Origins and nature of vessels in monocotyledons: 8. Orchidaceae

Xylem of the orchids studied provided unusually favorable material to demonstrate how conductive tissue evolves in monocotyledons. In the end walls of tracheary elements of many Orchidaceae, remnants of pit membranes were observed with scanning electron microscopy and minimally destructive methods. The full range from tracheids to vessel elements, featuring many intermediate stages, was illustrated with SEM in hand sections of fixed roots, stems, and inflorescence axes of 13 species from four subfamilies. Pit membranes in end walls of tracheary elements are porose to reticulate in roots of all species, but nonporose in stems of Cypripedioideae and Vanilloideae and porose to reticulate in stems of Orchidoideae and Epidendroideae. The distribution pattern of pit membranes and pit membrane remnants in end walls of tracheary elements of orchids parallels the findings of others. The position of Cypripedioideae and Vanilloideae as outgroups to Orchidoideae and Epidendroideae, claimed by earlier authors, is supported by clades based on molecular studies and by our studies. Little hydrolysis of pit membranes in tracheary element end walls was observed in pseudobulbs or inflorescence axes of epidendroids. The pervasiveness of network-like pit membranes of various extents and patterns in end walls of tracheary elements in Orchidaceae calls into question the traditional definitions of tracheids and vessel elements, not merely in orchids, but in angiosperms at large. These two concepts, based on light microscope studies, are blurred in light of ultrastructural studies. More importantly, the intermediate expressions of pit membranes in tracheary element end walls of Orchidaceae and some other families of angiosperms are important as indicators of steps in evolution of conduction with respect to organs (more rapid flow in roots than in succulent storage structures) and habitat (less obstruction to flow correlated with a shift from terrestrial to epiphytic).     

Origin and nature of vessels in monocotyledons. 5. Araceae subfamily Colocasioideae

Tracheary elements from macerations of roots and stems of one species each of five genera of Araceae subfamily Colocasioideae were studied by means of SEM (scanning electron microscopy). All of the genera have vessel elements not merely in roots, as previously reported for the family as a whole, but also in stems. The vessel elements of stems in all genera other than Syngonium are less specialized than those of roots; stem vessel elements are tracheid-like and have porose pit membrane remnants in perforations. The perforations with pit membrane remnants demonstrate probable early stages in evolution of vessels from tracheids in primary xylem of monocotyledons. The vessel elements with such incipient perforation plates lack differentiation in secondary wall thickenings between perforation plate and lateral wall, and such vessel elements cannot be identified with any reliability by means of light microscopy. The discrepancy in specialization between root and stem vessel elements in genera other than Syngonium is ascribed to probable high conductive rates in roots where soil moisture fluctuates markedly, in contrast with the storage nature of stems, in which selective value for rapid conduction is less. Syngonium stem vessels are considered adapted for rapid conduction because the stems in that genus are scandent. Correlation between vessel element morphology and ecology and habit are supported. Although large porosities in vessel elements facilitate conduction, smaller porosities may merely represent rudimentary pit membrane lysis.     

The tapetum and systematics in monocotyledons

This paper critically reviews the homologies and distribution of tapetum types in monocotyledons, in relation to their systematics. Two main types of tapetum are widely recognised: secretory and plasmodial, although intermediate types occur, such as the “invasive” tapetum described inCanna. In secretory tapeta, a layer of cells remains intact around the anther locule, whereas in the plasmodial type a multinucleate tapetal plasmodium is formed in the anther locule by fusion of tapetal protoplasts. In invasive tapeta, the cell walls break down and tapetal protoplasts invade the locule without fusing to form a plasmodium. When examining tapetum type, it is often necessary to dissect several developmental stages of the anthers. Secretory and plasmodial tapeta are both widely distributed in monocotyledons and have probably evolved several times, although there may be some systematic significance within certain groups. Among early branching taxa,Acorus andTofieldia have secretory tapeta, whereas Araceae and Alismatales are uniformly plasmodial. The tapetum is most diverse within Commelinanae, with both secretory and plasmodial types, and some Zingiberales have an invasive tapetum. Lilianae (Dioscoreales, Liliales, and Asparagales) are almost uniformly secretory.     

海韭菜的花器官发生

运用扫描电镜（SEM）观察了海韭菜（Triglochin maritimum）的花器官发生发育过程。结果表明：海韭菜花发育是典型的单子叶植物发生模式,即两轮花被片、两轮雄蕊和两轮心皮以三基数轮状交替发生,花器官是以向心向顶的方式发生的,未发现“花被片—雄蕊复合原基”。 发育后期雄蕊和与之对生的花被片之间的共同基部可能是相继向上居间生长的结果。花被片轮和雄蕊轮二者之间在发育位置、时间和速率上存在差异,内轮花被片原基和外轮雄蕊原基的不同发育时间和发育速度使得在成熟花中内轮花被片位于外轮雄蕊的内方。观察结果不支持水麦冬属植物的花是退化（或压缩）的花序侧分枝等假花的观点。     

Nomenclatural adjustments in some European monocotyledons

New nomenclatural combinations are validated for fifteen taxa belonging to the generaDanthonia, Stipa, Lolium, Phippsia, Elymus, Schoenoplectus andAllium.     

Systematics and biology of silica bodies in monocotyledons

Many plants take up soluble monosilicic acid from the soil. Some of these plants subsequently deposit it as cell inclusions of characteristic structure. This article describes the distribution and diversity of opaline silica bodies in monocotyledons in a phylogenetic framework, together with a review of techniques used for their examination, and the ecology, function and economic applications of these cell inclusions. There are several different morphological forms of silica in monocot tissues, and the number of silica bodies per cell may also vary. The most common type is the “druse-like” spherical body, of which there is normally a single body per cell, more in some cases. Other forms include the conical type and an amorphous, fragmentary type (silica sand). Silica bodies are most commonly found either in the epidermis (e.g., in grasses, commelinas and sedges) or in the sheath cells of vascular bundles (e.g., in palms, bananas and orchids). Silica-bearing cells are most commonly associated either with subepidermal sclerenchyma or bundle-sheath sclerenchyma. Silica bodies are found only in orchids and commelinids, not in other lilioid or basal monocots. In orchids, silica bodies are entirely absent from subfamilies Vanilloideae and Orchidoideae and most Epidendroideae but present in some Cypripedioideae and in the putatively basal orchid subfamily Apostasioideae. Among commelinid monocots, silica bodies are present in all palms, Dasypogonaceae and Zingiberales but present or absent in different taxa of Poales and Commelinales, with at least four separate losses of silica bodies in Poales.     

Lateral meristems and stem thickening growth in monocotyledons

Although monocotyledons lack a vascular cambium of the type found in dicotyledons and conifers, lateral meristems still play an important role in the establishment of their growth habits. The presence near the shoot apex of a primary thickening meristem (PTM), which is probably plesiomorphic in monocotyledons, predisposes evolution into the many pachycaul forms. A PTM occurs in virtually all monocotyledons, whereas the secondary thickening meristem (STM), which is morphologically similar, is limited to a few genera of Liliiflorae. these records are reviewed in a systematic context. To a greater or lesser extent in different taxa, the PTM is responsible for primary stem thickening, adventitious root production, and formation of linkages between stem, root and leaf vasculature. The STM largely contributes to the body of the stem. The sometimes obscure distinction between the two meristems, and their relationship with other stem meristems are discussed. For systematic purposes stem thickening in monocotyledons is separated into two characters: diffuse growth (as in palms), and growth by means of lateral meristems. The three states of the second character are represented by the first three of Mangin’s (1882) four categories (two herbaceous, the third arborescent): (1) The lateral meristem is limited in extent, and ceases activity after root formation. (2) It remains active for a limited period after cessation of root formation, contributing to the plant body. (3) It remains active throughout the life of the plant, contributing the bulk of the plant body.     

Nuclear DNA content of monocotyledons and related taxa

Nuclear DNA content of 62 species of angiosperms including 52 monocotyledons and ten dicotyledons has been estimated by flow cytometry using Nicotiana tabacum var. Xanthi as the internal standard. These data, considered together with previous data on diploid species, suggest the following: 1) Most families and orders of monocotyledons have small genomes. Contrary to the general impression that monocotyledons are a group characterized by large genomes, genomes of over 20 pg/2C nucleus occur only in the Liliiflorae, Commelinales, Alismatales, and Araceae. 2) Variation within families ranges from two- to 56-fold, but is two- to fivefold in most families. Thus extraordinary variation in genome size appears to be limited to particular lineages, perhaps owing to some shared feature that facilitates such variation. 3) Endopolyploidy is not observed in the leaves of the species studied, although it has been reported to occur in the roots of several monocotyledons. This suggests that an examination of the basis for this difference between the roots and leaves of monocotyledons may provide clues to the mechanisms that regulate endopolyploidization in these organs.     

Triaperturate pollen in the monocotyledons: configurations and conjectures

Triaperturate pollen are known in at least twenty seven genera of monocotyledons. Differences between aperture type and polarity indicate that the development of three apertures has occurred a number of times. Mode of cytokinesis during microsporogenesis is compared with differences in aperture configuration, to assess the extent to which this appears to influence aperture arrangement. Triapertury in monocot pollen tends to fall into one or another of three situations: 1) it is the normal state, 2) it is fairly common, but pollen with more or less apertures also occur in the taxon or sample, 3) it is a rare, or abnormal state for pollen which usually has less than three apertures. The various forms of triaperturate pollen are described, as well as monosulcate pollen of the orchid genera Cypripedium and Paphiopedilum, often misinterpreted as tri-sulcate, and the unusual extended trichotomosulcate pollen of Agrostocrinum (Hemerocallidaceae). Monosulcy, trichotomosulcy, and zonasulcy, with unusual and rare exceptions of zonasulcy in the eudicots, are aperture states shared exclusively with the basal dicots. Furthermore, to some extent all have links with the triaperturate condition in monocots and basal dicotyledons. This is discussed, as well as the association of tripory with polypory in monocots and basal dicots. The fossil pollen record is considered.This paper is dedicated to Klaus Kubitzki in recognition, not only for his extensive contribution to systematic botany, but also for his firm belief that pollen characteristics contribute to a better understanding of plant systematics and evolution.     

Systematic root anatomy of Asparagales and other monocotyledons

Root anatomy of several taxa of Asparagales and some taxa formerly included in Asparagales is described in a systematic context together with a literature review. The presence of a dimorphic outer layer with long and short cells is widespread in monocotyledons, indicating that it originated early in the monocot lineage, but whereas this layer is rhizodermal in most monocotyledons, in Asparagales and Araceae it is usually hypodermal. There may be a correlation between the presence of a velamen or a persistent rhizodermis in many Asparagales and Araceae and the presence of a dimorphic hypodermal layer. Many other root anatomical characters, such as the presence of vascular bundles in the central pith and a multi-layered sclerenchymatous cylinder, are probably xeromorphic and developed convergently.     

The Nucellus and Chalaza in monocotyledons: Structure and systematics

The majority of monocotyledons are crassinucellate, including some early-branching taxa (sensu Chase et al., 1995a, 1995b) such asTofieldia, although Araceae are predominantly tenuinucellate. The tenuinucellate condition occurs in a taxonomically wide range of monocotyledons, and there is some congruence between this character and existing monocot topologies at higher levels. For example, present evidence indicates a few tenuinucellate asparagoid clades, including Alliaceae sensu stricto and Hypoxidaceae, possibly two tenuinucellate lilioid lineages, and at least two tenuinucellate commelinoid lineages. Proximal nucellar structures arise from a multi-layered region of the ovule and include hypostase, enlarged dermal cells and conducting passage (Zuleitungsbahn), haustoria, postaments, podia, and perisperm. In some cases they may represent the same tissues at different developmental stages; in general the last three are seed structures. For example, a postament may be a resistant conducting passage from which the surrounding dermal cells have degenerated, or alternatively a resistant hypostase, although both are nucellar in origin. Such terminological confusions cause problems in establishing homologies. Several characters relating to the nucellus are outlined.     

SEM studies on vessels in ferns. 2. Pteridium

Xylem from roots and rhizomes of two infraspecific taxa of Pteridium aquilinum was studied by means of scanning electron microscopy (SEM). All tracheary elements proved to be vessels. End wall perforation plates were all scalariform, lacked pit membrane remnants in at least the central part of the perforation plate, and varied with respect to width of bars, from wide to tenuous, and with respect to presence of pit membrane remnants. In addition, porose pit membranes on walls that are likely all lateral vessel-to-vessel walls must be considered to be perforations also, although different from those on end walls. Lateral wall perforation plates, hypothesized by one worker on the basis of tylosis presence but denied by another on the basis of light microscopy, were confirmed by demonstration of pores with SEM. In addition, lateral walls of Pteridium vessels bear some grooves interconnecting pit apertures; this feature is newly figured by SEM for ferns. Lateral wall pitting that is not porose may either have striate thickenings of the primary wall or be smooth. Vessel presence and degree of specialization in Pteridium vessels may bear a relationship to the wide ecological tolerances of the genus.     

Origins of the neurovascular bundle: interactions between developing nerves and blood vessels in embryonic chick skin

Growth cones of nerves and endothelial cells of blood vessels are closely analogous in their migratory behavior, and they are both set a similar task during the early development of a limb. Both must invade the mesenchyme to form ramifying networks of large nerves and vessels. Both systems must densely pervade certain regions of the developing limb, such as muscle rudiments, and both form dense cutaneous plexuses at precisely the same depth beneath the epidermis. Moreover, adult tissues show many examples of neurovascular bundles in which nerves and blood vessels run closely parallel and branch in a correlated fashion, suggesting some interdependence during development. We have examined the interrelationship between developing nerves and blood vessels in chick wing skin because it allows a particularly convenient two-dimensional analysis of the two systems which can be revealed simultaneously in the same preparation by injection of Indian ink combined with silver-staining. We show that nerves do not use blood vessels as pathways along which to crawl, but that there are two other ways in which neurovascular associations arise: in some situations nerves and blood vessels follow the same route because they are responding independently to the same mesenchymal cues; and in some situations nerves induce blood vessels to remodel around them.     

Numerical analyses of distributions of Iberian and Balearic endemic monocotyledons

Chorological information concerning 182 taxa of monocotyledons endemic to the Iberian Peninsula and Balearic Islands was compiled and related to the 100×100 km, 50×50 km and 10×10 km UTM grids. Distributions were analysed using multivariate methods (two-way indicator species analysis and detrended correspondence analysis) for each scale. Comparison of results allows recognition of several floristic elements and sectors (i.e. Balearic, Murcian-Almerian, south western) common to all three scales, whereas other regions are assigned to different sectors depending on the grid size considered. As a consequence of the increase in detail, characteristics such as number of sectors, the outline of boundaries and continuity or fragmentation of the areas also change. These factors are discussed.     

Morphological traffic between the inflorescence and the vegetative shoot in helobial monocotyledons

Previous studies of reproductive structures in the helobial monocotyledons (Alismatidae) indicate that partitioning between flower and inflorescence is not always clear (e.g.,Lilaea,Scheuchzeria) and that this may be the result of ancestral, unisexual modules coming together to form flowers and/or inflorescences. Later evolutionary changes may have included the inflorescence becoming involved or mixed in with vegetative growth. Substitution of vegetative buds for flowers is the simplest version, and there can be additional modifications to the growth behavior of the inflorescence, such as horizontal growth and dorsiventrality. In the Alismataceae and Limnocharitaceae the derivation of stolonlike structures from inflorescences is obvious: vegetative features have been incorporated into structures that are recognizably inflorescences. In the Hydrocharitaceae the interrelationships between the inflorescence and the vegetative body are much less well defined. We previously suggested forHydrocharis, where a single axillary complex can contain both inflorescence and stolons, that the stolon is basically a sterilized inflorescence and that features of the inflorescence have become incorporated into the vegetative body. Here we will explore this theme further for the Hydrocharitaceae, using information from within and outside the family.