|The tree is a typical form of ligneous plants, having a trunk from wood with a deciduous crown.
According to the type of leaves, trees are divided into coniferous and deciduous.
Coniferous trees are usually distinguished by rigid, generally evergreen, needle-shaped or flaky leaves, called conifers or needles, strobiles or juniper berry types, the derivatives of their structures, are formed. For example, pines, firtrees, firs, larches, cedar, sequoienes belong to this group.
Deciduous (broad-leaf) trees have broad and flat leaves – the thickness is much less than the length and width and usually fall off once a year. The broad-leaf (or simply deciduous) trees usually blossom and fructify. Maples, beeches, ash-trees, eucalyptus etc belong to this group.
Apart from the classification of trees with respect to leaves, trees are divided with regard to the life of the leaves, into – therophyllous and evergreen.
Therophyllous trees have a clear change of deciduous cover: all leaves of the tree lose the green color and fall off, the tree stands without leaves for some time (in winter), then new leaves grow from leaf buds (in the spring).
Evergreen trees have no clear change of deciduous cover: the foliage is found on the tree during any period of the year and the leaf changing takes place gradually during the entire life of the tree.
Besides the biological classification, trees are classified with respect to other features: for example, fruit-trees (the fruits of which are used by human beings in food), valuable (the wood of which is used for industrial purposes), marine (used in shipbuilding), tropical (the habitation area which is near equator), northern (the habitation area which is far from the equator) etc.
Three main parts are distinguished in a tree: the root, trunk and crown.
The root of the tree, as a rule, is an underground part of the plant. Main functions – keeping the tree in a vertical position and the absorption of nutrients from the soil and passing them to trunk. The roots can spread to huge extents – penetrate up to a 30m depth and spread up to 100m to the sides. Some trees have air roots, which are located above the ground and their function is similar to the function of leaves.
The tree trunk accomplishes the function of support for the crown and also fulfills the function of passing-on substances between the roots and crown. During winter, it becomes a storage place of moisture and nutrients. The wood trunk consists of the core, wood, which accrues from cambium inside, forming annual rings - dark and bright sections, visible on the lateral saw end of the tree. The number of annual rings corresponds to the age of the tree and their thickness, the living conditions of the tree in each concrete year. From outside, the trunk is covered with bark. During its life span, the tree has, as a rule, a single trunk. If the main trunk is damaged (cut), sister trunks can grow in some trees from resting buds.
The tree crown is a set of branches and leaves in the top part of the plant, extending from the trunk branching up to the top of the tree or bush with all lateral branches and foliage. The characteristics such as a crown shape are distinguished - from columnar to sprawling and crown density - from dense to rare and delicate. Under the influence of light, synthesis of the required substances takes places in the leaves as a result of photosynthesis.
Trees in the news: The city of McKinney, Texas, ran into a conflict between the mayor Bill Whitfield and the workers who were contracted to cut the tree limbs by a power company. The workers were hired by the TXU Electric Delivery Company to trim trees in the city which are getting closer to the power lines. Most trimming was done on the golf course north of Valley Creek Trail and in the historical districts of the city. When Mr. Whitfield looked at this, he was upset with the work of the contracted crew. "While I may understand that the tree branches need to be cut to prevent interfering with power lines, the job contractors have done made the beautiful trees look ugly," said the mayor. "It will take nature years to recover from such trimming job," he added.
However, the electric company admitted no wrongdoing and replied that they have the contacts and obligation to their consumers and can not take chances of power supply being interrupted because the beautiful trees will interfere with the power lines. "We did everything by the book, the company director said. "The tree limbs were cut according to the national standards 10 feet from the power lines, and the contractors did the best they could to preserve the city look within these guidelines."
McKinney city has been long known for its tree ordinance and preservation in its recreational and historic areas. It was recently named Tree City USA by the National Arbor Day Foundation.