SOME FINE STRUCTURAL OBSERVATIONS ON DEVELOPING AND MATURE SIEVE ELEMENTS IN THE BROWN ALGA LAMINARIA SACCHARINA

The differentiation of sieve elements from inner cortical cells of the stipe of Laminaria saccharina (L.) Lamour. involves the development of a well-structured protoplast and an end wall possessing evenly spaced pores which are visualized by electron microscopy. The protoplast consists of organelles which are commonly found in brown algal cells, including nuclei, cup- or horseshoe-shaped chloroplasts, dictyosomes, mitochondria, and ER. Mitochondria and clusters of small vacuoles, presumably redistributed by the surging effect which occurs in sieve elements, were routinely observed in the vicinity of the end wall. Chloroplasts were seen in progressively degenerated states in older sieve elements, yet nuclei were determined to be non-necrotic. Numerous pores along the end walls interconnect adjacent sieve elements. Each pore is traversed by a strand of cytoplasm and surrounded by plasmalemma. The pores are open and possess no callose. In this paper the sieve element ultrastructures of L. saccharina are compared to those in L. groenlandica, Alaria marginata, Nereocystis lutkeana and Macrocystis pyrifera, and a possible phylogenetic specialization of sieve elements is presented in table form and discussed.     

CALLOSE SUBSTANCE IN SIEVE ELEMENTS

The secondary phloem of 6 species of woody dicotyledons was examined for the occurrence of callose on the sieve plates of active sieve elements. Fluorescence and bright-field staining methods were used to detect callose. Tissue from the 6 species was killed and fixed in each of 5 solutions. Some tissue of each species was submerged in the killing solutions as quickly as possible, the remainder within 15 min after removal from the tree. In each species, some active sieve elements of the quick-killed tissue gave negative callose reactions. All active sieve elements of the delay-killed tissue gave positive callose reactions. These and other results suggest that the active sieve elements in the secondary phloem of the species studied normally lack callose and that the extent of callose deposition in these cells depended primarily upon the rapidity with which the sieve-element protoplasts were killed after wounding of the phloem. In addition, bright-field observations of sieve plates of large numbers of sieve elements from a seasonal collection of Tilia americana secondary phloem suggest that the active sieve elements normally lack callose during the growing season and that the inactive sieve elements normally possess it (dormancy callose).     

SLIME SUBSTANCE AND STRANDS IN SIEVE ELEMENTS

Studies of the secondary phloem of 6 species of woody dicotyledons revealed that slime is not normally dispersed throughout the vacuole of mature sieve elements, but occurs in the form of discrete strands that traverse the cell and run from cell to cell through the sieve-plate pores. As many as 5 fine strands, each measuring less than 0.5μ in diameter, were observed in a single pore. Less than 30% of pore area was occupied by strands. Thus, the pores are mostly open, and intervacuolar continuity exists between cells. These structural characteristics of pores offer strong support for the concept of mass flow.     

THE NACREOUS WALLS OF SIEVE ELEMENTS IN SEAGRASSES

The nacreous walls of sieve elements occur in seagrasses in all three genera of the family Zosteraceae and the genus Halodule of the family Cymodoceaceae but are absent from another eight seagrass genera belonging to the families Hydrocharitaceae, Cymodoceaceae, and Posidoniaceae. They occur in leaf blades, leaf sheaths, rhizomes, and erect stems but are not present in root tissues. The nacreous wall is uneven along the inner limits reflecting irregular thickness. The wall consists of hemicellulose or pectin and cellulose, but no protein, lignin, or lipid. Ultrastructurally, the wall contains parallel microfibrils or loose fibrils embedded in an amorphous matrix. Open pores occur in sieve plates and branching plasmodesmata are present in enlarged sieve areas. Mitochondria, endoplasmic reticulum, and plastids are also present in these sieve elements.     

三种利什曼原虫的组织化学观察

本文报道了杜氏、砂鼠及墨西哥利什曼原虫体内糖原、DNA,RNA。蛋白质结合的a-氨基、碱性蛋白质、酪氨酸、色氨酸及组氨酸的定位及含量。     

ULTRASTRUCTURAL FEATURES OF DEVELOPING SIEVE ELEMENTS IN LEMNA MINOR L.—SIEVE PLATE AND LATERAL SIEVE AREAS

Both intact and cut duckweed plants were prepared for electron microscopy. Plants which are prepared intact do not exhibit callose formation during development of sieve-plate pores. Future pore sites can be recognized by the presence of median cavities that are unassociated with callose platelets. These cavities are first seen in the region of the compound middle lamella and are lined by a plasmalemma. As end walls thicken, the cavities increase in size until open pores of uniform width are formed. Mature sieve plates of intact-prepared plants are also devoid of callose. Fully opened pores are lined by a plasmalemma and are only traversed by an occasional tubule of endoplasmic reticulum. Plants which have been cut prior to fixation possess mature sieve plates containing callose. The pores of developing sieve plates in cut plants exhibit small amounts of callose. Except for the lack of callose, lateral wall connections between sieve elements and contiguous cells are similar in development and mature state to those reported for other species.     

新疆多年生小麦族植物染色体数的观察

本文对1987年采集于新疆的多年生小麦族(Triticeae Dum. )属种进行了细胞学观察。该地区多年生小麦族各属种的染色体数目变化范围是从2n=14到2n=84,前者主要存在于大麦属(Hordeum)、新麦草属(Psathyrostachys),而后者全部集中于赖草属(Leymus).其中染色体数目为2n=28和2n=42的频率最高,主要存在于鹅冠草属(Roegneria)和披碱草属(Elymus)。Roegneria gobicola, Roegneria kuqaensis, Roegneria tahelacona, Roegneria zhoasuensis的染色体数为首次报道。     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENTS IN PSILOTUM NUDUM

Shoot tissue of Psilotum nudum (L.) Griseb. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids, the presence of refractive spherules, and the overall dense appearance of their protoplast. The refractive spherules apparently originate in the intracisternal spaces of the endoplasmic reticulum (ER). With increasing age the sieve-element wall undergoes a marked increase in thickness. Concomitantly, a marked increase occurs in the production of dictyosome vesicles, many of which can be seen in varying degrees of fusion with the plasmalemma. Other fibril- and vesicle-containing vacuoles also are found in the cytoplasm. In many instances the delimiting membrane of these vacuoles was continuous with the plasmalemma. Vesicles and fibrillar materials similar to those of the vacuoles were found in the younger portions of the wall. At maturity the plasmalemma-lined sieve element contains a parietal network of ER, plastids, mitochondria, and remnants of nuclei. The protoplasts of contiguous sieve elements are connected by solitary pores on lateral walls and pores aggregated into sieve areas on end walls. All pores are lined by the plasmalemma and filled with numerous ER membranes which arise selectively at developing pore sites, independently of the ER elsewhere in the cell. P-protein and callose are lacking at all stages of development.     

STRUCTURE AND DEVELOPMENT OF SIEVE ELEMENTS IN THE LEAF OF ISOETES MURICATA

Leaf tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. The very young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids and the presence of crystalline and fibrillar proteinaceous material in dilated cisternae of the rough ER. During differentiation, the portions of ER enclosing this proteinaceous substance become smooth surfaced and migrate to the cell wall. Along the way they apparently form multivesicular bodies which then fuse with the plasmalemma, discharging their contents to the outside. At maturity, the sieve element contains an elongate nucleus, which consists of dense chromatin material, and remnants of the nuclear envelope. In addition, the mature sieve element is lined by a plasmalemma and a parietal, anastomosing network of smooth ER. Both plastids and mitochondria are present. P-protein is lacking at all stages of development. Tonoplasts are. not discernible in mature sieve elements. The end walls of mature sieve elements contain either plasmodesmata or sieve pores or both, but only plasmodesmata occur in the lateral walls.     

ULTRASTRUCTURE OF DEVELOPING SIEVE ELEMENTS IN THLASPI ARVENSE L. II. MATURATION

Developing sieve elements of pennycress (Thlaspi arvense L.) were studied with the electron microscope. The maturation of sieve elements involved loss of ribosomes from cytoplasm; degeneration of nulcei; modification of endoplasmic reticulum (ER); loss of tonoplast; and disappearance of dictyosomes and dictyosomes vesicles, coated vesicles, microtubules, and microbodies. Such changes produce a mature, presumably conducting cell that contains no nucleus or central vacuole but which retains a thin layer of peripheral cytoplasm with plastids, mitochondria, and smooth ER. Some similar changes have been described in a variety of developing sieve elements of angiosperms, but coated vesicles and microbodies previously have not been followed through sieve-element maturation. Likewise, few developmental studies have been made of plant sieve elements that exhibit two types of P-protein, the tubular type and the granular P-protein body.     

OBSERVATIONS ON BLINDNESS IN OATS

The relative importance in North Wales of various factors said to cause blindness in oats was investigated. It is concluded that a direct attack on the developing panicles by frit fly larvae and by adults and larvae of thrips was of little importance in causing the blindness encountered. Most of the blindness appeared to result from adverse physiological conditions probably operative during early growth. Varying varietal susceptibility to blindness was noted, and the effect on the subsequent panicles of an early insect attack on the vegetative parts of the plant is discussed.