In photosynthesis, only a small part of the pigment molecules (less than 1%) is directly involved in the primary photochemical reactions. These specialized molecules are located in definite centers (reaction centers: RC). The other pigment molecules form an antenna which first collects the light energy and then distributes the singlet excitation energy to the RC's. A RC and its (by relation of vicinity) associated part of the antenna are called a “photosynthetic unit” (PSU). The concept of PSU was born after the experiments of Emerson and Arnold1and has been developed in the thirties2–3. Franck and Teller4were the first to introduce in photosynthesis the concept of exciton migration. From that time an important theoretical work has been devoted to the problem of energy transfer in photosynthesis (see the review of Knox5). In this field the goal of the theoretician is to establish the master equation that governs teh excitation motion. The structural parameters of this equation have an important part from the biological point of view. As a matter of fact the collection antenna exhibits a short and a long distance degree of order. This spatial organization conditions the distribution of excitation energy to the Rc's. Consequently the study of energy transfer is inseparable from a structural analysis of the primary photochemical apparatus. In the following we will linit the discussion to the problem of gree plants.