Variation in Two Sorbus Species Endemic to the Isle of Arran, Scotland

A study of the morphology of plants from four species of Sorbus,S. arraensis, S. pseudofennica, S. aucuparia and S. rupicola,two of them endemic to North Arran, suggested that the originof the two endemic groups could have been by hybridization. Variation between individuals within the hybrid groups indicatesthat the two groups, probably of hybrid origin (S. arranensisand S. pseudofennica), could each consist of a set of geneticallyisolated clones reproducing by apomixis, with the possibilityin the case of S. pseudofennica of occasional genetic interchangeby sexual reproduction. The overlap of characters between the two hybrid groups suggeststhat the inflow of genetic material by hybridization and geneticmodification in these groups is continuing. Sorbus arranensis Hedl., Sorbus pseudofennica E. F. Warb., Sorbus aucuparia L., Sorbus rupicola (Syme) Hedl., interspecific hybridization, apomixis     

Effect of Wind on the Transpiration of Young Trees

The effects of wind on the transpiration rates of four plantspecies, Pinus sylvestris L., Quercus robur L., Fagus sylvaticaL. and Sorbus aucuparia L., were studied in a controlled environmentwind tunnel. Transpiration declined with increasing wind speedin a manner consistent with predictions of the Penman-Monteithequation. The stomatal resistance declined with increasing windspeed in two species and increased in one, but this effect wassmaller than reported in other studies. In all cases the magnitudeof the stomatal response was over-shadowed by the decliningleaf to air vapour pressure difference. Pinus sylvestris L., Quercus robur L., Fagus sylvatica L., Sorbus aucuparia L., wind, transpiration, cooling curve technique, stomatal resistance, aerodynamic resistance, Penman-Monteith equation     

Plasmodesmata and pit development in secondary xylem elements

Developing pit membranes of secondary xylem elements in Drimys winteri, Fagus sylvatica, Quercus robur, Sorbus aucuparia, Tilia vulgaris and Trochodendron aralioides have been examined by transmission electron microscopy. Absence of plasmodesmata from the membranes of vessel elements and tracheids indicates that their pits develop independently of these structures. On the other hand, plasmodesmata are abundant in pit membranes between fibres, parenchyma cells, and combinations of these cell types in Fagus, Quercus and Tilia. In each case the plasmodesmata pass right through the developing pit membrane. In the case of Sorbus fibres, however, plasmodesmata were absent from the majority of pit membrane profiles seen in sections. Occasionally they were observed in large numbers associated with a swollen region on one side of the pit membrane between fibres and between fibres and parenchyma, radiating from a small area of the middle lamella. In the case of fibre to parenchyma pitting, this swelling was always found on the fibre side of the membrane, while on the other side a small number of plasmodesmata were present completing communication with the parenchyma cytoplasm. These observations are discussed with regard to the role of plasmodesmata in pit formation, and in the differentiation of the various cell types in secondary xylem. The significance their distribution may have for our understanding of xylem evolution is also discussed.     

Electron Microscopic Observations of the Structure of Sieve-connexions in the Phloem of Angiosperms and Gymnosperms

The sieve elements of Pinus silvestris L., Sorbus aucupariaL., Vitis vinifera L., and Cucurbita pepo L. have been examinedelectron microscopically in ultra-thin section, and the structuresof the corresponding sieve areas or sieve plates have been describedand compared. In Pinus the sieve areas contain groups of connectingstrands which enter the wall from the lumen side as individualsbut coalesce within it in the median cavity. This cavity hasdeveloped by wall breakdown in the middle lamella and primarywall region and corresponds to the median nodule visible undera light microscope. Neither in this nor in the other speciesobserved is there any visible closing membrane. Structural differences between the four species are shown tosuggest that the major evolutionary trend in the evolution ofspecialized conducting strands has been the enlargement of theconnecting strands from groups of small separate strands toa smaller number of larger strands as the median cavity becomesenlarged to form a canal through the wall. The connecting strands appear invariably to be dense, highlyosmiophilic and continuous with the cytoplasmic surface of thecell. No signs of micropores or of other tubular structure inthe strands have been found. The structures thus revealed aremore nearly compatible with active transport of materials acrossthe sieve plate than they are with any purely physical mechanism.It is suggested that they are incompatible with any mass flowhypothesis.     

Carotenoids of Rowan Berries

The berries of Sorbus aucuparia have been investigated for theircarotenoid contents. The pigments identified were: phytofluene,-, ß-carotene, cryptoxanthin, monoepoxy--carotene,monoepoxy-ß-carotene, aurochrome, and mutatochrome.Usually when green fruits ripen the control of carotenoid synthesisis removed when the chlorophylls disappear, there is a rapidincrease of carotenoids in an over-all oxidative manner anddifferent carotenoids appear. However, the results obtainedsuggest that a different mechanism takes place in S. aucuparia.This may be the exception that proves the rule.     

Scanning electron microscopy in nematode-induced giant transfer cells.

A study of giant cells induced by the root-knot nematode, Meloidogyne incognita, in roots of Impatiens balsamina was made by scanning electron microscopy. The cytoplasmic contents of giant cells were removed by a procedure based on KOH digestion, to reveal inner wall structure. Wall ingrowths typical of transfer cells are present in giant cells from six days onwards after induction. They develop on walls adjacent to vascular tissues, and their distribution and development was examined. Pit fields contianing plasmodesmata become elaborated in walls between giant cells, but pit fields are lost between giant cells and cells outside them. The distribution of plasmodesmata in pit fields suggests that de novo formation of plasmodesmata occurs in walls between giant cells. Various aspects of giant cell formation and function are discussed and wall ingrowth development is compared in giant cells and normal transfer cells.     

Occurrence of plasmodesmata between differentiating vessels and other xylem cells inSorbus torminalis L. Crantz and their fate during xylem maturation

Summary The development of pit-pairs between differentiating xylem cells has been examined by transmission electron microscopy in young shoots ofSorbus torminalis. In some vessel-to-tracheid pits, as well as in previously studied intertracheid pits, a thickening of the pit membrane containing branched plasmodesmata was observed. A secondary wall-like cap was deposited over the thickening prior to cytoplasmic autolysis; some plasmodesmata, parallel to the plane of section, appeared to perforate the cap. At the end of the cell maturation stage, the central part of the primary wall thickening was hydrolysed, while the cap, including plasmodesmata remnants, appeared unaltered. In half-bordered pit-pairs between a parenchyma cell and a vessel or a tracheid, similar structures could be observed beside the conducting elements. When the vessel or tracheid matured, sealing of the pit membrane plasmodesmata resulted from the formation of a protective layer on the parenchyma-side rather than from the deposition of a cap on the conducting cell-side. These observations provide the first information on the presence of symplasmic connections in pits between differentiating vessels and neighbouring xylem cells. InS. torminalis, xylem differentiation is probably highly coordinated within a symplasmic domain; the persistence of such connections may account for the lack of specialization ofSorbus wood.     

The occurrence of plasmodesmata-like structures in a non-division wall

Summary The fine structure of the interspecific junction in the periclinal chimeraCytisus adami has been investigated. This non-division wall shows the occurrence of pits of normal morphology. The pit membrane is partially penetrated by structures closely resembling plasmodesmata. These plasmodesmata, however, do not show direct continuity across the wall. Connections where they are seen appear to be established by a process of bridging between two half-plasmodesmata. The results are discussed with reference to the reversibility of the differentiation of plasmodesmata and their mode of formation.     

Rapid detection of plasmodesmata in purified cell walls

Summary Plasmodesmata are complex channels within the plant cell wall, which create plasma membrane and symplastic continuity between neighbouring cells. To detect plasmodesmata in cell wall preparations fromNicotiana cle elandii, we have used 3,3-dihexyl-oxacarbocyanine iodide (DiOC₆), a cationic amphiphilic fluorescent probe, widely employed for general studies of membrane structure and dynamics. Punctate fluorescent staining was readily seen in pit fields, small depressions within the cell wall known to be rich in plasmodesmata. Scanning electron microscopy was used to demonstrate that the punctate staining corresponded to plasmodesmata. Treatment of cell wall fragments with chloroform-methanol to remove lipids did not alter the staining of plasmodesmata. In contrast, pronase E-sodium dodecyl sulfate treatment completely abolished staining, indicating that the DiOC₆ labelling of plasmodesmata may be protein rather than lipid specific. Although not membrane mediated, DiOC₆ staining of plasmodesmata is a simple, rapid, and specific tool for the detection of plasmodesmata in isolated cell walls and will prove useful for studies of plasmodesmal location, structure, and composition.     

Stomata and plasmodesmata

Summary In developing epidermal tissue ofPhaseolus vulgare L. complete plasmodesmatal connections occurred between guard cells and epidermal cells and between sister guard cells of a stoma but they were not seen in fully differentiated tissue. However, incomplete, aborted plasmodesmata were occasionally seen in the common guard/epidermal cell wall, usually connected to the epidermal cell protoplast, in mature tissue. Plasmodesmatal connections between neighbouring epidermal cells were commonly observed in tissue at all stages of development. In all locations, the plasmodesmata were usually unbranched occurring singly or in small pit fields; very rarely branched, incomplete plasmodesmata were also seen in the wall between mature guard and epidermal cells. The significance of these findings were related to stomatal functioning and to the development of plasmodesmata in general.     

Das gelbmosaik der eberesche (Sorbus aucuparia L.)

An Ebereschen (Sorbus aucuparia L.) wurden Blätter mit leuchtend gelben Flecken und Rändern sowie vollständig vergilbte Blätter beobachtet. Diese Symptome konnten durch Rinderschiidpfropfung auf gesunde Sämlinge von 5. aucuparia übertragen werden, nicht jedoch auf andere Gehölzarten. Die mechanische Übertragung eines Virus von kranken S. aucuparia‐Sämlingen auf krautige Testpflanzen gelang nicht. In kranken Blattgeweben konnten auch keine virus‐ oder phytoplasmaähnlichen Partikeln festgestellt werden.     

Arrangement of cortical microtubules during formation of bordered pit in the tracheids ofTaxus

Summary The arrangement of cortical microtubules during the development of the secondary wall and bordered pits in the tracheids ofTaxus was examined by immunofluorescence and electron microscopy. The cambial region of radial longitudinal sections of developing young shoots (2–3 years old) contains cells at various stages of differentiation from cambial cells to tracheids. At the early stage of formation of bordered pits, circular bands of microtubules were seen to be associated with the inner edge of the border of the developing pit. In other regions than the pit secondary wall of uniform thickness was laid down, and obliquely oriented cortical microtubules ran parallel to one another. These cortical microtubules also covered the surface of the border of the developing pit on the side facing the center of the cell. As the border of the pit developed, a circular band of MTs remained associated with the inner edge of border, suggesting that the MTs were involved in the formation of the rim of the bordered pit, extending the initial border thickening, which consisted of concentrically oriented cellulose microfibrils. After completion of the formation of the bordered pit, helical thickenings became apparent. The obliquely oriented microtubules were organized in bands parallel to one another, being superimposed on the helical thickenings. The involvement of MTs in the formation of bordered pits and helical thickening is discussed.     

Periodicity of Wood Formation in Twigs of some Tropical Trees in Nigeria

The periodicity of wood formation was studied in twigs of treesgrowing in three ecological areas in Nigeria, namely lowlandrainforest, Southern Guinea savanna and mangrove swamp. Initiationof cambial activity was correlated with bud break, and wheremultiple vegetative bud formation occurred in the lowland rainforestand the Southern Guinea savanna, the number of growth ringscorresponded with the number of times the vegetative buds opened.In the mangrove swamp, although there may be a clear correlationbetween vegetative bud growth and wood formation in 1 to 2 year-oldtwigs of Drepanocarpus lunatus and Ormacarpum verrucosum therewas no such correlation in Dalbergia ecastaphyllum. Short axillarybranches and inflorescences which developed irregularly in plantsof the mangrove swamp induced multiple growth ring formation.In D. ecastaphyllum, the pattern of unfolding of young leavesseems to play a major role in the formation of multiple growthrings. In each of the areas multiple growth ring formation couldbe induced by injury. In Daniellia oliveri there was periodicdeposition of prismatic crystals. The crystals were depositedeach time wood formation stopped. In plants in which flowersopened before the opening of the vegetative buds, there wasinitiation of cambial activity by the opened flowers in thetwigs on which the flowers were borne.     

Serial Development of Foliar Gemmae in Tortula (Pottiales, Musci), an Ultrastructural Study

The development and liberation mechanism of foliar gemmae havebeen studied by electron microscopy in two mosses, Tortula latifoliaBruch and Tortula papillosa Wils. The gemmae develop on theadaxial surface of mature leaves from single initial cells onboth the lamina and costa in T. latifolia but only on the costain T. papillosa . Elongation of the initial cell is associatedwith the deposition of a highly extensible new wall whilst theold wall and cuticle in the apical dome rupture. The first divisionis transverse and separates a short basal cell embedded in thefoliar tissue and a distal cell, or gemma primordium, protrudingfrom the leaf surface. Subsequent divisions of the gemma primordiumgive rise to a six-to-eight-celled globose gemma with mucilaginousouter walls. During gemma development the basal cell producesa new wall and elongates again whilst the common wall with thegemma splits apart centripetally along the boundary betweenthe old and new wall in the basal cell; plasmodesmal connectionsare gradually severed and eventually the young gemma remainsconnected to the basal cell only by mucilage. After separationof the first-formed gemma, the basal cell may expand and producea second gemma by the same mechanism. The whole process maybe repeated several times resulting in the formation of a chainof gemmae stuck together by mucilage and which are liberatedonly when the leaves are fully hydrated. Accumulation of abundantlipid deposits in the gemmae after symplasmic isolation reflectsconsiderable photosynthetic autonomy. Abscission; bryophytes; cell wall formation; plasmodesmata; vegetative reproduction     

Structure, Composition and Physiological State of the Endosperm of Phoenix dactylifera L

The date endosperm consists of living cells with the same generalcellular structure throughout the seed. The major storage products,as shown by histochemical staining, are lipid, stored as numeroussmall lipid bodies which fill the cytoplasm, and protein, aslarge but variably-sized protein bodies. Nuclei are presentbut lack large amounts of heterochromatin. Plastids and mitochondriaare present but infrequently seen and have poorly developedinternal membranes. No endoplasmic reticulum or dictyosomesare present before or after hydration. The cell wall is thickexcept in areas of pit fields and consists of three layers whichdiffer in their staining behaviour: middle lamella, thickenedwall and inner wall. Both the endosperm and embryo of the imbibedseed undergo aerobic respiration, but the embryo respires ata more rapid rate than does the endosperm. A small area of theendosperm around the distal pole of the cotyledon shows histochemicallydetectable levels of succinic dehydrogenase. Phoenix dactylifera L, date palm, endosperm, ultrastructure, histochemistry, respiration, succinic dehydrogenase     

The Ultrastructure and Computer-enhanced Digital Image Analysis of Plasmodesmata at the Kranz Mesophyll-Bundle Sheath Interface of Themeda triandra var. imberbis (Retz) A. Camus in Conventionally-fixed Blades

The ultrastructure of the plasmodesmata at the Kranz mesophyll-bundlesheath (KMS-BS) interface in Themeda triandra, and the substructureswithin the plasmodesmata were investigated, using conventionallyfixed leaf-blade material, enhanced by the addition of 0·1%tannic acid to the primary fixative. Examination of high-resolution electronmicrographs, and computer-enhanceddigital images suggests that these plasmodesmata are complexstructures, comprised of helically-arranged particulate material.The electron-dense particles are between 2·5 and 3·0nm in diameter. These particles are specifically associatedwith the inner face of the inner plasmalemma membrane leaflet,and the outer region of the desmotubule wall. The electron-denseparticles are presumably proteinaceous and embedded in a lipidmatrix. In the constricted median portion of the KMS-BS plasmodesmata,the space between the desmotubule and the inner plasmalemmamembrane leaflet and areas surrounding the proteinaceous particlesthereof (the cytoplasmic sleeve) is about 3 nm in cross-section,and constitutes what we believe to be the space through whichintercellular transport takes place.Copyright 1993, 1999 AcademicPress Themeda triandra, Poaceae, cytoplasmic sleeve, image analysis, plasmodesmatal structure, desmotubule, Kranz mesophyll     

Ultrastructural study of plasmodesmata in the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae)

Plasmodesmata are intercellular bridges that directly connect the cytoplasm of neighboring cells and play a crucial role in cell-to-cell communication and cell development in multicellular plants. Although brown algae (Phaeophyceae, Heterokontophyta) are phylogenetically distant to land plants, they nevertheless possess a complex multicellular organization that includes plasmodesmata. In this study, the ultrastructure and formation of plasmodesmata in the brown alga Dictyota dichotoma were studied using transmission electron microscopy and electron tomography with rapid freezing and freeze substitution. D. dichotoma possesses plasma membrane-lined, simple plasmodesmata without internal endoplasmic reticulum (desmotubule). This structure differs from those in land plants. Plasmodesmata were clustered in regions with thin cell walls and formed pit fields. Fine proteinaceous "internal bridges" were observed in the cavity. Ultrastructural observations of cytokinesis in D. dichotoma showed that plasmodesmata formation began at an early stage of cell division with the formation of tubular pre-plasmodesmata within membranous sacs of the cytokinetic diaphragm. Clusters of pre-plasmodesmata formed the future pit field. As cytokinesis proceeded, electron-dense material extended from the outer surface of the mid region of the pre-plasmodesmata and finally formed the nascent cell wall. From these results, we suggest that pre-plasmodesmata are associated with cell wall development during cytokinesis in D. dichotoma.     

A quantitative analysis of the interspecific plasmodesmata in the non-division walls of the plant chimera Laburnocytisus adamii (Poit.) Schneid.

Non-division walls in petals of the chimera Laburnocytisus adamii (Poit.) Schneid, were screened for the occurrence and distribution of symplasmic connections. The secondary plasmodesmata (PD) between epidermal cells of Cytisus purpureus Scop, and subepidermal cells of Laburnum anagyroides Medik. were compared with the PD of corresponding cell walls in petals of the two parental species. The non-division walls in the petals of L. adamii were traversed mainly by continuous PD and a few half-PD, both being grouped in pit fields. The secondary PD were characterized by a high percentage of branching (82%), with more than 40% consisting of a single strand at the Cytisus cell side interconnected by a median cavity with two strands of the Laburnum subepidermal cell. In addition, more than 30% of all PD showed secondary branching in the subepidermal wall portion. As a consequence, the cross-sectional areas of plasmodesmatal strands on each side of the central cavity differed remarkably in size, representing a “bottleneck” in the epidermal wall portion. In contrast, PD in the petals of the parental species were symmetrically branched. The comparison of cross-sectional areas of PD in the cell wall between the epidermis and subepidermis of petals of L. anagyroides showed a well-tuned system. The occurrence of half-PD in the intraspecific wall indicates a secondary origin. We conclude that, in the chimera, both genotypically different cells take part in the formation of the interspecific PD.