ULTRASTRUCTURE OF DEVELOPING SIEVE ELEMENTS IN THLASPI ARVENSE L. I. THE IMMATURE STATE

Immature sieve elements of pennycress (Thlaspi arvense, Brassicaceae) were studied with the electron microscope in connection with studies on virus-infected plants. Immature sieve elements contained cytoplasm rich in organelles and other components: endoplasmic reticulum, dictyosomes and associated smooth and coated vesicles, mitochondria, plastids, ribosomes, microtubules, microfilaments, vacuoles, and nuclei that were sometimes lobed. Tubular P-protein (phloem protein) and one to three granular P-protein bodies also were present in the cytoplasm. Coated vesicles may be involved in formation of the granular P-protein body and in some aspect of cell wall development, for in the latter case, they were often seen united with the plasmalemma. The association of coated vesicles with the P-protein body is discussed with reference to proposed concepts of the origin and function of these vesicles.     

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.     

ULTRASTRUCTURAL FEATURES OF DEVELOPING SIEVE ELEMENTS IN LEMNA MINOR L.—THE PROTOPLAST

The occurrence and precise arrangement of phloem tissue is reported for the first time in the root of the duckweed, Lemna minor L. Since these plants are small enough to be prepared intact for electron microscopy, the induction of artifacts as a result of mechanical manipulation during specimen preparation is virtually eliminated. Ultrastructural features of the complete sequence of sieve-element development are presented and are in some respects similar to what has been described in other monocotyledons (e.g., nuclear degeneration, aggregation of ER, elaboration of electron-dense crystalline plastid inclusions). Mature sieve elements in Lemna are enucleate, lack P-protein, contain a plasmalemma, ER-aggregations, mitochondria, plastids and, in many instances, intact vacuoles. It is suggested that structural modification required for the sieve element to assume a transport function may not be very drastic, apart from the degeneration of the nucleus.     

TUBULES IN DILATED CISTERNAE OF ENDOPLASMIC RETICULUM OF THLASPI ARVENSE (CRUCIFERAE)

Tubular inclusions in dilated cisternae of endoplasmic reticulum (ER) were found in healthy and virus diseased pennycress (Thlaspi arvense L., Cruciferae). The dilated cisternae were encountered more often in virus-infected cells than in comparable cells from healthy control plants. Dilated ER cisternae were found in vascular parenchyma cells and were shown to be interconnected. The tubules measured 30 nm in diam and cisternae that contain them may be more than 10 μm in length.     

ENVIRONMENTAL CONTROL OF SEED GERMINATION IN THLASPI ARVENSE (CRUCIFERAE)

The effect of environmental conditions during storage and imbibition on germination was investigated in field pennycress (Thlaspi arvense L.), a weed species that can behave as a winter or a summer annual. Freshly harvested seeds of an inbred line with a cold requirement for flowering exhibited primary dormancy that was rapidly lost following 1 month of afterripening in a dry state. Nondormant seeds were positively photoblastic. The light effect was mediated through phytochrome since germination was promoted by red light and inhibited by far red light. Seedling emergence was also inhibited by light filtered through a canopy of wheat leaves. Germination of field pennycress seeds was considerably more sensitive to moisture stress than two sympatric species, wild oat (Avena fatua L.) and wheat (Triticum aestivum L., cv. ERA). Seeds of the latter two species were chosen in order to compare the effect of water potential on germination in field pennycress with that in sympatric species. It was concluded that the major environmental factor limiting nondormant field pennycress seeds on the soil surface was water availability. Imbibition of fully afterripened seeds at low temperatures (6 C) induced a deep secondary dormancy. In contrast to primary dormancy, cold-induced dormancy was not alleviated by red light, alternating temperatures (21/5 C), or 2 months of dry storage at 6, 15, or 35 C. However, exogenous gibberellin A₃ or 24 weeks of dry storage resulted in germination in cold-induced dormant seeds. Secondary dormancy was not observed in fully afterripened seeds that were preincubated at 21 C for 1 or 2 days prior to the cold treatment. These results may explain the failure in field experiments to observe the cold-induced secondary dormancy that limits spring emergence in other winter annuals (J. Baskin, C. Baskin, Weed Res. 1979 19: 285–292).     

降香黄檀着叶期和无叶期次生韧皮部筛分子的超微结构

利用透射电镜技术研究了生长在海南岛的热带落叶树降香黄檀(Dalbegia odorifera T.Chen)1—2年生枝条着叶期和无叶期次生韧皮部筛分子的超微结构,并就这两个时期的筛分子进行了比较。着叶期每个成熟筛分子内有一个带尾的纺锤形P-蛋白质体,主体由稠密而散乱的P-蛋白质细纤维组成,尾部呈结晶状;筛分子具有横向端壁和单筛板,在邻近筛板处,细胞壁向筛分子腔内形成明显的突起。无叶期仍然保持着与着叶期大致相同厚度的有功能韧皮部,筛分子具有正常的原生质体,P-蛋白质和筛板孔的结构也与着叶期的相同,但筛分子内有较多的淀粉粒和囊泡。     

ULTRASTRUCTURE OF THE NACREOUS LEPTOIDS (SIEVE ELEMENTS) IN THE POLYTRICHACEOUS MOSS ATRICHUM UNDULATUM

The physiological phloem equivalents, leptoids, of the polytrichaceous moss Atrichum undulatum appear to be similar to the nacreous sieve elements that occur in many higher plants. These leptoids are elongated cells with nacreous thickenings on their radial and tangential walls. Their oblique end walls, which lack such thickenings, are traversed by numerous pores through which the plasmalemma, endoplasmic reticulum, and cytoplasm are continuous between adjacent leptoids of a longitudinal file. These end walls closely resemble the simple sieve areas of the sieve elements found in Polypodium vulgare. The leptoid sieve pores have a median expanded area and frequently are occluded by small amorphous protein plugs at each end. Also, callose was observed as electron-luscent areas both on the faces of the end walls and as a thin cylinder surrounding the lateral area of each pore. Amorphous and granular cytoplasmic contents of the leptoids appear to be morphologically similar to the slime (P-protein) found in the sieve-tube elements of many angiosperms. Differentiating leptoids are characterized by the formation of numerous membrane-bound protein bodies in close association with polysomes and endoplasmic reticulum. As the leptoid matures, the contents of the protein bodies become dispersed in the cytoplasm. Ultrastructurally and ontogenetically the leptoids in the gametophores of A. undulatum appear almost identical to the sieve elements of P. vulgare and therefore should be considered sieve elements rather than phloem-like equivalents.     

SOME ASPECTS OF SIEVE CELL ULTRASTRUCTURE IN PINUS STROBUS

The immature sieve cell of Pinus strobus contains all of the protoplasmic components commonly encountered in young cell types. In addition, it contains slime bodies with distinct double-layered limiting membranes. The mature sieve cell is lined by a narrow layer of cytoplasm consisting of a plasmalemma, one or more layers of anastomosing tubules of endoplasmic reticulum, numerous mitochondria, starch granules and crystal-like bodies. Each mature cell contains a necrotic nucleus. Ribosomes and dictyosomes are lacking. Strands derived ontogenetically from the slime bodies of the immature cell traverse the central cavity and are continuous with those of neighboring sieve cells through the plasmalemma-lined pores of the sieve areas. Sieve-area pores are also traversed by numerous endoplasmic membranes. A membrane was not found separating the parietal layer of cytoplasm from the large central cavity.     

玉米叶片细脉原生韧皮部筛分子的分化和超微结构 : 原生质体的变化

玉米（ＺｅａｍａｙｓＬ．）叶片细脉原生韧皮部筛分子开始分化时,首先出现长的粗面内质网潴泡和增厚的细胞壁,随后在质体中出现其特征性的拟晶体内含物。随着分化的进行,长的粗面内质网潴泡转化为较短的形态,最后聚集成一些小的堆叠并失去其核糖体。细胞核发生退化,但常保持到成熟期的后期,此时的细胞核由双层核膜或某些部位仅由内核膜包被,内含电子致密的不定形染色质团块。随后双层核膜破裂而变得不连续。在细胞核开始退化时,核周腔局部膨大。有的膨大核周腔的外核膜破裂,并伴随邻近的部分细胞质解体。在核退化过程中,除内质网外,质体和线粒体的结构也发生变化,而核糖体、细胞质基质、液泡和高尔基体则解体消失。成熟原生韧皮部筛分子的原生质组分分布在细胞边缘,由质膜、线粒体、小的滑面内质网堆叠及具拟晶体的Ｐ型质体组成。随着邻近后生韧皮部筛分子分化的进行,成熟原生韧皮部筛分子的原生质组分逐渐退化,最后消失。     

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.     

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).     

EFFECTS OF APEX REMOVAL AND NUTRIENT SUPPLEMENTATION ON BRANCHING AND SEED PRODUCTION IN THLASPI ARVENSE (BRASSICACEAE)

The shoot apex of a plant may be damaged by herbivory, weather conditions, or other factors, leading to growth of some axillary buds into branches. This alteration of branching pattern can affect the location and extent of seed production. I examined the effects of removal of the shoot apex and of mineral nutrient addition on branching and seed production in Thlaspi arvense. Individual plants received no, early, or late apex removal and no, early, or late nutrient addition in a two-way factorial experiment. Apex removal led to greater production of secondary branches. Total seed weight per plant was higher for early- than late-removal plants under all nutrient treatments, but intact controls produced the highest total weight of seeds in two of three nutrient treatments, because their seeds were heavier. Addition of nutrients increased the numbers of secondary branches, fruits, and seeds, and the average and total weight of seeds produced. Apex removal resulted in increased seed production only when it occurred early and when nutrients were added at the same time. Resource availability and plant phenology can be influential in determining the effects of apical damage on seed production, and their effects in combination may differ from their individual effects.     

THE FINE STRUCTURE OF SIEVE ELEMENTS OF NEREOCYSTIS LÜTKEANA

The sieve elements of Nereocystis from the base of phylloids contain numerous small vesicles, cytoplasm, ribosomes, and the usual organelles and membrane systems, including nuclei, plastids, mitochondria, dictyosomes, and endoplasmic reticulum. They have a thick secondary wall layer which is deposited along the longitudinal walls and at the sieve plate excluding the sieve pores. The sieve pores range in diameter from 100 to 400 nm and are lined by plasmalemma. The sieve elements from the hollow basal parts of the pneumatocyst show essentially the same features but have larger and fewer vesicles, relatively little cytoplasm, larger sieve pores, 400–900 nm in diameter, and may lack a nucleus. In old sieve elements there are large deposits of callose on the sieve plate and along the longitudinal wall; the vesicles seem to break down, and the protoplast appears necrotic. It is concluded that the trumpet hyphae and sieve tubes are basically the same type of cell, and that the trumpet-shape of the sieve elements is due to their passive stretching during extension growth of the organ in which they occur. There are minor but significant differences among the sieve elements from different regions of the thallus which may reflect possible levels of structural specialization of the sieve elements within the same plant.     

HORMONAL CONTROL OF OOCYTE MATURATION IN ARENICOLA MARINA L. (ANNELIDA, POLYCHAETA) II. MATURATION AND FERTILIZATION

In Arenicola marina (Annelida, Polychacta) the oocytes arc arrested in the first prophase stage of meiosis until spawning. Oocyte maturation is under hormonal control: when incubated in vitro in a brain extract oocytes proceed to the first metaphase at which they remain arrested until fertilization. The prophase arrested oocytes can neither be fertilized nor parthcnogenetically activated by ionophore A23187 or 1 M glycerol. On the contrary the metaphase-arrested oocytes can be fertilized and parthenogenetically activated. Fertilizability thus appears during maturation; it seems to be linked to microvilli retraction. A study of spermatozoa "capacitation" and oocyte fertilization or activation is reported. A scanning electron microscope study of early contact and penetration of spermatozoa is presented.     

ON THE ULTRASTRUCTURE OF CAMBIUM AND ITS VASCULAR DERIVATIVES. III. THE SECONDARY WALLS OF THE SIEVE ELEMENTS OF PINUS STROBUS

The sieve elements of Pinus strobus have thick, lamellate secondary walls, which are composed predominantly of cellulose and lesser amounts of polyuronides and pectins. Eight to 10 lamellae may be present in walls 2-3 μ in thickness. Each lamella represents a plane of high cellulose density which results from intersection of two parallel sets of fibrils. Polyuronides and pectins are more or less evenly distributed in the wall, possibly with a greater concentration near the middle lamella and the inner surface. Resemblance of these walls to the so-called nacré walls is indicated, and it is possible that the two represent the same structure.     

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.