It still is unclear exactly why plants have mostly evolved to be green. Green plants reflect mostly green and near-green light rather than absorbing it. Other parts of the system of photosynthesis still allow green plants to use the green light spectrum (e.g. through a light-trapping leaf structure, carotenoids, etc.). Green plants do not use a large part of the visible spectrum as efficiently as possible. A black plant can absorb more radiation, and this could be very useful, notwitstandanding the problems of disposing of this extra heat (e.g. some plants must close their openings, called stoma, on hot days to avoid losing too much water). More precisely, the question becomes why the only light absorbing molecule used for power in plants is green and not simply black.
The biologist John Berman has said that evolution is not an engineering process, so it often has limits that an engineer or other designer does not. Even if black leaves were better, evolution's limitations can prevent species from becoming as efficient as possible. Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants (embryophytes) are believed to have evolved from a common ancestor that is a sort of green algae – so chlorophyll has evolved only once (common ancestor).
Shil DasSarma, a microbial geneticist at the University of Maryland, has pointed out that species of archaea use another light-absorbing molecule, retinal, to get power from the green spectrum. Some scientists believe that green-light-absorbing archae were once the most common in the earth environment. This could have left open a "niche" for green organisms which would absorb the other wavelengths of sunlight. This is just a possibility, and Berman wrote that scientists are still not convinced of any one explanation.
Chlorophyll is a chlorin pigment, which is structurally similar to and produced through the same metabolic pathway as other porphyrin pigments such as haem. At the center of the chlorin ring is a magnesium ion. For the structures depicted in this article, some of the ligands attached to the Mg center are omitted for clarity. The chlorin ring can have several different side chains, usually including a long phytol chain. There are a few different forms that occur naturally, but the most widely distributed form in terrestrial plants is chlorophyll "a". The general structure of chlorophyll "a" was worked out by Hans Fischer in 1940. By 1960, when most of the stereochemistry of chlorophyll "a" was known, Robert Burns Woodward published a total synthesis of the molecule. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming, and in 1990 Woodward and co-authors published an updated synthesis. In 2010, a near-infrared-light photosynthetic pigment called chlorophyll f might have been discovered in cyanobacteria and other oxygenic microorganisms that form stromatolites.
The different structures of chlorophyll are summarized below:
Chlorophyll Content meters measure the optical absorption of a leaf to estimate its chlorophyll content. Chlorophyll molecules absorb in the blue and red bands, but not the green and infra-red bands. Chlorophyll content meters measure the amount of absorption at the red band to estimate the amount of chlorophyll present in the leaf. To compensate for varying leaf thickness, Chlorophyll Meters also measure absorption at the infrared band which is not significantly affected by chlorophyll.
The chlorophyll content of leaves can be non-destructively measured using hand-held, battery-powered meters. The measurements made by these devices are simple, quick and relatively inexpensive. They now have large data storage capacity, averaging and graphical displays.
Measurement of the absorption of light is complicated by the solvent used to extract it from plant material, which affects the values obtained,
In Angiosperms, the last step in the synthesis of chlorophyll is light-dependent. Such plants are pale (etiolated) if grown in the darkness. Non-vascular plants and green algae have an additional light-independent enzyme and grow green in the darkness instead.
Chlorosis is a condition in which leaves do not make enough chlorophyll, so they become yellow. Chlorosis can be caused by a not having enough iron—called iron chlorosis—or by not having enough magnesium or nitrogen. Soil pH sometimes affects these kinds of chlorosis. Many plants are adapted to grow in soils with specific pH levels and their ability to absorb nutrients from the soil can depend on this. Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.
Oak
The oak tree is a tree or shrub in the genus "Quercus". There are about 600 living species. The common name "oak (Oaks)" may also appear in the names of species in related genera, such as "Lithocarpus".
The genus "Quercus" is native to the Northern Hemisphere. It includes deciduous and evergreen species from cool temperate to tropical latitudes in Asia and the Americas. North America has the largest number of oak species: about 90 occur in the United States. Mexico has 160 species, of which 109 are endemic. The second greatest center of oak diversity is China, which has about 100 species.
Oaks have spirally arranged leaves, with rounded edges in many species; some have leaves with jagged edges or entire leaves with smooth margins. Many deciduous species do not drop dead leaves until spring. In spring, a single oak tree produces both male flowers (as catkins) and small female flowers. The fruit is a nut called an acorn, carried in a cup-like structure. Each acorn has one seed (rarely two or three) and takes 6–18 months to mature, depending on species. The so-called "live oaks" are evergreen. They are not a taxonomic group, just a life style which occurs across the genus.
The oak is a kind of hardwood forest tree. They are well known as a climax vegetation in the temperate zone of the northern hemisphere. That means, left untouched by humans, it would be the dominant tree. Much of England was covered by oak forests before modern farming took over the land. The last extensive oak woodlands were cut down to build ships for the Royal Navy in the 18th century.
Some kinds of oak wood are very hard. That is why people in past centuries cut them down to make ships, furniture and other things. The wood is now scarce and expensive and only used to make a few things anymore. Much cheaper are softwoods like pine.
Oak trees grow slowly and can live up to 1000 years.
A mature oak tree stands about 100 feet tall (~30 metres). It is a home for more animals than any other European tree. 30 species of birds, 45 different bugs and over 200 species of moth have been found on oaks. Beetles burrow under the bark, and some drill holes into the wood. The leaves are eaten by many caterpillars. Many leaves carry strange little bumps on the underside. These are insect galls, caused by many little animals. Midges, moths, worms and tiny wasps lay their eggs in leaves or leaf buds. The leaf reacts by forming a growth around the eggs. Inside the gall, larvae develop. The leaf falls, but the larvae may come out only the next spring. Small galls only have one larva, but larger galls may contain as many as 30 larvae.
Oak trees produce acorns once a year which ripen in autumn. Oak trees may start producing acorns when they are about 20 years old. A mature oak may produce 90,000 acorns a year; this is several millions in its lifetime.
Glucose
Glucose is a simple carbohydrate, or "sugar". It is one of several kinds of sugars. It is important because cells in an organism use it as a source of energy. Turning glucose into energy is called cellular respiration, which is done inside the cells of a living organism. Excess glucose is converted to fats and are stored in adipose tissues.