MEMBRANES OF MESOPHYLL CELLS OF NICOTIANA RUSTICA AND PHASEOLUS VULGARIS WITH PARTICULAR REFERENCE TO THE CHLOROPLAST

Weier , T. E., and W. W. Thomson . (U. California, Davis.) Membranes of mesophyll cells of Nicotiana rustica and Phaseolus vulgaris with particular reference to the chloroplast. Amer. Jour. Bot. 49(8): 807–820. Illus. 1962.—The endoplasmic reticulum in mesophyll cells is represented by short lengths of irregularly disposed, paired membranes. It is occasionally associated with a typically double nuclear envelope. Groups of irregularly parallel, paired membranes suggesting disorganized dictyosomes occur infrequently. Mitochondria are unevenly distributed in mesophyll; they are large and have sparse tubular cristae around their periphery. In the great majority of instances the bounding membrane is diffusely stained with KMnO₄. When it is sharp and distinct, it may be double as usually pictured, or it may have well-delineated stretches of a single membrane bounding 25–50% of its circumference. The tonoplast and ectoplast are very fragile, the former appearing as a single dark line. In young leaves the ectoplast is visualized as a continuous single membrane adjacent to the cell wall, but in our micrographs of mature leaves it is always discontinuous. The plastid membrane sometimes is distinctly double, having 2 dark components bounding a light component. In the great majority of cases, however, this membrane is either a solid dark line, or the clear component of the double membrane is crossed by delicate dark lines giving the membrane a braided, or scalariform appearance. The various appearances of the membrane may intergrade with each other. The width of the plastid membrane is variable, ranging from 200 to 400 A. The inner component may invaginate into the stroma, and bodies may form in the clear space between the 2 outer membrane components. Micrographs suggest that these bodies, and others formed by small masses of stroma, may be expelled into the hyaloplasm, where they exist as spherical single-membraned particulates. The reality of the variable structure of the plastid membrane is discussed in light of concepts of membrane activity, molecular structure, and the relation of these factors to possible artifacts.     

THE EXCITABILITY OF FROG MUSCLE WITH PARTICULAR REFERENCE TO ITS LATENT ADDITION

With a view to indicating that the α excitatory state in muscle is not of a special nature it is shown that the α strength-duration curves are of the same form as those determined for nerve and other tissues except that in about two-thirds of all cases the rheobase appears to be slightly too low. Also from experiments in latent addition it is found that the α excitatory state following an inadequate stimulus subsides exponentially at a rate which is related to the α excitability in the same way, approximately, as the subsidence rate in nerve is related to the nerve''s excitability. In both tissues the subsidence as measured directly is 2–3 times as fast as it appears to be from the strength-duration curve. The α refractory period is at least as short as that of nerve so the α chronaxie is unusually long compared to the refractory period. There is no reason at present, however, to consider this as having any bearing on the problem at issue. It is concluded therefore, that the α excitability differs from others in the rates of its reactions rather than in its fundamental nature and that any conclusions about excitability drawn from its study will probably be valid quite generally.     

THE TRANSMISSION OF PLANT VIRUSES BY BITING INSECTS, WITH PARTICULAR REFERENCE TO COWPEA MOSAIC

Experiments on the virus-vector relationship of the Trinidad cowpea mosaic virus, transmitted by Ceratoma ruficornis , gave the following results: ability to infect decreased with increasing time after ceasing to feed on infected plants, but vectors remained infective for 14 days (much longer than the longevity in vitro of the virus at glasshouse shade temperatures of 23–31°C.); the beetles transmitted more consistently after longer feeding on infected plants, though feeds of under 5 min. made them efficient vectors; the proportion of plants infected increased with the amount of feeding damage on them; fasting the vectors before feeding on infected plants increased voracity but had no effect on their ability to transmit; beetles became infective immediately after feeding on infected plants. Cowpeas were infected by inoculation with macerated infective vectors or with juice regurgitated by vectors. There is no evidence that aphids or other sucking insects can transmit the virus. It seems similar to squash mosaic and turnip yellow mosaic, for vectors of all three viruses probably transmit by regurgitating infective juice during feeding.     

THE ULTRASTRUCTURE OF THE SENSORY HAIRS AND ASSOCIATED ORGANELLES OF THE COCHLEAR INNER HAIR CELL, WITH REFERENCE TO DIRECTIONAL SENSITIVITY

From the apical end of the inner hair cell of the organ of Corti in the guinea pig cochlea protrude four to five rows of stereocilia shaped in a pattern not unlike the wings of a bird. In the area devoid of cuticular substance facing toward the tunnel of Corti lies a consistently present centriole. The ultrastructure of this centriole is similar to that of the basal body of the kinocilium located in the periphery of the sensory hair bundles in the vestibular and lateral line organ sensory cells and to that of the centrioles of other cells. The physiological implications of the anatomical orientation of this centriole are discussed in terms of directional sensitivity.     

THE FINE STRUCTURE OF NORMAL AND NEOPLASTIC MELANOCYTES IN THE SYRIAN HAMSTER, WITH PARTICULAR REFERENCE TO CARCINOGEN-INDUCED MELANOTIC TUMORS

The dermal melanocyte system of the Syrian hamster is particularly responsive to the melanogenetic and tumor-inducing effects of 7,12-dimethylbenz(a)anthracene (DMBA). The melanocytes of the hair follicles appear to be susceptible to the melanogenetic effect of DMBA but not to its tumor-inducing effect. The epidermal melanocytes are non-pigmented and are unresponsive to both melanogenetic and carcinogenic effects of DMBA. The pigmented granules of the dermal melanocytes of both the golden and the white hamster have an identical substructure and pattern of melanization which occurs in an orderly fashion on a delicate fibrillar component. The hair melanocytes have larger pigment granules with a more complicated fibrillar substructure. The epidermal melanocytes do not possess pigment granules but are recognized by their dendritic shape, the absence of desmosomes and tonofilaments, and the presence of racket-shaped or rod-shaped organelles. The melanin granules in neoplastic melanocytes of the golden hamster differ from corresponding normal melanocytes only in their larger size. In the white hamster, however, the melanin granules in tumors produced under identical experimental conditions are so bizarre and atypical that consideration was given to the possibility that a genetic difference in the melanization pattern between the two varieties becomes apparent in carcinogen-induced melanotic tumors. No definite conclusions could be reached as to the precise origin of the melanin granules in either normal or neoplastic melanocytes.