\Pho`to*syn"the*sis\, n. (Plant Physiol.)
The process of constructive metabolism by which carbohydrates
are formed from water vapor and the carbon dioxide of the air
in the chlorophyll-containing tissues of plants exposed to
the action of light. It was formerly called {assimilation},
but this is now commonly used as in animal physiology. The
details of the process are not yet clearly known. Baeyer's
theory is that the carbon dioxide is reduced to carbon
monoxide, which, uniting with the hydrogen of the water in
the cell, produces formaldehyde, the latter forming various
sugars through polymerization. Vines suggests that the
carbohydrates are secretion products of the chloroplasts,
derived from decomposition of previously formed proteids. The
food substances are usually quickly translocated, those that
accumulate being changed to starch, which appears in the
cells almost simultaneously with the sugars. The chloroplasts
perform photosynthesis only in light and within a certain
range of temperature, varying according to climate. This is
the only way in which a plant is able to organize
carbohydrates. All plants without a chlorophyll apparatus, as
the fungi, must be parasitic or saprophytic. --
{Pho`to*syn*thet"ic}, a. -- {Pho`to*syn*thet"ic*al*ly}, adv.