Although dehydrated foods and tanked water and oxygen will suffice for space flights of relatively short duration, long-term operations will be possible only if these necessities can be derived from the environment within the space vehicle. Conservation of environmental mass will make mandatory some type of closed ecological system kept in operation by a source of continuous energy. Conceivably, such a system will involve a carbon-dioxide exchange between humans and plants, waste reutilization, and the growth of plants for human consumption. Several sustenance systems have been proposed. In each case, attention was centered on the use of algae as the photosynthetic component of the closed cycle. Algae has been used as a dietary supplement, but no instance has been reported where the diet of man, or any other terrestrial mammal, has consisted entirely of algae. In contrast, higher plants have long been a primary source of human food material. Thus the study reports the use of higher plants as the photosynthate in the human sustenance system.