Monsi, Uchijima and Oikawa (1973) have pointed out that the mathematical description of canopies with non-randomly distributed foliage is much more difficult than of those where foliage is randomly distributed; the calculation of light interception by such canopies poses correspondingly serious problems, and the major deficiency in accurate working models of light interception and photosynthetic productivity is (therefore) for forests and fruit tree orchards. This is serious because the time trees take to reach maturity, and their size, makes it difficult to determine optimum planting density and tree form for maximum productivity over an orchard lifetime by field experimentation. The extreme form of the non-random canopy is one in which there are major gaps in ground cover. This type of discontinuous canopy is common in agricultural crops in the early stages of growth prior to canopy closure or where access for machinery or people is required throughout the life of the crop. A number of computer models of light interception by discontinuous canopies have been produced, e.g. by Charles-Edwards and Thorpe (1976), Palmer (1977), Oikawa (1977) but tend to be limited to canopies which can be defined in simple geometrical terms or to require a very great deal of detailed information on canopy structure. In considering the relevance of these models to the choice of plant spacing, arrangement, form and, in the case of orchard crops, pruning, at different latitudes and with different growing seasons it has become clear that it is advantageous to view light transmission (T) through discontinuous canopies as consisting of two separate and additive components: