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.     

THE LATERAL MERISTEM AND ITS DERIVATIVES IN THE CORM OF ISOETES MURICATA

Corm tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Very young secondary sieve elements can be distinguished from contiguous cambial cells by their distinctive plastids and by the presence of crystalline and/or fibrillar proteinaceous material in dilated cisternae of rough endoplasmic reticulum (ER). At maturity, the sieve elements are lined by the plasmalemma and a parietal, anastomosing network of smooth ER. Degenerate nuclei persist in all mature sieve elements. In addition, mature sieve elments contain plastids and mitochondria. Sieve-area pores are present in all walls. The lateral meristem of I. muricata consists of 2–3 layers of cells year-round. Judging from numerous collections made between October 1972 and July 1975, new sieve-element differentiation precedes cambial activity by about a month. Early in May, 1–2 cells immediately adjacent to already mature sieve elements differentiate directly into sieve elements without prior division. In early June, at about the time sieve-element differentiation is completed, cambial division begins. Division is sporadic, not uniform throughout the meristem. Dormancy callose accumulates in the secondary sieve elements in late October, and is removed in early May, at about the same time new sieve-element differentiation begins. Cells of the dormant cambium are characterized by the presence of numerous small vacuoles and large quantities of storage materials, including lipid droplets, starch grains, and tannin. By contrast, active cambial cells contain few large vacuoles with little or no tannin, and they have little storage material.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENT IN THE STEM OF LYCOPODIUM LUCIDULUM

Stem tissue of Lycopodium lucidulum Michx. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Although their protoplasts contain similar components, immature sieve elements can be distinguished from parenchymatous elements of the phloem at an early stage by their thick walls and correspondingly high population of dictyosomes and dictyosome vesicles. Late in maturation the sieve-element walls undergo a reduction in thickness, apparently due to an “erosion” or hydrolysis of wall material. At maturity, the plasmalemma-lined sieve elements contain plastids with a system of much convoluted inner membranes, mitochondria, and remnants of nuclei. Although the endoplasmic reticulum (ER) in most mature sieve elements was vesiculate, in the better preserved ones the ER formed a tubular network closely appressed to the plasmalemma. The sieve elements lack refractive spherules and P-protein. The protoplasts of contiguous sieve elements are connected with one another by pores of variable diameter, aggregated in sieve areas. As there is no consistent difference between pore size in end and lateral walls these elements are considered as sieve cells.     

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.     

STRUCTURE AND DEVELOPMENT OF THE SHOOT IN DOLICOTHELE

Boke , Norman H. (U. Oklahoma, Norman.) Structure and development of the shoot in Dolicothele. Amer. Jour. Bot. 48(4): 316–321. Illus. 1961.—A study of 2 species of Dolicothele reveals that although they have dimorphic areoles and a pattern of spine development similar to those of certain mammillarias, they share a significant number of ectomorphic and endomorphic characters with coryphanthas of the “vivipara group.” These include a tendency toward a cespitose habit; relatively large flowers; green fruits; pitted seeds; medullary vascular systems; forking of the main tubercle traces in the bases of the tubercles; lack of mucilage cells; thin-walled epidermis and hypodermis, both devoid of crystals; and large, druse-like crystalline aggregates in older parts of the pith and cortex. The evidence suggests that Coryphantha vivipara and closely allied species are the nearest extant relatives of Dolicothele. It would, therefore, seem inconsistent to return Dolicothele to Mammillaria unless an author's viewpoint were so conservative that he was willing also to return most, if not all, coryphanthas and escobarias to that genus.     

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.     

GROWTH AND GRAVITATIONAL RESPONSE IN THE DEVELOPMENT OF LEAF BLADE HYPONASTY

Rapid (4 hr) auxin-induced hyponastic curvature of primary leaves of Phaseolus vulgaris is shown to depend on a positive increase in growth of the lower portion of the blade. The curvature involves laminar growth as well as vein growth and is not due to simple turgor changes. The response is sensitive to gravitational orientation, as inversion and horizontal rotation reduce the auxin-induced curvature. The ethylene-generating compound, 2-chloroethylphosphonic acid, had no hyponastic effect on the leaves when applied to either the upper or lower surface and it inhibited auxin-induced hyponasty. This inhibition was additive to that of inversion. Long-term (24–48 hr) effects of 1 mM auxin depend on the surface of the leaf treated. Application to the upper surface results in epinasty, lower surface application in hyponasty, although the initial response in each case is a hyponastic curvature. A dorsi-ventral auxin transport system and differential auxin sensitivity of upper and lower portions of the leaf blade are postulated to account for these responses.     

STRUCTURE AND DEVELOPMENT OF THE PERIANTH IN DOWNINGIA BACIGALUPII

The structure and ontogeny of the calyx and corolla of Downingia bacigalupii Weiler (Campanulaceae; Lobelioideae) were investigated for the purpose of comparing perianth development with previous observations on the floral bract, as well as elucidating the mechanism of development of the zygomorphic, sympetalous corolla. Sepals are uni-traced with a palmate, reticulate venation. They have basal and apical hydathodes, as well as storage tracheids. Sepals show a reduction in size, venation and hydathode number when compared to the bract. The pentamerous, zygomorphic corolla is bilabiate, consisting of a three-lobed adaxial lip and a two-lobed abaxial lip connected by a short tubular region. The constituent petal lobes are also uni-traced and have a reticulate venation, resembling that of the sepal and bract, but lack storage tracheids and hydathodes. Sepals arise in an adaxial to abaxial succession and are initiated in the outer corpus layer of the floral apex. Expansion of the floral apex follows and is accompanied by the establishment of a second tunica layer. Sepals undergo apical, marginal, and intercalary growth accompanied by acropetal differentiation of procambium. The petals arise simultaneously and are initiated in the second tunica layer and the outer corpus cells. After initiation, the petals exhibit a period of apical and marginal growth followed by intercalary growth. Apical growth in petals is less protracted than in sepals, but plate meristem activity is more extensive. The free petal lobes become temporarily fused by an interlocking of marginal epidermal layers, but they separate at anthesis. Zonal growth beneath the originally free lobes forms the tube and lip regions of the sympetalous corolla. Zygomorphy is evident from the time of initiation of petals and is accentuated by later differential growth. Comparative observations of corolla ontogeny in autogamous species of Doumingia indicate that the reduced corollas in these taxa are derived by a simple process of neoteny.     

DEVELOPMENT AND STRUCTURE OF PHLOEM IN THE PETIOLE OF LUFFA CYLINDRICA

Differentiation of external phloem is earlier than that of internal phloem in the young petiole of Luffa cylindrical. For a single sieve-tube element, one to six companion cells are present. The young sieve element shows many globular slime bodies which fuse longitudinally and disperse into the cytoplasm. Simultaneously the nucleus loses its stainable contents and later disorganizes. The contents of the sieve element are in the form of plugs, strands or a granular mass. Undispersed slime in the form of discrete bodies along the lateral walls is also observed. During one stage, at least, the dispersed slime and other contents of a mature sieve element lie at the periphery around a central cavity. A special type of phloem-parenchyma cell shows disorganizing chloroplasts, an extruded nucleolus, and callose on primary pit fields.     

叶表皮结构在榆属分类中的应用

本文利用扫描电子显微镜对榆属9种1变种的成熟叶表皮结构进行了观察,认为榆属叶表皮结构在扫描电镜下存在着明显的差别,可以作为榆属分种的辅助特征。     

STRUCTURE AND DEVELOPMENT OF THE SIEVE-ELEMENT PROTOPLAST IN THE HYPOCOTYL OF PINUS RESINOSA

Hypocotyl tissue of Pinus resinosa Ait. was fixed in glutaraldehyde-paraformaldehyde and postfixed in osmium tetroxide for electron microscopy. Although young sieve cells contain all the components characteristic of young, nucleate cells, they can be identified early in their development. Increase in wall thickness occurs early and rapidly. Concurrently, the plastids, which already contain starch granules, form both crystalline and fibrillar inclusions. As the sieve cell approaches maturity, an extensive network of smooth, tubular endoplasmic reticulum (ER), which becomes mostly parietal in distribution, is formed. At maturity, massive aggregates of this ER occur on both sides of sieve areas. These ER aggregates are interconnected with one another longitudinally by the parietal ER. In addition, the mature, plasmalemma-lined sieve cell contains a degenerate nucleus, mitochondria, and intact plastids. Dictyosomes, ribosomes, and vacuolar membranes are lacking. P-protein is not present at any stage of development.