Ontogeny of pollen in Poinciana (Leguminoseae). II. Microspore and pollen grain periods

Our interpretations for development of exine form in Poinciana gilliesii Hooker are correlated with information, published separately, on the initiation and sequence of development of the exine template. The exine develops exactly in accordance with the template in the following respects: attachment position of foot layer rods, size of the rod components of the foot layer, size of tectal components on the aperture, height of bacules both on aperture and interaperture, and craggy inner surface of the interapertural tectum. Thickness of the interapertural tectum increased after the tetrad period, and the entire endexine was formed only subsequently.The endexine, we find, consists of tubules. The central core of these tubules is low in contrast and has a diameter similar to the thickness of the “white line lamellation” common for these endexine components as seen in oblique and longitudinal views.The bacules over the entire exine, including the extensive synaperture and its prominent margin, are all about the same height. The synaperture is marvellously adapted to accommodate contraction and expansion. Each bacule is cross-connected at the top by tectal straps long enough for rather great separation of neighboring bacules and flexible enough to be folded for close packing of bacules. At their base bacules are attached to one or several rods of the endexine. These rods are either entirely separate or can become separated over apertures.     

Exine ontogeny in Borago officinalis pollen

The primexine matrix is finely granulo-fibrillar up to callose digestion; it becomes distinctly fibrillar at the free microspore stage. The columellae and the tectum are initiated at the middle tetrad stage, the foot layer and the endexine are initiated when the callose wall digestion begins. The columellae are initiated by the deposition of spiral elements around a clear central zone. This hollow aspect of columella disappears when thickening. The foot layer and the endexine are built by the expansion of plasmalemma derived components. The foot layer appears first at the poles, then at the interapertural levels and at last at the apertures while the endexine appears first at the mesoapertures, then it spreads laterally towards the interapertural levels and, at last, at the poles. The gemmae are formed at the free microspore stage over all the tectum. The thickening of the exine takes place essentially during the free microspore stage and continues during the vacuolate microspore one. Apertures are entirely formed before the complete digestion of the callose wall. The ectoapertures are determined by the lacking of the columellae; the sites of the pericolpal cavities and the mesoapertures result from the plasmalemma retraction even before the setting up of the foot layer and the endexine by which they will be delimited respectively afterwards. The endoapertures are determined by the lacking of compact endexine at their level, and merge into a continuous equatorial belt.     

A Comparative study on pollen exine ultrastructure of Nothofagus and the other genera of Fagaceae

The genus Nothofagus is mainly distributed in South America and New Zealand. The present paper describes its pollen exine ultrastructure and compares the exine ultrastructure with that of the other genera of Fagaceae. The pollen grains were examined using ultrathin sectioning technique under transmission electron microscope. The study shows that the pollen exine ultrastructure of Nothofagus differs from that of the other genera of Fagaceae by its exine structure and thickness, type of aperture, and ornamentation. The pollen exine of Nothofagus is thin and possesses granular bacules, regular foot layer and tectum, spinulate ornamentation, and the endexine is usually visible at poral area, and 5～8 colpate. The pollen exine of the other genera of Fagaceae possesses entire bacules, irregular foot layer and tectum, granulate and tuberculate ornamentation, thicker endexine, and is 3-colporate ( 3-colpate or 3-colporoidate). The pollen exine ultrastructure of Nothofagus may belong to primitive type. The pollen exine ultrastructure data support Kuprianova’s opinion that Nothofagus should be separated from Fagaceae and established as a monogenetic family, i.e. Nothofa-gaceae.     

Microspore and tapetal development in Echinodorus cordifolìus (Alismaceae)

In the microspore tetrad period the exine begins as rods that originate from the plasma membrane. These rods are exine units that on further development become columellae as well as part of the tectum, foot layer and “transitory endexine”. The primexine matrix is very thin in the future sites of the pores. At these sites the plasma membrane and its surface coating (glycocalyx) are without exine units and adjacent to the callose envelope. The exine around the aperture margin is characterized by units of reduced height. After the exine units and primexine matrix have become ca 0.2 μm in height a fibrillar zone forms under the aperture margin. It is the exine units around the aperture that are templates for exine processes on apertures of mature pollen. Oblique sections of the early exine show that the tectum consists of the distal portions of close-packed exine units. The exine enlarges in the free microspore period but initially its substructure (tectum, columellae, foot layer and transitory endexine) is not homogeneous and unit structures are visible until after the vacuolate microspore period. There are indications of a commissural line/plane (junction plane) which separates the foot layer from the endexine during early development. Our observations of development in Echinodorus pollen extend a growing number of reports of “transitory endexines” in monocot pollen. The exine unit-structures become 0.2 μm or more in diameter and many columellae are composed of only one exine unit. Spinules become exceptionally tall, many protruding ca 0.7 μm above the level of the tectum as units only ca 0.1 μm in diameter. The outer portion of the tectum fills in around spinules and by maturity they are microechinate with their bases spread out to ca 1 μm or more. Unit structures can be seen with SEM in mature pollen following oxidation by plasma ashing and in the tapetum these units are arranged both radially, as in spinules, and parallel with the tapetal surfaces. There are clear indications of such an arrangement of units in untreated fresh pollen. Units comprising the basal part of the exine are not completely fused by sporopollenin accumulated during development. This would seem to be a characteristic feature, based on published work, of the alismacean pollen. Our use of a tracer shows, however, that there is considerable space within or between exine structure of mature Echinodorus pollen. Based upon the ca 0.1 μm size of exine-units formed early in development and exine components seen after oxidative treatment it seems that the early (primary) accumulated sporopollenin has greater resistance to oxidation than sporopollenin added, secondarily, around and between units later in development. Both primarily and secondarily accumulated sporopollenin are resistant to acetolysis but published work indicates that acetolysis alters exine material. At the microspore tetrad time and until the vacuolate stages tapetal cells are arranged as in secretory tapetums. During early microspore stages there are orbicules at the inner surface of tapetal cells. At free microspore period tapetal cells greatly elongate into the loculus and surround the microspores. By the end of the microspore vacuolate period tapetal cells release their cellular contents and microspores are for a time enveloped by tapetal organelles and translocation material.     

Pollen and anther ontogeny in Cabomba caroliniana (Cabombaceae, Nymphaeales)

Cabomba is a small water lily genus that is native to the New World. Studies of pollen development and associated changes in the anther yield valuable characters for considering the evolution of reproductive biology in seed plants. Here we characterized the complete ontogenetic sequence for pollen in Cabomba caroliniana. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using scanning electron, transmission electron, and light microscopy. Tetragonal and decussate tetrads both occur in C. caroliniana, indicating successive microsporogenesis. The exine is tectate-columellate, and the infratectal columellae are the first exine elements to form, followed by a continuous tectum and a thin foot layer. A lamellate endexine initiates in the early free microspore stage, but becomes compressed in mature grains. Tectal microchannels and sculptural rods also initiate during the early free microspore stage, and significant pollenkitt deposition follows, supporting the hypothesis that these elements function in entomophily. The tapetum is morphologically amoeboid, with migratory tapetal cells directly contacting developing free microspores within the anther locule. Results from this study illustrate the importance of including ontogenetic data in analyzing pollen characters and in developing evolutionary and ecological hypotheses. The new palynological data also emphasize the character plasticity that occurs in basal angiosperms.     

Pollen wall ultrastructure and ontogeny in Heliotropium europaeum L. (Boraginaceae)

The pollen grains of Heliotropium europaeum are heterocolpate, with alternation of 3 colpori and 3 pseudocolpi. The exine is characterized by a scabrate and thick tectum, massive columellae with a granular appearance and a thick nexine. The thickening of the intine at the apertural level makes the interpretation of this zone difficult. The ontogenetic study helped to understand the ultrastructure of the exine and the apertures. The different steps are as follows. The primexine matrix is formed during the beginning of the tetrad stage; it consists of an outer thick and electron dense zone and an inner one, less dense to electrons. The tectum and the infratectum begin to form in the outer zone of the matrix, towards the middle of the tetrad stage. The infratectum consists of a network of columellae variable in thickness and oriented in different directions. The foot layer is lacking. The endexine is formed on a lamella system during the callose loss and microspore separation. The endexine becomes compact very early on its inner part. The apertures are initiated during the tetrad stage; a granulo-fibrillar oncus develops. At the free microspore stage, the oncus gets fibrillar and is bordered by endexine lamellae on its outer side and by endexine granulations on its inner one and laterally. The intine is set at the end of this stage. At the vacuolated microspore stage, the intine shows three layers: two thin, clear and homogeneous layers, one outside and the other inside, and a thick middle layer that forms the zwischenkörper, crossed by trabecula, in the apertural areas.     

Pollen morphology in thePortulacaceae (Centrospermae)

Pollen of 110 species from 18 genera in thePortulacaceae has been examined by light and scanning electron microscopy, and a representative number by transmission electron microscopy. Three basic pollen types were found: 3-colpate with thick tectum and foot layer with prominent unbranched columellae and an extremely thin endexine; pantoporate with thick tectum and foot layer with branched columellae enclosing pores and an endexine that is one to two layers thick; pantocolpate with thin tectum and foot layer with broad, short unbranched columellae and an inconspicuous endexine. All pollen types, however, have a spinulose and tubuliferous/punctate ektexine. Also, all the genera except three,Calandrinia H.B.K.,Montia L. andTalinum Adanson are stenopalynous. There is, however, no absolute correlation between pollen morphology and geographical distribution, although both the major centre of palynological diversity and the majority of all species with tricolpate grains occur in South America.     

Exine development in Nymphaea colorata (Nymphaeaceae)

We show a sequence of developmental events in microspores and tapetal cells in Nymphaea colorata based upon transmission and scanning electron microscopic observations. There are parallel cytoplasmic processes and surface coatings in microspores and tapetal cells. Uptake is indicated by the passage of lanthanum as a tracer from anther locule into the microspore cytoplasm and by the condition of the cytoplasmic surface of microspores. The callose envelope is not a barrier to transfer of lanthanum. During formation of the proexine glycocalyx tiny spiral elements, components of the exine substructural units, were oriented in different directions in the surface coating of microspores and tapetum. Lipoidal globules are associated with the spiral elements. After the uniform proexine stage, three regions of different exine structure and their gradations become differentiated in the sporoderm: 1) a proximal region with thick tectum and foot layer, thin columellae and a compact layer of lamellated endexine; 2) a distal pole region with separately disposed endexine lamellae; and 3) an equatorial encircling-sulcate aperture region which consists of infratectal layer, foot layer, and endexine lamellae. Based upon the presence of structurally comparable surface coats in microspores and tapetal cells, experimental uptake of lanthanum nitrate, and the co-ordinated processes in tapetum and microspores, we conclude that there is probably a reciprocal controlling influence between the microspores and the tapetum and other sporophytic tissues.     

A unique pollen wall mutation in the family Compositae: ultrastructure and genetics

During a routine screening of pollen fertility in the n = 2 chromosome race of Haplopappus gracilis, a spineless pollen wall mutation was discovered that renders the otherwise functional pollen grains completely unrecognizable as Compositae pollen. Normal Haplopappus pollen is characterized by an outer layer, the ektexine, consisting of large spines supported by a roof (tectum), which in turn is supported by collumellae that are joined basally. A large cavity (cavea) stretches from aperture to aperture and separates columellae bases from the final ektexine unit, the foot layer. The spines, tectum, columellae, and columellae bases are filled with perforations (internal foramina), while the foot layer is without them. Immediately underlying the foot layer is a thickened, lamellate, disrupted, internal foramina-free second exine layer, the endexine. In contrast, the mutant pollen ektexine is a jumble of components with randomly dispersed spines as the only clearly definable unit. The endexine layer is similar to the endexine in normal pollen. The mutation apparently disrupts only the organization of ektexine units, and mutant pollen appears to be without the caveae and foot layer characteristic of normal pollen. In genetic tests, the mutant allele is recessive. There is a simple Mendelian pattern of inheritance of the mutant gene, and its phenotype is under sporophytic control.     

Pollen ontogeny in Brasenia (Cabombaceae, Nymphaeales)

Brasenia is a monotypic genus sporadically distributed throughout the Americas, Asia, Australia, and Africa. It is one of eight genera that comprise the two families of Nymphaeales, or water lilies: Cabombaceae (Brasenia, Cabomba) and Nymphaeaceae (Victoria, Euryale, Nymphaea, Ondinea, Barclaya, Nuphar). Evidence from a range of studies indicates that Nymphaeales are among the most primitive angiosperms. Despite their phylogenetic utility, pollen developmental characters are not well known in Brasenia. This paper is the first to describe the complete pollen developmental sequence in Brasenia schreberi. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using combined scanning electron, transmission electron, and light microscopy. Both tetragonal and decussate tetrads have been identified in Brasenia, indicating successive microsporogenesis. The exine is tectate-columellate. The tetrad stage proceeds rapidly, and the infratectal columellae are the first exine elements to form. Development of the tectum and the foot layer is initiated later during the tetrad stage, with the tectum forming discontinuously. The endexine lamellae form during the free microspore stage, and their development varies in the apertural and non-apertural regions of the pollen wall. Degradation of the secretory tapetum also occurs during the free microspore stage. Unlike other water lilies, Brasenia is wind-pollinated, and several pollen characters appear to be correlated with this pollination syndrome. The adaptive significance of these characters, in contrast to those of the fly-pollinated genus Cabomba, has been considered. Brasenia does not produce pollenkitt nor develop tectal microchannels as does Cabomba. Instead, the discontinuity of the tectum reduces the amount of sporopollenin in the wall, which may allow for more effective wind dispersal. The importance of reassessing palynological characters in light of new ontogenetic data and the phylogenetic implications of this reevaluation are also discussed.     

Pollen wall ontogeny in <Emphasis Type="Italic">Polemonium caeruleum</Emphasis> (Polemoniaceae) and suggested underlying mechanisms of development

By a detailed ontogenetic study of Polemonium caeruleum pollen, tracing each stage of development at high TEM resolution, we aim to understand the establishment of the pollen wall and to unravel the mechanisms underlying sporoderm development. The main steps of exine ontogeny in Polemonium caeruleum, observed in the microspore periplasmic space, are spherical units, gradually transforming into columns, then to rod-like units (procolumellae), the appearance of the initial tectum, growth of columellae in height and tectum in thickness and initial sporopollenin accumulation on them, the appearance of the endexine lamellae and of dark-contrasted particles on the tectum, the appearance of a sponge-like layer and of the intine in aperture sites, the appearance of the foot layer on the base of the sponge-like layer and of spinules on the tectum, and massive sporopollenin accumulation. This sequence of developmental events fits well to the sequence of self-assembling micellar mesophases. This gives (together with earlier findings and experimental exine simulations) strong evidence that genome and self-assembly probably share control of exine formation. It is highly probable that self-assembly is an intrinsic instrument of evolution.     

Pollen morphology and ultrastructure of selected species from Annonaceae

The morphology and ultrastructure of fresh pollen from nine species, one including two varieties representing seven genera of Annonaceae are described based on observations with scanning and transmission electron microscopy. The pollen grains are elliptic with a single furrow, or disulculate. Some are globose with no visible aperture or any indication of a pole. Ornamentation is smooth, rugulate, echinate or verrucate. The tectum is usually continuous and of the same thickness over the whole grain except for the aperture zone, where the exine elements are very often imperceptible. The infratectum may be granular, or columellae and granules are mixed together. The foot layer consists of continuous or irregularly contorted foliations. The endexine is distinct and thin, and varies slightly in thickness in some species, but is vaguely distinguishable in others. The intine is two-layered and consists of an entexine with many vesicular-fibrillar components with tubular extensions, and a more homogeneous endintine. The controversy around the presence of an endexine in Annonaceae is discussed, but whether its presence is ancestral cannot be determined. Data on fresh pollen are compared with those from similar studies on dried pollen.     

FINE STRUCTURAL STUDIES OF ZEA MAYS POLLEN I: CELL MEMBRANES AND EXINE ONTOGENY

Electron microscopy was used to study pollen wall ontogeny in Zea mays. The initial stage of development consisted of compartmentalization of microspores within callose special walls. Microspore plasma membranes retracted and tubular elements of the endoplasmic reticulum became perpendicularly oriented to the plasma membranes. Evaginations of the endoplasmic reticulum into the microspore plasma membrane resulted in the establishment of a template or blueprint of the mature pollen wall. Sporopollenin deposition upon the template began immediately after dissolution of the callose special walls and release of the microspores into the anther locule. The columellae were the first pollen wall units to be formed; the tectum and foot layer became established shortly thereafter. The granular endexine was the last-formed unit. The relationships of membrane systems to the ontogeny of the pollen wall units and the mode of pollen wall growth are discussed.     

木通科、大血藤科花粉壁的超微结构研究

应用透射电子显微镜(TEM)观察了木通科Decaisnea,Sinofr-anchetia,Holboellia,Stauntonia属以及大血藤科Sargentodoxa属共18种植物花粉壁的超微结构。所观察的木通科和大血藤科植物具较发达的覆盖层和柱状层;外壁内层以及内壁均在萌发沟处明显增厚;基层通常不甚发达。与扫描特征相对应的覆盖层结构特征,显示出类群的特异性。在Stauntonia属,覆盖层富于形态变化,反映出该属在木通科中较进化的地位;大血藤(Sarg-entodoxa cuneata)花粉壁结构隶属木通型花粉结构,表明大血藤科与木通科的密切关系。     

前胡族花粉外壁超微结构及其系统学意义

报道了前胡族(Peucedaneae Drude)当归亚族(Angelicinae Drude)、阿魏亚族(Ferulinae Drude)和环翅芹亚族(TordyliinaeDrude)等3亚族18属18种植物的花粉外壁表面和内部的超微结构.根据花粉外壁超微结构资料,论述了18种及其所在属的系统位置.对现时尚存疑的问题,诸如当归亚族中的山芹属(Ostericum Hoffm.)等7属(种),经分析认为统归于当归属(Angelica L.)不恰当;阿魏亚族中的球根阿魏(Schumannia turcomanica Kuntze)、伊犁芹(Talassiatransiliensis(Herd.)Korov.)、胀果芹(Phlojodicarpus villosus(Turcz.ex Fisch.et Mey.)Turcz.ex Ledeb.)等属的代表种显示其外壁演化程度相差甚大,应以与阿魏属(Ferula L.)分别独立为宜;环翅芹亚族(Tordyliinae Drude)中大瓣芹属(Semenovia Regel et Herd.)与独活属(Heracleum L.)的代表种其外壁演化程度相距甚远,也以分立为宜.     

前胡族花粉外壁超微结构及其系统学意义(英文)

Transmission electron microscopy (TEM) observation of pollen grains of 18 species belongingto 18 respective genera of the tribe Peucedaneae Drude revealed distinct ultrastructural difference in thepollen exine, including the thickness and features of rectum, columnar layer, foot layer and endexine. Thesystematic position of those 18 species are re-evaluated based on their ultrastructural characteristics ofpollen exine observed in this study. Seven genera related to Angelica L. in Angelicinae Drude have beendeduced to genus Angelica L. For example, Osterfcum grosseserratum (Maxim.) Kitagawa was changed toAngelica L. by some authors, but it differs from Angelica sinensis (Oliv.) Diels in having well developedtectum which is thicker than the columnar layer and foot layer. Its columnar layer is quite well-developedwith long and branched columellae. Besides its surface is tuberculated. Evidently, its development exceedsthat of Angelica sinensis. However, it is only a moderate evolutional species in its genus, and the Angelicasinensis accounts as the most advanced species in Angelica Diels, thus, placing genus Ostencum Hoffm.in Angelica L is not suitable, it may be more appropriate to keep its original position. In addition, Ferulaakitschkensis B. Fedtsch. ex K.-Pol. differs greatly from those of Talassia transiliensis (Herd.) Korov. andSchumannia turcomnnia Kuntze, because of its very well-developed columnar layer, being about four tofive times thicker than the total of tectum and foot layer, tuberculated tectum surface and complicatedstructure of columnellae. As another species Heracleum forrestJ‘iWolff also differs from Semenovia rubtzovii(Schischk.) Monden. in having an even thickness of exine and well-developed columnar layer, it seemssuitable that they should also be treated as two independent genera.     

Development of the pollen wall in Tsuga canadensis (Pinaceae)

In discussions of exine structural types, Tsuga is often mentioned as an exception, since no infratectal layer is present in the ektexine. The present investigation documents the formation of this pollen wall type at the ultrastructural level in T. canadensis . All layers of the exine are formed during the tetrad period, when the microspores are surrounded by a callose wall. The outer layer (ektexine) is elaborated on a fibrillar microspore surface coat, while the inner layer (endexine) is elaborated on lamellated structures. The deposition of the pretectum is followed by the appearance of endexine lamellae. In the initial stages, the two layers—pretectum and endexine—appear to be separated from each other only by a dense microspore surface coat. As additional wall materials are deposited, the tectal elements become convoluted and come to rest, in places, on the now recognizable footlayer. Upon release from the tetrad, intine formation begins and continuous accumulation of sporopollenin leads to an increase in ektexine thickness. The mature pollen wall of Tsuga canadensis , with a convoluted tectum resting directly on the footlayer, is characteristic of the genus.     

Development and cytochemistry of pollen and tapetum in Mitriostigma axillare (Rubiaceae)

The development of pollen grains and tapetum in Mitriostigma axillare (Rubiaceae) was studied from anther primordium to dehiscence. Anthers were freeze-cracked and studied with SEM. Embedded anthers were sectioned and studied with LM and TEM. Cytochemistry was performed in order to distinguish the different layers of the sporoderm and to determine its chemical nature at different development stages. The pollen grains remained as tetrads by partial fusion of the exine, probably because of reduced callose septa during the stage of microspore tetrads within callose envelopes. Characteristic features of the sporoderm were an irregular foot layer, an endexine composed of amalgamated granules, a transient granular-fibrous layer beneath the endexine, and a thin intine. During maturation of the exine, the endexine became chemically different from the ectexine. All layers of the sporoderm were reduced in thickness due to stretching during the engorgement of the pollen grains prior to dehiscence. The pollen grains were colpoidorate with a reticulate to microreticulate tectum covered with a scanty surface coating. The mature pollen grains were binucleate and contained a lot of starch grains. Thick intineous onci protruded through the apertures and formed papillae. About 50% of the microspores were aborted. The tapetum was of secretory type, probably with cycles of hyperactivity and protrusions of the cells into the locular cavity. No syncytium was formed and there were neither orbicules nor tapetal membrane.     

Comparative exine development from the post-tetrad stage in the early-divergent lineages of Ranunculales: the genera <Emphasis Type="Italic">Euptelea</Emphasis> and <Emphasis Type="Italic">Pteridophyllum</Emphasis>

Studies of pollen wall development produce a great deal of morphological data that supplies useful information regarding taxonomy and systematics. We present the exine development of Euptelea and Pteridophyllum, two taxa whose pollen wall development has never previously been studied using transmission electron microscopy. Both genera are representatives of the two earliest-diverging families of the order Ranunculales and their pollen data are important for the diagnosis of the ancestral pollen features in eudicots. Our observations show these genera are defined by having microechinate microreticulate exine ornamentation, perforate tectum, columellate morphology of the infratectum and the existence of a foot layer and endexine. The presence of lamellations is detected during the early stages of development in the nexine of both genera, especially in the apertures. Euptelea presents remains of the primexine layer during the whole maturation process, a very thin foot layer, and a laminate exinous oncus in the apertural region formed by ectexine and endexine elements. Pteridophyllum has a thicker tectum than Euptelea, a continuous foot layer and a thicker endexine. In the apertures, the exinous oncus is formed by islets and granules of endexine, in contrast to the Euptelea apertures. The secretory tapetum produces orbicules in both genera, but they have different morphology and electron-density. Comparisons with pollen data from related orders and families confirm the ancestral states for the pollen of eudicots proposed in previous studies: reticulate and echinate surfaces, columellate infractectum and a thin foot layer relative to the thickness of the ectexine. According to our observations, we propose considering the possibility of a polymorphic state for the aperture number in the ancestor of Ranunculales, and suggest the development of orbicules as the ancestral state in this order.     

Ontogeny of the exine in pollen of Aristea (Iridaceae)

The ontogeny of the pollen wall was studied in four species of Aristea , from the vacuolated stage of the microspores, to observe the possible formation of an endexine. At this stage, the ectexine is completely formed (tectum, columellae, and structurally homogeneous foot layer), but its maturation is incomplete and variable depending on the species. In all cases, there are one or several tripartite lamellae with a white line under the foot layer, in the apertural and extra-apertural regions. In A. major , and A. pauciflora , the exintine is not yet present, whereas in A. macrocarpa and A. glauca , it has started to initiate. In mature pollen of the four species, the tripartite endexine lamellae of the vacuolated stage disappear and there is no trace of endexine. The tripartite intine is completely formed. Maturation of exine is complete and it appears homogeneous and of medium electron density, except in A. glauca , which has particularly fragile exine, where it remains incomplete with a granular and highly electron dense appearance, which contrasts with the usually mature exine. Despite the very clear presence of endexine lamellae at the vacuolated stage, it is thus very difficult to conclude that endexine exists in pollen of the genus Aristea .