SUMMARY PIPES, an organic based buffer, was used in the preparation of a variety of marine and mammalian tissues for electron microscopic study. The ultrastructural results demonstrate that this buffer renders superior ultrastructural details to those obtained with some of the more common inorganic buffers especially when long fixation times are required. Furthermore, this buffer system does not appear to contribute extraneous anions or cations to the tissues and thus permits accurate elemental determinations to be performed on thin sections of the plastic embedded specimens by means of energy dispersive X‐ray analysis.

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Tannin accumulations in tissue culture cells of slash pine occurred in the smooth endoplasmic reticulum, in vesicles, and within cell vacuoles. Dense osmiophilic granules were observed also throughout the cytoplasm of many cells. Vacuolar deposits appeared as homogeneous spherules or as a froth-like material. Generalized degeneration of the cytoplasm and the organelles was observed in many of the cells synthesizing tannins. Accumulation of the tannin within the cell vacuoles did not necessarily result in cell degradation. Synthesis and deposition of tannin in slash pine cells closely resembles synthesis and storage of aromatic products in mammalian tissues.

The ultrastructure, absorbance, and elemental content of lipochondria present in the cytoplasm of Aplysia giant neurons have been investigated before and after 30-1,200 sec doses of white light at intensities which produce saturated light responses. The effects of exposure to the calcium ionophore A-23187 and to EGTA were also examined. The lipochondria of nonilluminated neurons are membrane-bound, and contain lipids, protein, Na, K, Mg Ca, Si, Cl, Br, P, and a pigment which is probably beta-carotene. The cytoplasm appeared to have little pigment. When neurons were illuminated for 20 min, 60-70% of the lipochondria showed marked ultrastructural alterations, the most notable being the appearance of membranous material. Earlier changes which occur after 30 sec of illumination include the appearance of paracrystalline arrays and mottling. Less than 10% of lipochondria in nonilluminated neurons have a similar appearance. These effects were greatly enhanced in illuminated neurons exposed to the calcium ionophore or EGTA. In nonilluminated neurons, the ionophore also produced ultrastructural changes. In frozen specimens, the calcium content of the most electron dense lipochondria of illuminated neurons was reduced. Other elements which were counted were also reduced. The lipochondria are the main intracellular site of photopigment. They may also act as an intracellular source for calcium which, as the accompanying paper indicated, may mediate phototransduction in Aplysia neurons.

Summary. The effect of paraquat (l,1′‐dimethyl‐4,4′‐bipyridylium‐24) on the ultra‐structure of mesophyll cells of honey mesquite (Prosopis juliftora (Swartz) DC. var. glandulosa (Torr.) Cockerell) was studied by means of electron microscopic techniques. The first visible change induced by the herbicide was a rapid disintegration of the plasmalemma, followed by rupturing of the chloroplast membranes and loss of chloroplast turgor. Changes that occurred in plants treated and sampled in the dark were similar to those in plants treated and sampled in the light. The only change of a biochemical nature detected was a marked reduction in the quantity of starch deposits in chloroplasts. No changes were noted in the mitochondria, Golgi bodies, endoplasmic reticulum or nucleus, or in the composition of the cytoplasm or cell walls of treated tissues. Effets du paraquat sur I'ultrastructure des cellules du mésophylle de Prosopis Resume L'action du paraquat (1,1′‐diméthyle‐4,4′‐bipyridylium‐2A) sur l'ultrastructure des cellules de mésophylle de Prosopis (P. juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell) a été eatudiée au moyen des techniques de microscopie électronique. Le premier effet visible provoqué par l'herbicide fut une deasintegration rapide de la membrane cytoplasmique suivie de la rupture des membranes des chloroplastes et de la perte de la turgescence des chloroplastes. Les transformations qui apparurent dans les plantes traitées et conservées a l'obscurité, furent analogues á ceux des plantes traitées et exposées a la lumière. Le seul changement de nature biochimique deacelé fut une reduction marquée de la quantité des dépôts d'amidon dans les chloroplastes. Aucun changement ne fut observe dans les mitochondries, l'appareil de Golgi, le réticulum endoplasmique ou le noyau, pas plus que dans la composition du cytoplasme ou des parois cellulaires des tissus traités. Der Einfluss von Paraquat auf die Feinstruktur von Mesophyllzellen des Honig‐Mesquitebaums