Ricerche Sull'ultrastruttura della Parete Cellulare

Abstract

A study about the cell wall of AVENA coleoptile epidermis cells. — A new type of lamellae structures embedded in the outer periclinal wall of oat coleoptile epidermis cells has been observed. These structures are present more frequently in the inner non-cutinized portion of the cell wall; their orientation, most often parallel to the cell surface, follows a regular pattern. They are formed by alternate layers of electrontransparent and electron-dense bands. The thickness of these lamellar bodies is about 200–300 Å; their length is rather difficult to determinate. They are bounded by a 30–40 Å thick membrane; the inner compartment is formed by a central highly manganophilic zone 50–70 Å thick where several thin lamellae can be seen and by two lateral zones about 40–50 Å in thickness.

Embedded in the cutinized portion of the wall some elliptical bodies have also been observed, surroundedd by a single membrane, 20 Å in thickness. The interior of these bodies shows thin lamellae enclosed in an electron-transparent stroma.

In experimental conditions unfavourable to growth, the frequency of these structures falls greatly. When the cell distension comes to end, their aspect undergoes deep changes. It is proposed in this paper that these new structures are involved in cell wall growth and development.     

薏苡种子胚芽鞘细胞的结构

观察了薏苡浸泡种子胚芽鞘的结构。胚芽 外,内表皮薄壁组织及２个侧位的维管束组成。在外表皮两处,观察到径向壁不边原细胞群,它们实际是合胞体。薄壁细胞含丰富的核糖体,内质网小泡和线粒体,说明代谢活动已经活跃。初生纹孔场内有胞间连丝,显示胞间已存在物质的共质运转。     

HISTOLOGICAL STUDIES ON THE COLEOPTILE. I. TISSUE AND CELL TYPES IN THE COLEOPTILE TIP

The uppermost 1-4 mm of 25-mm coleoptiles of oats and wheat have been studied at the optical microscope level, using newer histological methods and sections 1-4 μ thick. The outer epidermal wall, which shows very fine wrinkling, is continuous with the thinner wall of the inner epidermis through the pore. The cells of both epidermal layers have acidophilic cytoplasm with long transvacuolar strands. Both inner and outer epidermis have stomata, those of the outer epidermis having kidney-shaped guard cells like those of dicotyledons. The guard-cell walls are lignified in their inner layers only and are thinly cuticularized. In the vascular bundles the sieve tubes terminate apically about 250 μ below the end of the xylem; the xylem in turn terminates about 400 μ below the extreme apex. A number of clearly undifferentiated cells, with highly basophilic cytoplasm and many mitochondria, separate the xylem elements from the inner epidermis. Towards the outer epidermis there are a few sieve elements, each of which is associated with a special cell having an elongated nucleus supported on fine cytoplasmic strands. The parenchyma of both the tip and the shaft of the coleoptile are generally interpenetrated by air-spaces, but where they are adjacent to the inner epidermis there is heavy interposition of readily stained intercellular material, especially in Triticum. Plastids are widely distributed throughout the tissue, but their greening in light takes place preferentially towards the phloem side of the vascular bundles. The observations are discussed in reference to earlier literature and with regard to the function of the coleoptile as a protecting and guiding organ for the shoot system of the seedling.     

Ricerche Sull'infrastruttura del Coleottile di Avena

Abstract

Researches on ultrastructure of Avena coleoptile. 3. The sieve elements. — A study on the ultrastructural organization of the mature sieve elements of Avena coleoptile has been carried out. Data suggest that functional phloem tubes are alive and remain alive until they are working. Judging on morphological basis, the metabolic activity of sieve elements should be of peculiar type and low in comparison to that of the companion cells. In fact the cytoplasm is located in a narrow parietal strand, mitochondria, Golgi apparatus and endoplasmic reticulum are present, but they appear very modified; plastids and nucleus are absent. The cytoplasm is bounded externally by a normal plasmalemma, whilst the vacuole has no visible limits: a tonoplast is, therefore not identifiable.

The strands connecting the superimposed sieve elements with one another through the sieve plate result to be made by a double membrane system very similar to the endoplasmic reticulum, which we believe to realize cytoplasmic continuity between phloem tubes.

The data reported are more favorable to the existence in the sieve tubes of an active mechanism of translocation of organic solutes than a passive mass-flow.

The collaboration of companion cells in the translocation mechanism has been discussed.     

Una Struttura Comparabile Con I Lomasomi Nelle Cellule Austoriali di Cuscuta Pentagona Engelm

Abstract

A structure comparable with the lomasomes in baustorial cells of « Cuscuta pentagona » Engelm. — Electron microscopic observations of haustorial cells in « Cuscuta pentagona » revealed to the A. the presence of some vesicular structures, outside the plasma membrane, comparable to the lomasomes of Moore and McAlear. The vesicles usually group in a bag of the plasma membrane and show an envelope consisting of unit membrane; their shape and size are variable and some of them are not well distinct while some others show ribosome-like bodies inside .In the examined cells the plasma membrane shows prominences which are in connection with the vesicle membrane, so that the A. thinks it possible the derivation of the vesicles from the peripherical cytoplasm. The function of these structures is not yet explained; they are only found in non photosyntetic cells, such as some Mycophyta, dark grown « Avena » coleoptiles and dodder's haustorial cells. The previously ultrastructural researchs on dodder have not pointed out lomasomes, which seem to be exclusive of the haustoria, so that it is possible to suppose a connection between the lomasome function and that of the haustoria.     

L'infrasTruttura di Dunaliella Salina

Abstract

The ultrastructure of Dunaliella salina. — The ultrastructure of Dunaliella salina is described. Its structural organisation is rather similar to that previously reported for other Volvocales. The lack of a cellulosic wall is confirmed; the protoplast is bounded only by the monolayered plasma membrane which covers also the flagella. The conspicuous cup-shaped chloroplast fills up the whole back part of the cell and its terminal part is close to the anterior pole: consequently all the other cellular organelles are confined in its deep anterior cavity. The pyrenoid is located in the back part of the chloroplast: communication between the pyrenoid and the surrounding chloroplast is established by a system of paired lamellae. The droplets of carotenoids which form the eyespot are not stacked in regular order. The Golgi apparatus is located in the anterior part and is formed by two conspicuous dictyosomes. A great number of vesciculae, perhaps formed by the dictyosome, are crowded around the insertion of the two flagella. Evidence for the existence of an endoplasmic reticulum is uncertain. Two unknown structures are observed and their morphological organisation is described.     

Osser Vazioni Anatomiche su Foglie di Olivo Sane ed Attaccate da Spilocaea Oleagina (Cast.) Hugh.

Abstract

Anatomical observations on sound leaves and by SPILOCAEA OLEAGINA (Cast.) Hug infected leaves in Olive-tree. — The author has studied the anatomical characters comparatively between two cv. of Olive tree growing in Messina district (Sicily): cv. Ogliarola, receptive to Spilocaea oleagina (Cast.) Hug, and cv. Leccino not receptive.

Anatomical differences of the leaf as a whole and particularly of the «cuticular membrane » have been observed between sound leaves and attacked leaves of cv. Ogliarola, and sound leaves of cv. Leccino.

The main conclusions drawn from this study are the following ones:

The mesophyll is thicker in cv. Ogliarola and thinner in cv. Leccino.

The cuticular membrane and its 4 layers are thicker in Ogliarola leaves than in Leccino's ones.

Ogliarola leaves attacked by the fungus show a cuticolar membrane thicker than the sound leaves of the same cultivar.

Hiperplastic and hipertrophic tissues have been observed in Ogliarola leaves attacked by Spilocaea, expecially epidermis and palisade.

The anatomical causes of resistence and receptivity are prospected.     

L'osmo-Resistenza di Dunaliella Salina Ricerche Ultrastrutturali

Abstract

The resistance of DUNALIELLA SALINA TO OSMOTIC STRESSES. ULTRASTRUCTURAL RESEARCHES. — The mechanisms which confer to Dunaliella salina its high resistance to osmotic stresses have been studied from a cytological point of view. For this purpose the Authors have examined the ultrastructural changes following a great rise or fall in the osmolarity of the normal culture medium.

When the tonicity of the nutrient medium is suddenly lowered the response of the alga is given by an instantaneous and general swelling. The whole cell and the single cytoplasmic structures appear equally swollen and the state of dispersion of the stroma of cellular organelles is highly increased. This shows clearly that a conspicuous amount of water has penetrated in every part of cell and suggests that the plasmatic cell membrane and the inner cytoplasmic boundaries may be considered as semipermeable membranes. The response of the different cellular organelles to a hypertonic shock is not quite identical; the mitochondria show for example a quite uniform type of swelling which does not alter deeply their morphological aspect, whilst in the inner space of the nuclear envelope wide sacks appear suggesting that the outer layer of the double membrane of the nucleus may be more permeable to water than the inner one. In the chloroplasts the flattened sacks formed by paired lamellae having blind endings in the peristromium swell up giving rise to conspicuous vesicles. This proves that the permeability to water of the lamellae is greater than that of the external plastidial boundary.

A sudden and strong hypertonic stress causes a great loss of water from every part of the cell. The alga appears indeed shrunken and shrinking phenomena can be see in all cytoplasmic organelles together with a greater electron density of their stroma. The two layers of the nuclear membrane and the lamellar pairs forming the sacculi in the chloroplasts stick closely together causing a nearly total disappearance of the inner space. This shows that the formation of large vesicles after a sudden hypotonic shock is really due to a different permeability of the two layers of the membrane and not to localised osmotic gradients.

In both cases the quite conspicuous structural variations appear reversible and the alga returns to the normal aspect in a comparatively short time.

From these observations it is concluded that the resistance of Dunaliella salina to severe osmotic shocks (which in our experimental conditions were probably more sudded than in the natural environment of the cells) is not due to a mechanism isolating the cells from the external medium or maïntaining homeostatic conditions but rather to a ready permeability of the plasmatic membranes to water and ions. As a consequence an equilibrium between internal and external osmotic conditions is established within the cell in a sufficiently short time allowing normal life conditions.

It has to be observed that during the short time span necessary for the restablishment of a new osmotic equilibrium and of a normal aspect of the plasmatic structures the cells show some clear symptome of suffering consisting in a loss of the motility and in an altered metabolism. Although this time is rather short, it appears clear that the hardness of Dunaliella to osmotic shocks involves a specific resistance of plasma proteins to denaturation.     

La Flora E La Vegetazione Pel Capo S. Elia (Sardegna Meridionale)

Abstract

Morphological organization of the lomasomes. — From a study on morphological organization of the lomasomes, in Avena coleoptile cells, it has been reported: 1) lomasomes form is quite variable, nevertheless, usually they resemble to a cone having a flattened tip and the base line against the cellular wall. 2) the external border of the lomasoma towards the cytoplasm is represented by the plasma membrane; such external profile results very sinuous and deep invaginations are present. 3) the internal structure is characterized by the presence of an unstructurated matrix containing spherical vescicles, flattened vescicles and tubules all showing an higher density to the electron radiations than the cytoplasmic matrix. Both vescicles and tubules are delimited by a single membrane.     

Immunolocalization of a plant glutathione peroxidase-like protein

We report on the localization of GPXle-1, a plant glutathione peroxidase (GPX)-like protein, in the internode of Lycopersicon esculentum Mill. GPXle-1 was detected in the cytoplasm near to the plasma membrane of trichomes, in the wall of collenchyma and in both the cytoplasm and wall of vascular tissues. GPXle-1 was not found in the epidermis or parenchyma. After mechanical stimulation, a change in its cell distribution was recorded. In stimulated plants, GPXle-1 was detected throughout the cytoplasm in the epidermis, collenchyma and cortical parenchyma.Abbreviations GPX Glutathione peroxidase - ROS Reactive oxygen species     

Chloroplast Development in Rye Coleoptiles

In the parenchyma cells of 1-d-old dark-grown rye coleoptiles (Secale cereale) proplastids occurred which sometimes contained starch grains. During coleoptile growth in darkness starch-filled amyloplasts are formed from the preexisting proplastids. No prolamellar bodies were observed in the stroma of the plastids of the etiolated coleoptile. After irradiation of 3-d-old etiolated coleoptiles with continuous white light three different types of plastids occurred. In the epidermal cells proplastids were observed. The parenchyma cells below the stomata of the outer epidermis (above the two vascular bundles) contained mature, spindle-shaped chloroplasts with a well-developed thylakoid system. In the parenchyma cells that surround the vascular bundles amyloplasts with some thylakoid membranes (chloroamyloplasts) occurred. The mesophyll cells of the primary leaves of dark-grown seedlings contained etioplasts with large prolamellar bodies. In the primary leaves of irradiated plants chloroplasts similar to those of the parenchyma cells of the coleoptile were observed. Our results show that the rye coleoptile, which grows underground as a heterotrophic organ, is capable of developing mature chloroplasts upon reaching the light above the soil surface. The significance of this expression of photosynthetic capacity for the carbon economy of the developing seedling is discussed.     

Studio Anatomico delle Reazioni Indotte dalla Decapitazione Combinata a 2,4-D in Turioni di Asparagus Officinalis L

Summary

1) Young stems of Asparagus officinalis L., from 12 to 46 cm. in length, were decapitated just below the shoot apex and 2,4-D was applied to the cut surface in concentrations 0,1% to 2% by weight in lanolin paste. Decapitated Asparagus stems treated with lanolin only served as controls. A total of 50 stems, of which 10 controls, were investigated.

2) Apart from the sprouting of lateral buds in two plants, no other macroscopical or histological reaction was noticed in the control material. Following treatment with 2,4-D, on the contrary, the following gross responses were evidently seen

a) Bending reaction (figs. 2, 3, 5). This was found to occur in 13 plants, on the second to fourth day since decapitation and at a distance of 1 to 5 cm. from the cut surface. Only in one case was the bending so pronounced as to bring about a complete change in polarity of the upper portion of the stem;

b) Tumor formation. In 8 plants, 7 of which had been treated with doses 1% or 2% of 2,4-D, a swelling appeared just below the surface of decapitation. Due to a difference in cell elongation between the cells of the fundamental parenchyma and those of the cortex, the cut surface took, in a few days, a spoon-like appearance (fig. 10). In two of the 8 plants a tumor-like swelling was noted at some distance from decapitation (fig. 11);

c) Unbalanced elongation in fasciated stems. This brought about the bending towards the cut surface of the more elongated portion in fasciated stems (fig. 15);

d) Longitudinal splits in fasciated stems. (figs. 12, 13). Longitudinal splits were noticed in some of the fasciated stems, which appeared also affected by a very strong gummosis;

e) Survival of decapitated stems. 2,4-D treatment clearly increases the survival of decapitated stems, the maximum duration of survival being 19 days in controls and 30 to 33 days in some stems treated with 1% or 2% phyto-hormone;

f) Sprouting of lateral buds (figs. 4, 5). The frequency of this phenomenon in 2,4-D treated material is not higher as compared with that of controls. However, the treatment seems to induce some auxinlike responses in lateral branches.

3) Concerning the histological reactions, the following can be said

a) No mitosis stimulation is induced neither by wounding alone nor by wounding plus 2,4-D in any portion of the Asparagus stem;

b) radial cell elongation of cortical cells and, in some cases, of pericycle cells (Borgstrom's «transverse reaction») is responsible for both sub-terminal tumors and the two tumor-like swellings of the type reported in fig. 11 (fig. 20 for histological details). Differential elongation in different portions of the stem is responsible for some of the gross responses referred to above;

c) extensive lignification and suberization of parenchyma and, sometimes, epidermal cells occur in 2,4-D treated material. Lignification begins, on the 12th to the 15th day since decapitation, in the cells of the pericycle and fundamental parenchyma surrounding the outermost bundles and gradually spreads centripetally reaching, in extreme cases, some portions of the pith (fig. 24). In these same cases, the lignification process (which is generally present in the first 2 or 3 cm. below the cut surface) may extend as far as 7 cm. below the surface of decapitation. The lignification process appears not to be spred evenly in the stem, being sometimes localized in one or more large areas (fig. 18) and expressing itself in highest degree in zones near the divergence of leaf traces (fig. 16). Lignification of cortical parenchyma and of isolated groups of epidermal cells occurs in a later stage, being concomitant with some suberization process in other cells of the same region of the stem. Photographs of different patterns of both suberization and lignification processes in the layers from epidermis to outermost bundles are reported in figs. 26 to 31, all taken from material treated with 2% 2,4-D. The lignification process takes place according to the well-known scheme of «vascular differentiation» (Sinnot e Bloch 1945);

d) gummosis is very common in 2,4-D treated material. The various stages of gummosis in the different tissues of the stem have been described and special attention has been paid to the extensive formation of lysigenous spaces, following cell degeneration (figs. 32 to 37).

4) In discussing the results of the present investigation, the point is stressed that by experimental means it has been possible to induce in Asparagus some patterns of tissue differentiation and distribution which are to be found in other species of Monocotyledons. After taking into special consideration the actually available data on «vascular differentiation», a twofold effect of 2,4-D on this process is assumed to have taken place, namely: a) favouring influence on cell wall lignification and b) prolongation of survival, probably bringing about a kind of «aging» of tissues. This prolongation of survival seems also to have been a determining factor in the occurrence of the strong gummosis noticed.     

Ricerche sull'infrastruttura del coleottile di avena: I. — Il fascio vascolare in corso di differenziazione

Abstract

STUDIES ON THE INFRASTRUCTURE OF THE OAT COLEOPTILE. I. - THE DEVELOPING XYLEM CELLS. — The writers are engaged with a research program dealing with the morphological changes effected in the cell infrastructure by growth substances. The problem, obviously, requires an intimate knowledge of cell infrastructure of all different tissues of the test used, the oat coleoptile in this case. With this first note, cell infrastructures of the xylem region of the bundle (parenchyma and tracheids) are described, limitedly to the portion 1 mm behind the coleoptile tip, where cell divisions are completely suppressed while cell elongation and differentiation are set on.     

Ricerche al Microscopio Elettronico sui Tubercoli Radicali di Alcune Leguminose

Abstract

Electron microscope studies on root nodules of some leguminous plants. — From this study of ultrathin sections of root nodules of Pea, Bean and Kidney-bean plants, it has been possible to recognize the infrastructural changes which occur during the development of the root nodules, both in the special cells (containing Rhizobia) and in the intermediary cells (free of Rhizobia). With regard to the symbiotic bacteria it has been possible to ascertain that their penetration into root nodules occurs by means of an infection's thread (delimited by its own wall), which contains numerous Rhizobia dipped in an abundant mucilage, with which they, at the beginning, are released into the host plant's cells. The Rhizobia appear enveloped by three membranes, namely a thin citoplasmic membrane, a cell wall, and a « membrane envelope », this last looking at the beginning rather detached from bacterial cell, by the presence of mucillaginous substances which afterward become less and less reduced. The membrane envelope, which is common for several bacteria in the kidney-bean plants, surrounds instead a single bacterium in the pea and in the bean plants. The importance of this membrane seems to rely on the fact that, according to some Authors, it would be formed around the bactetia which are active in nitrogen fixation. Moreover, it has been possible to find out that, among the varous Leguminous plants studied, there are remarkable differences in the morphology of the bacteria as well as in their modifications during the evolution to bacteroid stage.     

Structure and distribution of lignin in primary and secondary cell walls of maize coleoptiles analyzed by chemical and immunological probes

Lignin is an integral constituent of the primary cell walls of the dark-grown maize (Zea mays L.) coleoptile, a juvenile organ that is still in the developmental state of rapid cell extension. Coleoptile lignin was characterized by (i) conversion to lignothiolglycolate derivative, (ii) isolation of polymeric fragments after alkaline hydrolysis, (iii) reactivity to antibodies against dehydrogenative polymers prepared from monolignols, and (iv) identification of thioacidolysis products typical of lignins. Substantial amounts of lignin could be solubilized from the coleoptile cell walls by mild alkali treatments. Thioacidolysis analyses of cell walls from coleoptiles and various mesocotyl tissues demonstrated the presence of guaiacyl-, syringyl- and (traces of)p-hydroxyphenyl units besidesp-coumaric and ferulic acids. There are tissue-specific differences in amount and composition of lignins from different parts of the maize seedling. Electron-microscopic immunogold labeling of epitopes recognized by a specific anti-guaiacyl/syringyl antibody demonstrated the presence of lignin in all cell walls of the 4-d-old coleoptile. The primary walls of parenchyma and epidermis were more weakly labeled than the secondary wall thickenings of tracheary elements. No label was found in middle lamellae and cell corners. Lignin epitopes appeared first in the tracheary elements on day 2 and in the parenchyma on day 3 after sowing. Incubation of coleoptile segments in H₂O₂ increased the amount of extractable lignin and the abundance of lignin epitopes in the parenchyma cell walls. Lignin deposition was temporally and spatially correlated with the appearance of epitopes for prolinerich proteins, but not for hydroxyproline-rich proteins, in the cell walls. The lignin content of coleoptiles was increased by irradiating the seedlings with white or farred light, correlated with the inhibition of elongation growth, while growth promotion by auxin had no effect. It is concluded that wall stiffness, and thus extension growth, of the coleoptile can be controlled by lignification of the primary cell walls. Primary-wall lignin may represent part of an extended polysaccharide-polyphenol network that limits the extensibility of the cell walls.Abbreviations G, S, H guaiacyl, syringyl andp-hydroxyphenyl constituents of lignin - HRGP hydroxyproline-rich glycoprotein - LTGA lignothioglycolic acid - PRP proline-rich protein Dedicated to Professor Benno Parthier on occasion of his 65th birthdayDeceased 7 November 1996     

Cultura in Vitro di Ovuli di Pinus Pinea L.

Abstract

Pinus Pinea ovules cultured in vitro. — The degree of growth and autonomical differentiation of Pinus pinea L. proembryo has been studied by means of controlled cultures in vitro of excised ovules.

Proembryos in vitro undergo involution and initials of their growth points change back into parenchimatoides cells.

Completely differentiated embryos cultivated in vitro behave as if they were not physiologicaly ripe in all their parts. Embryos cultured in august, september and october develop into rocotless seedlings. Only embryos cultured in november have roots able to elongate where germinating, but in a still scarce degree in comparison with hypocotile and cotiledons. Hypocotil root ratio is inverted as regards what happens in nature.

The primary endosperm of Pinus pinea L. cultivated in vitro undergoes surface diffuse proliferation.

A case of polyembriony has been observed.     

宁夏枸杞果实韧皮部及其周围细胞超微结构研究

应用透射电镜技术研究了宁夏枸杞果实韧皮部细胞的超微结构变化。结果表明:(1)随着枸杞果实的发育成熟,果实维管组织中的韧皮部筛分子筛域逐渐变宽,筛孔大而多,通过筛孔的物质运输十分活跃;筛分子和伴胞间有胞间连丝联系,伴胞属传递细胞类型,与其相邻韧皮薄壁细胞和果肉薄壁细胞连接处的细胞界面发生质膜内突,整个筛分子/伴胞复合体与韧皮薄壁细胞之间形成共质体隔离,韧皮部糖分的卸载方式主要以质外体途径进行。(2)韧皮薄壁细胞间的胞间连丝较多,而韧皮薄壁细胞与果肉薄壁细胞的胞间连丝相对较少,但果肉薄壁细胞间几乎无胞间连丝;果肉薄壁细胞之间胞间隙较大,细胞壁和质膜内突间形成较大的质外体空间,为质外体的糖分运输创造了条件。(3)筛管、伴胞、韧皮薄壁细胞和果肉薄壁细胞中丰富的囊泡以及活跃的囊泡运输现象,暗示囊泡也参与了果实糖分的运输过程。研究推测,枸杞果实韧皮部同化物的卸载方式以及卸载后的同化物运输主要以质外体途径为主。     

Osservazioni Sulle Infrastrutture di Anabaena Cycadinae E delle Cellule dei Tubercoli Radicali di Cycas Revoluta

Abstract

Observations on the infrastructures of ANABAENA CYCADINAE and of root-nodules cells of CYCAS REVOLUTA. — Light and electron microscope observations carried on the fine and ultrafine sections of root-nodules of Cycas revoluta, brought in evidence differences structural and infrastructural of cells. These differences concern mainly the modifications presented by reticulum endoplasmic which in middle-aged cells takes a fragmentary and vescicular aspect, and the mitochondria which, in the same cells, show short cristae starting by outside wall of the organul.

About the plastyds, I could bring in evidence that there is plenty of leucoplasts but the proplastyds are still numerous, as the cloroplastyds showing some development of the lamellar system and of grana. Plastyds green are present either in while nodules, either in the root-tops.

In no nodules it has been possible to point out the presence of bacteria while it has been possible to verify that Algae have some polymorphism of structure because the chromatoplasma may result essentially of tree different aspects, by me, indicated as reticular type (fig. 12), lamellar type (fig. 10) and type at mixed structure (fig. 11).     

Ultrastructure of the Pollen Grain and Taxonomy

Abstract

The fine structure of mature pollen grains of several Monocotyle-dons and Dicotyledons plants was studied at the electron microscope. It was observed that inside the pollen grain each generative cell is always clearly separated from romaining cytoplasmic portions. The ways by which the generative cell is delimited are vary in systematically different plants. There may be either a cell wall, quite similar to the one MARUYAMA (1965) reported in the pollen of Tradescantia paludosa. He believed, it to be of a pectocellulosic nature, or a passage from a wall to a two-layered membrane. Multiple membranes, or simply two-layered ones have also been found. These different structures seems to be related to the possible evolutionary trends of the pollen grain. The most primitive forms have grains with an evidently walled generative cell, while in the more evolued ones, there is only a two layered membrane. The fact that in both Monocotyledons and Dicotyledons the generative cell in the more primitive species has a distinct wall, while in the most advanced types it has a double membrane, is very interesting from a phylogenetic point of view.     

Cell biology and genetics of root hair formation inArabidopsis thaliana

Summary In this review we integrate the information available on the cell biology of root hair formation with recent findings from the analysis of root hair mutants ofArabidopsis thaliana. The mature Arabidopsis root epidermis consists of root-hair-producing cells and non-root-hair-producing cells. Root hair growth begins with a swelling of the outer epidermal wall. It has been postulated that this is due to a pH-mediated localised cell wall loosening. From the bulge a single root hair emerges which grows by tip growth. The root hair tip consists of a vesicle-rich zone and an organelle-rich subapical zone. The vesicles supply new plasma membrane and cell wall material for elongation. The cytoskeleton and its associated regulatory proteins such as profilin and spectrin are proposed to be involved in the targeting of vesicles. Ca²⁺ influxes and gradients are present in hair tips, but their function is still unclear. Mutants have been isolated with lesions in various parts of the root hair developmental pathway from bulge identity and initiation, to control of tip diameter and extent and polarity of elongation.Abbreviations [Ca²⁺]_c cytosolic calcium concentration - MT microtubule - PM plasma membrane - VRZ vesicle-rich zone - WT wild type Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday