Plants are the primary producers of energy in any ecosystem, meaning that they bring in new energy to it which supports the life that lives there. For plants the produce their own energy, they require sunlight to conduct photosynthesis.
In today's diversified culture of plants' varying species require different amounts of light in a day. The amount of time that a plant requires is called the critical period. Some plants, for instance, require more than 14 hours of light, and without this amount of light on a regular basis, their stalks become etiolated and the leaves of the plant fail to grow to their potential.
This is partly because their is not enough light available for photosynthesis and therefore not enough ATP available for cellular processes and growth.
Auxin also plays a part, as light destroys auxin, plants that are immersed in light have cells that do not become as elongated producing a weak stem. Plants who require more than 12 hours of light are deemed 'long day short night plants' because of their light dependant nature.
On the other side of the coin, some plants require longer periods of darkness, these being 'short day long night' plants. These type of plants only flower when the level of sunlight is below 12 hours, or more precise, there is more darkness than sunlight.
Photoperiodism (the length of light that each plant will react to) is stimulated by a pigment called phytochrome.
Various forms of phytochrome react to these varying wavelengths, depending on which wavelength of light is absorbed by the plant. There is a constant reaction involving phytochrome in plants, which is the plants' indicator of whether it is daytime or night time.
Phytochrome 660 is in abundance in the plant during the day, and converts itself to phytochrome 730 on absorbing light. This unstable form of phytochrome reverts to its original form during the day and once again the cycle starts all over again. This instance can control the degree of photosynthesis that occurs in the plant, while phytochrome is visibly one of the ways that plants react to sunlight.
Phytochrome therefore has a regulating control on how and when photosynthesis occurs, and phytochrome levels during different times of the day will react variously within different species of plant.
When looking at Darwin's outlook on speciation and adaptive radiation, it is more plausible now to see why plants possess dithering characteristics suited to their external environment and their needs. The latitude of where a plant exists also has great bearing on how they will cope in their environment, because, for instance, in the extreme latitudes, there is not sunlight for 6 months, and the following six months their is constant light. Seasons also play a part, and considering both seasons and latitude into account, it is evident why different climates make homes for different varieties of plants.
Plants have a balancing act between flowering, growing and producing energy for the mass already present on the plant. These chemicals, i.e. indole acetic acid, gibberellin and phytochrome all play their part internally, while reacting to the outside environment. With this in hand, all these reactions originate from one thing, the genetic coding that made the plant operate the way it does.
Previous pages have studies the basics of cells, the genes that control them, and some of the cellular processes that occur in animals. The following pages describe how these processes are regulated, by genetic coding and a suitable environment for the reactions to occur in (i.e. enough sunlight to reach a critical period).