=Biological Clocks= [image:] Many animals and plants seem to be able to respond to environmental events before they even occur. For instance, many animals that are active during the day come out just before the sun rises. Plants can anticipate the sunrise, orientating their leaves towards the horizon before the sun actually rises. Likewise, plants that exhibit sleep movements tend to ‘wake up’ (raise their leaves) a little before sunrise. These organisms must have some internal awareness of time. This is often referred to as a ‘Biological Clock’ '''A biological clock is an internal (endogenous) timing system that continues without any external cues and controls the activities of plants and animals''' '''Advantages of a Biological Clock:''' *Enables plants to anticipate environmental changes such as sunrise and maximise photosynthesis *Enables plants to prepare for periodic environmental extremes (winter, drought etc) *Enable plants to flower at the same time / when pollinators are available, increasing the chance of pollination ==Biological Rhythms== Because the function of a biological clocks is to anticipate environmental changes they tend to be set to environmental rhythms such as daily, tidal and seasonal changes. While, the behaviour or response itself is mediated by the endogenous (internal) biological clock, the clock is set to these exogenous (external) environmental cycles. As a result the following are commonly observed periods for biological rhythms: *'''Circadian''' –daily activity (~24hrs) ''E.g. Sleep movements, the opening and closing of flowers and solar tracking'' *'''Circatidal''' –tidal activity period (~12.4hrs) *'''Circalunar''' –monthly activity period (~29days) *'''Circannual''' –Yearly activity period (~365 days) ''Seed germination, flowering, and leaf fall'' ==Biological Rhythms in Plants== [image:] If the biological rhythm is controlled by a biological clock it will continue even in the absence of the exogenous (external) stimulus. The behaviour is not a direct response to an environmental factor, but is controlled '''endogenously''' -by an internal timing mechanism that is ultimately genetic in origin. Many of the circadian (daily) rhythms, such as sleep movements, the opening and closing of flowers and solar tracking are controlled endogenously (by an internal biological clock). Circannual (yearly) rhythms, such as seed germination, flowering, and leaf fall are often controlled purely by exogenous (external) factors such as a change in temperature. For instance '''deciduous''' plants lose their leaves each year in response to changes in the day length.