PLASTID ULTRASTRUCTURE IN THE BARREL CACTUS, ECHINOCACTUS ACANTHODES

Summary The unique feature of the plastids in the stem of the barrel cactus is the presence of exceedingly large and often numerous plastoglobuli. In the outer green tissue the chloroplasts are similar in ultrastructure to those of other angiosperms except for the presence of numerous plastoglobuli of various sizes and the absence of starch. In the inner, light‐green transition zone adjacent to the cortical tissue, the chloroplasts have large and small plastoglobuli but also contain some starch. The dominant features of the plastids in the cortical and pith tissues are plastoglobuli, accumulations of phytoferritin and the reduction or absence of grana and starch. These observations suggest that the primary storage product in cactus plastids are lipids which accumulate into plastoglobuli. In the transition zone the plastoglobuli are fewer in number than in the outer green tissue and the large plastoglobuli contain regions of electron translucency both of which are indications that the plastoglobuli are being degraded. A possible relationship between the plastoglobuli and starch through gluconeaogenesis is suggested by the tight association of microbodies with the chloroplasts in this tissue. The plastids in the light‐green transition zone, cortical and pith tissue are irregular in shape which is interpreted to indicate that the form of the plastids in these tissues is dynamic.