can occur any time of year and varies by region of the world. Even within a small local area there can be variations in the timing and duration of leaf drop; different sides of the same mountain and areas that have high water tables or areas along streams and rivers can produce a patchwork of leafy and leafless trees.

You can also compare trees by the types of flowers they produce.

Flowerless Trees - Gymnosperms.

The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos, meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the seeds or ovules of flowering plants (angiosperms) which are enclosed during pollination. Gymnosperm seeds develop either on the surface of scale- or leaf-like appendages of cones, or at the end of short stalks (Ginkgo).

The gymnosperms and angiosperms together comprise the spermatophytes or seed plants. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, Gnetales (Gnetophyta, Ephedra and Welwitschia), and Ginkgo (a single living species).

In early classification schemes, the gymnosperms (Gymnospermae) were regarded as a "natural" group. There is conflicting evidence on the question of whether the living gymnosperms form a clade. The fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology (the so-called seed ferns or pteridosperms.) When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids are considered, it is clear that angiosperms are nested within a larger gymnosperm clade, although which group of gymnosperms are their closest relatives remains unclear.

It is widely accepted that the gymnosperms originated in the late Carboniferous period. Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 380 million years ago. It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms were by extinct species of scorpionflies that had specialized proboscis for feeding on pollination drops. The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms.

Gymnosperms have major economic uses. Pine, fir, spruce and cedar are all examples of conifers which we use for lumber. Some other common uses for them are as soap, varnish, paint, food and perfumes.

Gymnosperms are spore-bearing plants (sporophytes), with a sporophyte-dominant life cycle; as in all other vascular plants the gametophyte (gamete-bearing phase) is relatively short-lived. Two spore types, microspores and megaspores, are generally produced in pollen cones or ovulate cones, respectively. A short-lived multicellular haploid, gamete-bearing phase (gametophyte) develops inside the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells; megagametophyte tissue develops in the megaspore of each ovule, and produces multiple egg cells. Thus, megaspores are enclosed in ovules (unfertilized seeds) and give rise to megagametophytes and ultimately to egg cells. During pollination, pollen grains are physically transferred between plants, from pollen cone to the ovule, being transferred by wind or insects.

Flower Bearing Trees - Angiosperms

The flowering plants (angiosperms), also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies (derived characteristics). These characteristics include flowers, endosperm within the seeds, and the production of fruits that contain the seeds.

The ancestors of flowering plants diverged from gymnosperms around 245 to 202 million years ago, and the first flowering plants known to exist are from 140 million years ago. They diversified enormously during the Lower Cretaceous and became widespread around 100 million years ago, but replaced conifers as the dominant trees only around 60 to 100 million years ago.

The flowers, which are the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from other seed plants. Flowers aid angiosperms by enabling a wider range of adaptability and broadening the ecological niches open to them. This has allowed flowering plants to largely dominate terrestrial ecosystems.

Land plants have existed for about 425 million years. Early land plants reproduced sexually with flagellated, swimming sperm, like the green algae from which they evolved. An adaptation to terrestrialization was the development of upright meiosporangia for dispersal by spores to new habitats. This feature is lacking in the descendants of their nearest algal relatives, the Charophycean green algae. A later terrestrial adaptation took place with retention of the delicate, avascular sexual stage, the gametophyte, within the tissues of the vascular sporophyte. This occurred by spore germination within sporangia rather than spore release, as in non-seed plants.

A current example of how this might have happened can be seen in the precocious spore germination in Sellaginella, the spike-moss. The result for the ancestors of angiosperms was enclosing them in a case, the seed. The first seed bearing plants, like the ginkgo, and conifers (such as pines and firs), did not produce flowers. Interestingly, the pollen grains (males) of Ginkgo and cycads produce a pair of flagellated, mobile sperm cells that "swim" down the developing pollen tube to the female and her eggs.

The apparently sudden appearance of relatively modern flowers in the fossil record initially posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin. However, the fossil record has considerably grown since the time of Darwin, and recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps.

Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245 to 202 million years ago). A close relationship between angiosperms and gnetophytes, proposed on the basis of morphological evidence, has more recently been disputed on the basis of molecular evidence that suggest gnetophytes are instead more closely related to other gymnosperms.

The amount and complexity of tissue-formation in flowering plants exceeds that of gymnosperms. The vascular bundles of the stem are arranged such that the xylem and phloem form concentric rings.

In the dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles (interfascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside. The soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.

Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They contain no cambium and once formed the stem increases in diameter only in exceptional cases.

The characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, and provide the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally on a shoot or from the axil of a leaf (where the petiole attaches to the stem). Occasionally, as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More typically, the flower-bearing portion of the plant is sharply distinguished from the foliage-bearing or vegetative portion, and forms a more or less elaborate branch-system called an inflorescence.

The reproductive cells produced by flowers are of two kinds. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls). The "female" cells called megaspores, which will divide to become the egg-cell (megagametogenesis), are contained in the ovule and enclosed in the carpel (or megasporophyll).

Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families plants, the Poaceae, or grass family, is by far the most important, providing the bulk of all feedstocks (rice, corn maize, wheat, barley, rye, oats, pearl millet, sugar cane, sorghum). The Fabaceae, or legume family, comes in second place.

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