Negative staining of chloroplast lamellae inAcanthus leaves below the compensation point

Summary Leaves ofAcanthus kept in an environment with a low concentration of carbon dioxide but connected to plants growing in open air show at electron microscopy level chloroplasts with anomalous stain of the thylakoids. Intra- and interthylakoidal spaces are electron opaque, while the outer protein layers appear formed by electron translucent globular units on which a dark deposit is visible in correspondence of the end-granal membranes and frets. It is suggested that the stain is in some way related to compounds active in light dependent photosynthesis which strongly reduce the osmium tetroxide.Supported by a grant of C.N.R. (Rome).