William A. Jensen

Ultrastructural changes during growth and embryogenesis in carrot cell cultures

Clumps in suspension cultures of wild carrot have two histological regions; an inner region of highly vacuolate, nondividing cells and a peripheral region of small meristematic cells. Significant ultrastructural features of the meristematic cells include the presence of many starch-filled plastids, a dense background of free ribosomes, rough ER bearing conspicuous polyribosomal aggregates, and numerous multivesicular bodies apparently derived from dictyosomes. Multivesicular bodies fuse with the plasmalemma, releasing their contents into the wall space. Cell enlargement is accompanied by an increase in rough ER, eventual decrease in free ribosomal content, and two types of structural alteration in the walls which eventually cause clump disintegration and release of smaller fragments. Ultrastructural changes accompanying formation of embryos in auxin-free media include a striking increase in free ribosomal content, decrease in ER, loss of polyribosomal aggregates, and appearance of microtubules. The mechanism of embryo polarization is discussed.

Cotton embryogenesis: The entrance and discharge of the pollen tube in the embryo sac.

SummaryAn analysis of the entrance and discharge of the pollen tube into the embryo sac of Gossypium hirsutum was made with the light and electron microscopes. The following sequence of events is seen in cotton: 1.One of the two synergids begins to degenerate following pollination but before the pollen tube reaches the embryo sac. This degeneration is marked by the swelling and darkening of the organelle membranes, the collapse of the vacuoles, and the disappearance of the plasma membrane. Striking chemical changes accompany the structural degeneration.2.The pollen tube grows into the degenerating synergid through the filiform apparatus. The tube ceases growth while the tip is still in the synergid. A pore develops on the chalazal side of the tube in a subterminal position.3.The pollen tube cytoplasm and the sperm are discharged into the degenerating synergid through the pore in the tube. Following discharge a plug forms at the pore. None of the discharge leaves the synergid except the sperm nuclei which enter the egg or central cell directly from the synergid. The X-bodies present in the synergid are the remains of the sperm cytoplasm. The data stress the importance of the degenerating synergid in pollen tube discharge and the entrance of the sperm nuclei into the egg and central cell. A hypothesis is presented to explain the passage of the sperm nuclei into the egg and central cell. The data show clearly that the pollen tube does not destroy the synergid it enters, and that the degenerating synergid following pollen-tube discharge is remarkably stable.

Capsella embryogenesis: The suspensor and the basal cell

SummaryThe suspensor and basal cell ofCapsella were examined with the electron microscope and analyzed by histochemical procedures. The suspensor cells are more vacuolate and contain more ER and dictyosomes, but fewer ribosomes and stain less intensely for protein and nucleic acids than the cells of the embryo. The end walls of the suspensor cells contain numerous plasmodesmata but there are no plasmodesmata in the walls separating the suspensor from the embryo sac. The lower suspensor cells fuse with the embryo sac wall and the lateral walls of the lower and middle suspensor cells produce finger-like projections into the endosperm. At the heart stage the suspensor cells begin to degenerate and gradually lose their ability to stain for protein and nucleic acids.The basal cell is highly vacuolate and enlarges to a size of 150μ X 70μ. An extensive network of wall projections develops on the micropylar end wall and adjacent lateral wall. The nucleus becomes deeply lobed and suspended in a strand of cytoplasm traversing the large vacuole. The cytoplasmic matrix darkens at the late globular stage and histochemical staining for protein becomes very intense. The basal cell remains active after the suspensor cytoplasm has degenerated. It is proposed that the suspensor and basal cell function as an embryonic root in the absorption and translocation of nutriments from the integuments to the developing embryo.

Studies of the Development of Wheat (Triticum Aestivum) Pollen

Abstract The origin and formation of the pollen wall and Ubisch bodies of wheat (Triticum aestivum) have been investigate d. Two layers of callose surround the developing sporocyst: the first layer formed [MATTER UNCLEAR IN PDF FILE]e ual in distribution and is believed to push the sporocyst against the tapetum, while [MATTER UNCLEAR IN PDF FILE]second layer completely surrounds each cell and is believed to act as a temporary wall [MATTER UNCLEAR IN PDF FILE] cell develops. The pollen wall is initiated in conjunction with the plasma membrane. The plasma membrane protrudes toward the microspore coat and at the end of each protrusion there is a small patch of electron dense material that is the beginning of the probacules. The probacules continue to enlarge and the protectal fibrillar-reticulate pattern becomes evident. A trilamellar structure forms on the surface of the plasma membrane. Material deposited on this structure appears to be the beginning of the foot layer. Eventually the foot layer increases i...

Cotton embryogenesis: The zygote

SummaryThe zygote of cotton (Gossypium hirsutum) remains undivided for approximately

Cotton embryogenesis: The tissues of the stigma and style and their relation to the pollen tube.

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The composition and ultrastructure of the nucellus in cotton

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Fertilization in Flowering Plants

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Cotton embryogenesis: The pollen tube in the stigma and style

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