Plant growth depends on the production of organic materials, mostly sugars and amino acids, by photosynthesis. At first consideration, it seems reasonable to suppose that the rate of plant growth should depend directly on the concentration of these substrates: the higher their concentration, the faster the rate of growth. However, this supposition is not supported by available evidence. Young seeds, especially, seem to maintain preset growth rates that show little relation to sugar and amino acid concentrations. Instead, growth rates appear to be controlled by the transport of these substances from conducting cells into the growing embryo, a distance of less than a millimeter. Developing wheat grains offer advantages for studying this transport, since the pathway is well- defined and growth rates and concentrations can be measured at key points along the path. These investigations will develop a better understanding of transport along this pathway and, consequently, of how the growth rate of seeds might be controlled.//
