Geographical Distribution and Plant Communities

Few places on the Earth can be found where at least some of the 400,000 species of plants are not adapted to live. Only the polar zones, the highest mountains, the deepest oceans, and the driest deserts are devoid of plants (other than bacteria). A given plant species has a limited distribution, however, depending on its own particular requirements. Some species are broadly distributed, being tolerant of a wide range of conditions. Narrowly restricted species have limited tolerance to a specific factor, such as soil type.
Climate is the major factor affecting the distribution of plants and determining their structural adaptions. The greatest number of species are found near the equatorial regions in the tropical climates, where moisture and temperature are seldom limiting. The number of species per area decreases toward the poles.
Plants assume various adaptations as they occupy drier or colder areas away from the tropics. In drier climates plants develop features, known as xeromorphic ("dry form") characteristics, such as smaller, thicker leaves, spines, dense hairiness, and water-storage organs. In colder zones plants become lower in stature, with the growing points protected at or just beneath the ground. Because of similar adaptations, plants of a given climatic zone form a characteristic vegetation type. A large area occupying a given climatic zone and with a characteristic vegetation and associated groups of animal species is called BIOME. The major biomes include the tropical rain forest, desert, and tundra.

Current Plant Research
Much of the emphasis in research is on the development and use of new techniques and equipment. In the field of classification especially, the use of new chemical methods and computers has become important in elucidating relationships and in handling data. The electron microscope, and in recent years the scanning electron microscope (SEM), are important tools of the plant at the subcellular level. The SEM is used in particular in studying the detail of leaf surfaces and pollen grains.
Studies of the structure and function of membranes, where much of the plant's activities take place, are widely pursued. Recent research in molecular botany has been in the synthesis of chlorophyll and the interrelationships of nucleic acids and hormonal functions. The role of hormones and their interactions with phytochromes in affecting flowering continues to be an intensively studied area in plant physiology. Nearly all photosynthetic plants utilize the carbon from carbon dioxide to manufacture sugar molecules by employing one specific set of chemical reactions to fix, or transfer, the carbon atoms. The series of chemical reactions is called the 
C3 (or Calvin-Benson) cycle because the three-carbon compound, phosphoglyceric acid, is formed during its operation. An area of recent interest has been the discovery of an alternative carbon-fixing pathway in a number of other plants.
This is called the C(or Hatch-Slack) cycle because four-carbon compounds are produced during this process.
In agriculture, the main emphasis is still on increased food production, with breeding programs to develop high-yield strains, especially those yielding more protein. Computers are also used to simulate the growth of several food crops and study the factors involved.