The growth, development and differentiation of plants growing in natural environments is determined by biotic, genetic and physical factors. Within each plant species the absolute limits of any growth response is established by inherent genetic information and the delineation of that response is, in turn, determined by the physiology of the plant. Among the most important physical factors in any natural environment are light quality, quantity and duration. Plant growth depends on a very narrow bandwidth of the electromagnetic spectrum which usually includes the near ultraviolet (down to 320 nm), the visible, and the near infra-red (up to 800 nm) regions. The radiation of this spectral range not only supplies the necessary energy for photosynthesis on which plant metabolism is based, but also by way of various photomorphogenetic processes, it controls, independently of photosynthesis, the way in which this captured energy is directed along the various metabolic pathways. Since for most processes other than photosynthesis, the amount of radiant energy initially absorbed is low, in relation to the response effect, these light reactions can be considered to belong to a group of photostimulus processes which are characterised by dose-effect relationships. These are exothermic in that they ultimately release, or direct an amount of stored energy, which may be very large as compared with the energy content of the radiation initially responsible for the stimulus (Wassink and Stolwijk, 1956).