Biogenic Opal in Three Species of Gramineae

Particulate biogenic opaline silica is concentrated in cellwalls, intercellular deposits and cell lumina of all portionsof the above-ground plant body of three species of PanicoidGramineae,Andropogon gerardi, Sorgastrum nutans and Panicumvirgatum. Morphologically distinct opal phytoliths form notonly in long cells, short cells, trichomes, stomatal elementsand bulliform cells of the epidermis but also within the cellularstructure of mesophyll, vascular, and sclerenchyma tissues.Roots and rhizomes contain measurable quantities of opalinesilica, and phytoliths develop in epidermal long cells, saddle-shapedshort cells, vascular cells, and intercellular deposits. A morphologicallyunique plate-phytolith, formed by silicification of the innertangential wall of the endodermis, is present in the roots ofall three species. Differences in the quantity of opaline silicaoccur between species and between parts of the same species.The amount of opal deposited in the soil annually by root systemsand above-ground parts is approximately equal in magnitud Andropogon gerardi, Sorgastrum nutans, Panicum virgatum, opaline silica deposition     

Silica Deposition in Some C3 and C4 Species of Grasses, Sedges and Composites in the USA

We report new information on silica deposition in 15 plant species,including nine grasses, two sedges and four composites. Thesilica depositional patterns found in seven of the grass speciesindicate that they are C₄ plants. However the festucoid grassCortaderia selloana is a C₃ plant with long leaf trichomes andoval silica structures in the leaves. In contrast the panicoidC₄ grasses Chasmathium latifolium, Chasmathium sessiflorum,Imperata cylindrica, Panicum repens, Panicum commutatum andSetaria magna, all produce dumb-bell-shaped silica structuresin the leaves. The chloridoid grasses Spartina patens and Spartinacynosuroides have saddle-shaped structures and no dumb-bellor oval shaped ones. The sedges Rhynchospora plumosa and Scirpuscyperinus were found to have oval phytoliths and may be C₃ plants.Our examination of these and other grasses strongly suggeststhat C₄ grasses tend to produce the same type of silica cells.Grasses and sedges with C₃ type photosynthesis tend to produceoval silica structures. The composite Grindelia squarrosa andsunflowers Helianthus angustifolia, Helianthus atrorubens andHelianthus tuberosus absorb relatively small amounts of siliconand larger amounts of calcium, where both elements deposit inleaf trichomes. We found no clear indicator for the C₃ sunflowersor C₄ types in the Asteraceae. Helianthus tuberosus leaves havemany trichomes on the adaxial surface. These trichomes havea higher concentration of silica than the surrounding leaf surface.Helianthus tuberosus leaves had much higher ash and silica contentsthan those of Helianthus angustifolia and Helianthus atrorubens.The composite Grindelia squarrosa has a usual deposition ofsilica in the basal cells around the guard cells. Silica depositionoften reflects the surface features of a leaf. An exceptionis Scripus cyperinus where the silica structures are deep inthe tissue and do not reflect the surface configurations. Theinforescence of Setaria magna had a 14.64 silica content. Thetufts of white, silky hairs characteristic of Imperata cylindricainflorescence have no silica. C₃ and C₄ plants, silica and ash content, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, spectra of elements in plants, trichomes, silica fibres, phytoliths     

Intracellular Silica Deposition in Mature and Senescent Leaves of Sieglingia decumbens (L.) Bernh

Following 1, 2, 4, or 8 weeks of growth in a silica-minimalsolution, tillers of Sieglingia decumbens (Heath Grass) weretransferred for 8 days to a nutrient solution which contained50 or 100 ppm silica. The resultant formation of intracellularopal phytoliths (silica bodies) was compared for the four developmentalstages of leaf 3. Characteristically different phytolith typesoccurred in leaves of different ages. Senescent, in contrastto younger, mature leaves, typically exhibited extensive extracellularsilicification of the mesophyll, in addition to deposits instomatal, long, and bulliform cells of adaxial epidermis; depositionin abaxial silica cells and long cells, characteristic of youngleaves, was much reduced, or absent. Physico-chemical factors and cytoplasmic changes associatedwith senescence are discussed in relation to intracellular opalphytolith formation in mature leaves of S. decumbens. In thisrespect, the tendency of silicic acid sols to polymerize inthe presence of an organic matrix and mineral cations is consideredto be significant.     

Silica and Ash in Native Plants of the Central and Southeastern Regions of the United States

Ash and silica contents and depositional patterns were determinedfor different tissues of 11 plants growing in the southeasternand central parts of the USA. Silica content was high in theleaves, sheaths and inflorescences of the grasses studied, especiallyso in the inflorescence of the C₃ grass, Stipa comata Trise.and Rupr. The ash content was especially high in leaves of Polymniauvedalia L., which are also high in calcium. Calcium depositionwas largely in trichomes and in veins of the leaf. Energy-dispersiveX-ray analysis showed that the distribution of the element siliconis closely related to certain epidermal structures such as ridges,cell walls, rows of irregularly-shaped structures lying lenghthwisealong the leaf, dumb-bell shaped structures and trichomes. Thesestructures also correspond to the phytoliths left behind afterdecay of the plant. The C₃ grasses differed from the C₄ in thatthey showed oval structures and produced correspondingly ovalphytoliths. Silicified trichomes (particularly in the C₃ grasses)and long, narrow, silica fibres were common in the inflorescencesof the grasses studied. These sharp particles could be irritatingto oesophageal and other tissues. Similar fibres in other grasseshave been implicated in certain cancers. High silicificationof the inflorescence structures might afford protection forthe seed, as reported for other grasses. C₃ and C₄ grasses, silica and ash content, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, spectra of elements in plants, trichomes, silica fibres, phytoliths     

Distribution of Silicified Cells in the Leaf Blades of Pleioblastus chino(Franchet et Savatier) Makino (Bambusoideae)

Distribution of silicified cells in the leaf blades of Pleioblastuschino was investigated using a light microscope and a scanningelectron microscope equipped with an energy dispersive X-raymicroanalyser. The most dense accumulation of silica was foundin epidermal tissues. Little silica was deposited in vascularbundles and chlorenchyma, while more was deposited in bundlesheath and fusoid cells. In the epidermis, silica density andfrequency of silicified cells differed depending on cell type,although silica deposition was observed in most cell types.Heavy deposition was found in silica cells, bulliform cells,micro hairs and prickle hairs. Silica cells were the cell typemost frequently silicified (96.9–99.7%) in the adaxialand abaxial epidermis. Silica may be deposited as leaf tissuesage.Copyright 2000 Annals of Botany Company Pleioblastus chino(Franchet et Savatier) Makino, bamboo, silicified cells, leaf blade, epidermis, chlorenchyma, silica, clearing method, freeze-fracturing, freeze-drying, light microscopy, scanning electron microscopy, X-ray microanalysis     

Silicon Deposition in the Roots, Culm and Leaf of Phalaris canariensis L

Silicon deposition in the roots, culm and leaf of canary grass(Phalaris canariensis L.) was investigated using light microscopy,scanning electron microscopy and electron probe microanalysis. In adventitious roots grown in solution silicon was concentratedin four endodermal walls. Silicon was not detected in the endodermisof aerial adventitious roots, but was present in the epidermisand outer cortical cell layers. Silicon deposition in the culm mainly took place in the epidermis,and particularly in epidermal papillae. The silica deposition pattern in the leaf was typical of thesub-group Festucoideae. The leaf blade showed deposits in costalprickle hairs and wavy rods, but few intercostal deposits. Inthe ligule deposition was confined to isolated groups of pricklehairs on the abaxial surface. The major sites of silica depositionin the leaf sheath were the stomatal subsidiary cells, papillaeand intercostal idioblasts. Prickle hairs were much less commonin the sheath than the blade, and costal wavy rods appearedto be absent in the sheath. Phalaris canariensis L., canary grass, silicification, root, culm, leaf, electron probe microanalysis     

Silica and Ash in Tissues of Some Coastal Plants

Ash and silica contents and their depositional patterns in differenttissues of 44 Mississippi coastal plants were determined. Silicacontent of dried plants varied from no more than a trace inChenopodium album L. leaves to 7.37 per cent in Zizanopsis miliacea(Michx) Doell & Aschers leaves. Ash content varied from2.50 per cent in Lythrum lineare L. stems to 28.24 per centin Borrichia frutescens (L.) DC leaves. Plants in the same familytend to be alike in their ability to absorb or not absorb silica.Poaceae and Cyperaceae had consistently high concentrationsof silica. In contrast, the Asteraceae studied had very lowsilica contents but often had high contents of other minerals.Dicotyledonous plants studied had consistently lower silicacontents than the monocotyledons. Plants growing in salt watercontained considerable sodium chloride. Spectra were obtainedfor major elements in four different plants. Energy-dispersiveX-ray analysis shows that distribution of the element siliconis clearly related to certain epidermal structures such as guardcells, ridges, dumb-bells and balls that appear in electronmicrographs. Silica was deposited differently in each type ofplant studied. In many of the plants silica was deposited inrows of irregular-shaped particles running lengthwise of theleaf and in guard cells. In others, like Zizanopsis miliacea(Michx) Doell & Aschers, the deposit was sheet-like. Zizaniaaquatica L. not only had a sheet-like deposit, but the depositwas ridged and there were rows of dumb-bell-shaped silica cells.Related plants had similar structures. Euchlaena mexicana Schrad.,Tripsacum dactyloides (L.) L and Manisuris rugosa (Nutt.) Kuntzeall had irregular phytoliths similar to those in Zea mays L. coastal plants, marsh plants, ash content, silica deposition, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, X-ray diffraction patterns, spectra of elements in plants     

Structure and Function of Silica Bodies in the Epidermal System of Grass Shoots

Silica (SiO₂.nH₂O) is deposited in large quantities in the shootsystems of grasses. In the leaf epidermal system, it is incorporatedinto the cell wall matrix, primarily of outer epidermal walls,and within the lumena of some types of epidermal cells. This biogenic silica can be stained specifically with methylred, crystal violet lactone, and silver amine chromate. At theultrastructural level, the silica in lumens of silica cells,bulliform cells and long epidermal cells is made up of rodsabout 2.5 µm in length and 0.4µm in width. Ultimateparticles in the rods range from 1 to 2 nm in diameter. In contrast,silica in the cell wall matrix of trichomes and outer wallsof long epidermal cells is not rod-shaped, but rather, formsroughly spherical masses. Detailed analyses are presented on the frequencies of occurrenceof the different types of epidermal cells that contain silicain the leaves of representative C₃ and C₄ grasses. The C₄ grasseshave higher frequencies of bulliform cell clusters, silica cells,and long epidermal cells, whereas the C₃ grasses have higherfrequencies of trichomes. No correlation was found in the frequencyof occurrence of silica bodies in bulliform cells for C₃ grassesas compared with C₄ grasses. Of all the grasses examined, Coix,Oryza, and Eleusine had the highest densities of such bodies,and some taxa had no silica bodies apparent in their bulliformcells. The idea that silica bodies in bulliform cells and silica cellsmight act as "windows’ and trichomes might function as‘light pipes’ to facilitate light transmission throughthe epidermal system to photosynthetic mesophyll tissue belowwas tested. The experimental data presented do not support eitherof these hypotheses. C₂ and C₄ grasses, biogenic silica, light pipes, window hypothesis, silica staining, silica ultrastructure     

Epidermal Patterning and Silica Phytoliths in Grasses: An Evolutionary History

Due to the immense ecological and economic significance of grasses, their highly characteristic long–short epidermal patterning and associated silica phytoliths represent significant diagnostic markers in studies of ancient climate change and agriculture. We explore the link between epidermal cell patterning and phytolith development and review the evolutionary history of phytoliths in the context of recent well-resolved phylogenetic analyses of grasses and allied Poales, focusing on early-divergent grasses and the subfamilies that constitute the BEP group (the bamboos and their allies). Dimorphic epidermal patterning is a common feature of Poaceae and the related family Joinvilleaceae, where phytoliths are located primarily in the short cells. However, Joinvillea lacks the short-cell pairs that occur in many grasses. The costal rows of phytoliths that characterize some grasses could represent loss of long–short cell patterning over the veins. Unlobed phytoliths probably represent the ancestral condition in grasses, though bilobate phytoliths evolved at an early stage. Either transverse-unlobed or transverse-bilobate phytoliths predominate in the early-divergent lineages, whereas axial-bilobates (or polylobates) primarily characterize the PACMAD clade and the BEP subfamily Pooideae.     

Some Factors in Relation to Bulliform Cell Silicification in the Grass Leaf

The formation of discrete ‘tablets’ of hydratedsilica in the bulliform cells of the leaf blade was followedover a 16-day period in three species of the Gramineae representingdifferent habitats. Seedlings of Oryza sativa (rice) and Cynodondactylon (Bermuda Grass) were cultured under growth-cabinetconditions at levels of 50 and 500 ppm dissolved silica (SiO₂)in the nutrient solution. In addition, bulliform depositionwas studied in mature tiller leaves of Sieglingia decumbens(Heath Grass). Attached leaves, as well as those excised fromthe culm, were used. Initial stages of tablet formation were observed by the 2-dayharvest in the central and basal zones of the fully expandedseedling blades. Deposition did not occur at a stage when bulliformturgor changes might influence blade evolvement. At the 16-dayharvest, deposition was heaviest in the tip zone, and decreasedprogressively towards the base of the blade. In addition, proportionatelyhigher tablet counts (P = 0.05) generally were absent from theleaves grown at the higher silica level. This indicated a limitedavailability of deposition sites. These results are discussed in relation to (i) cellular maturation;(ii) internal leaf anatomy; (iii) leaf expansion; (iv) a basipetalsenescence gradient within the leaf blade. Certain of theseare considered to be possible limiting factors to silica depositionin the grass leaf.     

Silica and Ash Content and Depositional Patterns in Tissues of Mature Zea mays L. Plants

Ash and silica content and their depositional patterns in differenttissues of the mature corn plant (Zea mays L.) were determined.Ash and silica were highest in the leaf blades (up to 16.6 and10.9 per cent, respectively) followed by the leaf sheath, tassel,roots, stem epidermis and pith, and ear husk. The percentageof ash as silica was also highest in the leaves. Silica wasextremely low in the kernels. The upper stem epidermis and pithcontained nearly twice the silica content as did the lower portion.The patterns of ash and silica distribution were similar inplants grown in two different areas of Kansas, but were in lowerconcentration in the leaves and leaf sheaths from the area withlower soluble silica in the soil. Silica was deposited in theepidermis in a continuous matrix with cell walls showing serratedinterlocking margins in both leaves and stem. Rows of lobedphytoliths of denser silica were found in the epidermis as wellas highly silicified guard cells and trichomes. The silica matrixof the epidermis appears smooth on the outer surface and porousor spongy on the inner surface. Zea mays L. Corn, maize, ash content, silica deposition, scanning electron microscopy     

Silica Deposition in the Grass Leaf in Relation to Transpiration and the Effect of Dinitrophenol

Characteristic opal phytolith (‘silica body’) formationwas demonstrated in detached leaves of Sieglingia decumbens(Heath Grass), cultured in 100 ppm dissolved silicon (silicondioxide), previously, the leaves were free from intracellulardeposits as a result of silica-minimal tiller growth. The formertechnique allowed the study of the leaf deposition processesindependently of apical and root tissues, under growth-cabinetand glasshouse conditions. Deposition in excised leaves wascompletely suppressed by a surface, monomolecular coating, thusindicating that total net water loss was a limiting factor,however, evaporation from the recipient, epidermal tissues perse was not a requirement for this in situ deposition Generally,apart from an apparent, cell site shift in one treatment, phytolith-formationwas unaffected by the presence of the metabolic inhibitor 2,4-dinitrophenol. Also, some evidence was obtained of the influxof germanium dioxide into epidermal ldioblasts, which indicateda non-specificity of the host cell for silica. These results and those of earlier studies suggest that passive,non-metabolic mechanisms could account for the transport, influx,and cell lumen polymerization of silica in the grass leaf.     

Detection of Biogenic Silica in Leaf Blade, Leaf Sheath, and Stem of Bermuda Grass (Cynodon dactylon) Using LIBS and Phytolith Analysis

Laser induced breakdown spectroscopy (LIBS) has been used to perform in situ analysis of major and minor elements present in the different parts of the Bermuda grass (Cynodon dactylon). In situ, point detection/analysis of the elements in plants without any sample preparation has been demonstrated. LIBS spectra of the different parts (leaf blade, leaf sheath and stem) of fresh C. dactylon plant have been recorded to study the pattern of silica deposition in its different parts. Atomic lines of Si, Mg, Ca, C, Al, Zn, N, Sr, etc. have been observed in the LIBS spectra of the C. dactylon. A close observation of LIBS spectra of the different parts of the plants shows that silica concentration is greater in leaf blades than leaf sheaths and stems. The results obtained with LIBS analysis are also compared with the number density of phytoliths deposited in different parts of C. dactylon. It is observed that the highest silicified cell frequency is present in leaf blades followed by leaf sheaths and stems which is in close agreement with LIBS analysis.     

Growth Pattern and Movement of Epidermal Cells within Leaves of Asphodelus tenuifolius Cav.

The pattern of growth (velocity field) in the intercalary growthzones of monocotyledon leaves can be determined from patternsof cell number density (number per unit length of cell file)and leaf elongation rates using theory based on a cell numberconservation equation. The case where elongation rate is non-steadywhile the pattern of cell number density is steady is discussedand a method for extending calculations into the meristem usingobservations of numbers of mitotic cells is outlined. Applicationof these methods is illustrated using data for epidermal cellsin the first leaf of Asphodelus tenuifolius Cav. During earlyleaf development, leaf elongation rate increased exponentiallybut cell number density and mitotic number density were steady.Cells 0.1 mm from the base of the leaf when leaves were 3.2mm long took 8.3 d to move through the growth zone. In leavesthat were 4 d older, similar cells took 5.1 d to traverse thegrowth zone. Increases in the rates of leaf elongation and ofcell movement appeared to be associated mainly with increasesin total rates of cell production in the epidermal meristem. Asphodelus tenuifolius Cav., Asphodelus fistulosus L., velocity field, meristem, mitotic cell number density, extension-only zone     

Silica and Ash in Seeds of Cultivated Grains and Native Plants

Silica and ash contents and the depositional patterns of opalinesilica have been determined in the seeds of 31 plant species.Included were 13 monocotyledons, eight dicotyledons and theseeds of eight common cereal grains. The cereal grains, exceptfor Oryza sativa L. (3.2%) and Avena sativa L. (1.4%), werequite low in silica. The silica in these cereals was in thelemma. In seeds with high silica content it often makes up morethan 50% of the ash. Silica in seeds occurs largely in the outercoating of the seed. Dicotyledon seeds tend to have less silicathan those of monocotyledons. Energy-dispersive X-ray analysisshows that the distribution of the element silicon is clearlyrelated to certain epidermal structures such as ridges, raisedareas, trichomes and hairs. It also occurs in cell walls. Membersof a specific plant family tend to have very similar silicadepositional patterns in their seeds. Small amounts of K, S,Cl and Ca are also found in seeds. Light-microscopy studiesshow that the silica in the lemma of seeds such as Oryza sativaL. is deposited in cellular sheet-like structures with crenateedges. Silica in seeds also occurs in fibres and in other cellularstructures (silica cells) that become phytoliths. Seeds, epidermis, seed coat, silica and ash content, scanning electron microscopy, energy-dispersive X-ray analysis, silica depositional patterns, trichomes     

Do lignification and silicification of the cell wall precede silicon deposition in the silica cell of the rice (Oryza sativa L.) leaf epidermis?

Aims

Rice is a well-known silica-accumulating plant. The dumbbell-shaped silica bodies in the silica cells in rice leaf epidermis are formed via biosilicification, but the underlying mechanisms are largely unknown.

Methods

Leaves at different developmental stages were collected to investigate silica cell differentiation by analyzing structures and silicon localization in the silica cells.

Results

Exogenous silicon application increased both shoot and root biomass. When silicon was supplied, silica cells in the leaf epidermis developed gradually into a dumbbell-shape and became increasingly silicified as leaves aged. Silicon deposition in the silica cells was not completed until the leaf was fully expanded. Multiple lines of evidence suggest that lignification of silica cell walls precedes silicon deposition in the lumen of silica cells. The organized needle-like silica microstructures were formed by moulding the inner cell walls and filling up the lumen of the silica cell following leaf maturation.

Conclusions

Two processes were involved in silicon deposition: (1) the silica cell wall was lignified and silicified, and then (2) the silicon was deposited gradually in silica cells as leaves aged. Silica body formation was not completed until the leaf was fully mature.     

Epidermal Cell Division and the Coordination of Leaf and Tiller Development

Initiation and development of grass leaves and tillers are oftendescribed individually with little attention to possible interrelationshipsamong organs. In order to better understand these interrelationships,this research examined epidermal cell division during developmentaltransitions at the apical meristem of tall fescue (Festuca arundinaceaSchreb.). Ten seedlings were harvested each day for a 9-d period,and lengths of main shoot leaves and primary tillers were measured.In addition, numbers and lengths of epidermal cells were determinedfor 0·5 mm segments along the basal 3 mm of each leafand tiller. Primordia development and onset of rapid leaf elongationwere characterized by an increase in the number of cells perepidermal file with mean cell length remaining near 20 µmper cell. After the leaf had lengthened to 1-1·5 mm,cells near the leaf tip ceased dividing and increased in length,at which time leaf elongation rate increased rapidly. Liguleformation, marking the boundary between blade and sheath cells,occurred prior to leaf tip emergence above the whorl of oldersheaths, while the earliest differentiation between blade andsheath cells probably began when leaves were < 1 mm long.Major transitions in leaf and tiller development appeared tobe synchronized among at least three adjacent nodes. At theoldest node, cessation of cell division in the leaf sheath wasaccompanied by initiation of cell division and elongation inthe associated tiller bud. At the next younger node the ligulewas being initiated, while at the youngest node cell divisioncommenced in the leaf primordium, as elongation of a new leafblade began. This synchronization of events suggests a key rolefor the cell division process in regulating leaf and tillerdevelopment.Copyright 1994, 1999 Academic Press Festuca arundinacea Schreb., tall fescue, cell division, leaf initiation, tillering, ligule development     

Silica deposition in abaxial epidermis before the opening of leaf blades of Pleioblastus chino (Poaceae, Bambusoideae)

BACKGROUND AND AIMS Silica deposition is one of the important characteristics of the family Poaceae. The distribution, deposition process and physiology of silica in this family have been extensively investigated. Bamboos among members of Poaceae have leaves with a fairly long life span, and the leaves continuously accumulate silica in their tissues throughout their life, not only during the course of leaf opening, but also after opening. It has been revealed that the silica deposition process in relation to ageing of the bamboo leaf after opening differed depending on the cell types comprising the tissues. However, silica deposition has never been examined during the development and maturation periods of bamboo leaves. Hence, to clarify the silica deposition process in a developmental stage of the bamboo leaf, distribution of silica was observed in the abaxial epidermis before the opening of the leaf blades of Pleioblastus chino. METHODS: Abaxial epidermal tissues of leaves were examined using a scanning electron microscope equipped with an energy dispersive X-ray microanalyser. KEY RESULTS: Among seven cell types comprising the abaxial epidermis, three types of cells, guard cells, prickle hairs and silica cells, deposited silica conspicuously, and another four types, cork cells, long cells, micro hairs and subsidiary cells, deposited only a little silica. Among the former group of cell types, silica cells and guard cells deposited silica over their entire surfaces, while prickle hairs deposited silica only in the point-tips. Silica deposition was detected firstly in prickle hairs, and then in silica cells and guard cells. Only silica cells were assumed to deposit silica conspicuously before leaf opening but not conspicuously after opening. CONCLUSIONS: Cell types in leaf epidermis of bamboo are classified into three groups according to the silica deposition pattern. Silica deposition in silica cells may be positive as a part of the physiological activities of leaves.     

Drought effects on cellular and spatial parameters of leaf growth in tall fescue

The effect of drought and recovery on cellular and spatial parametersof the growth process in tall fescue leaves was studied in twoexperiments. In both experiments plants grown on vermiculiteand maintained in a controlled environment were submitted toa 7 d drought period generated by withholding water. Droughtwas followed by a 3 d recovery period in experiment II. As leafelongation rate (LER) decreased during developing drought boththe growth zone length (initially 40 mm) and the maximum relativeelemental growth rate (initially 0.09 mm mm^–1 h^–1during the dark period of diurnal cycles) within the growthzone declined. But the growth zone still exhibited a lengthof approximately 15 mm when LER approached 0 under severe drought(–2.0 MPa predawn leaf water potential). The growth potentialof the basal 15-mm-long portion of the leaf was conserved duringthe period when drought effected the complete arrest of leafelongation. A (retrospective) analysis of the position-timerelationships of epidermal cells identified on leaf replicas(experiment II) indicated that the cell flux out of the growthzone responded very sensitively to drought. Before drought theflux was maximum at approximately 3.2 cells (cell file h)^–1during the dark period. Flux decreased to 0 when leaf elongationstopped. Flux also varied diurnally both under well-wateredand droughted conditions. In well-watered conditions it wasabout 30% less during the light than the dark period. Cell elongationwas also sensitive to drought. Under well-watered conditionsepidermal cell elongation stopped when cells attained a lengthof approximately 480 µm. During developing drought cellsstopped elongating at progressively shorter lengths. When LERhad decreased to almost nil, cells stopped elongating at a lengthof approximately 250 µn. When drought was relieved followinga 2 d complete arrest of leaf elongation then cells shorterthan 250 µm were able to resume expansion. Following rewateringcell flux out of the growth zone increased rapidly to and abovethe pre-drought level, but there was only a slow increase overtime in the length at which cell elongation stopped. About 2d elapsed until the leaf growth zone produced cells of similarlength as before drought (i.e. approximately 480 µm). Key words: Epidermal cell length, cell flux, (leaf) growth zone, leaf elongation rate, relative elemental growth rate, position-time relationships (path line, growth trajectory), drought, water deficit     

Phytoliths of Indian grasses and their potential use in identification

Phytoliths are amorphous silicon dioxide (SiO₂.nH₂O) inclusions abundant in leaves, in-ternodes and glumes in members of Poaceae. They may occur as inclusions filling the entire lumen of the silica cells, bulliform cells and trichomes or may be part of the outer epidermal cell walls. Since phytoliths are resistant to fungal or animal digestive juices, a large quantity of phytoliths accumulate in the soil where grasses grow. Compared with the pollen grains of grasses which tend to be uniform, phytoliths vary in sue and morphology and can be of value in identification at different taxonomic levels and in the dating of past vegetation. The size and shape of phytoliths of about 100 species of grasses from Tamil Nadu, India, have been determined. Silica bodies were observed either after isolation or in cleared leaf blades. Size and shape of phytoliths were determined under a microscope or from micrographs of the specimens. Size and shape can be used to assign the phytoliths to their respective subfamilies and to distinguish some of the grasses at the generic level. Drawings of silica cells and an identification key are provided for 80 species.