Most chloroplasts inArisarum proboscideum(L.) Savi show one or two stromatic protrusions, which may be short and thick but also very long and thin. Thylacoids have never been observed in the matrix of these protrusions, whereas small vesicles are often present whose membrane sometimes appears to be continuous with the inner membrane of the chloroplast envelope. Frequently, near to the chloroplast ends, dep hollows are present which include a conspicuous portion of cytoplasm, generally with mitochondria. In many instances, formations with the same structural features as the protrusions are found close to the chloroplasts but separate from them. Such phenomena were observed in sections of leaves of different ages and on isolated chloroplasts as well, both sections and in replicas of freeze-etched samples. The same phenomena, sometimes even in a more striking form, also occur inArisarum vulgareTarg. Tozz.,Arum italicumMill.,A. maculatumL.,A. pictumL. f.,Dracunculus vulgarisSchott andZantedeschia aethiopicaSpreng. The functional significance of these protrusions, which inArisarum proboscideumare the rule rather than the exception, is possible to be related to the close spatial relationship between chloroplasts and mitochondria. The protrusions could be regarded as the expression of a tendency of the chloroplast to extend its outer surface, which fact could favour metabolic exchanges between functionally cooperating organelles.