The distribution and systematic relevance of the androecial character polymery

RONSE DECRAENE L. P. AND SMETS E. F., 1993. The distribution and systematic relevance of the androecial character polymery . Two characters, viz. oligomery and polymery, have been previously proposed to circumscribe the localization of the androecium. Their distribution is more or less correlated with two groups of taxa: polymery is found in Magnoliidae, Caryophyllidae, Liliatae and part of Hamamelidae; oligomery is found in Dilleniidae, Rosidae, part of Hamamelidae and Asteridae. Polymery can be described by a number of character states, which are presented in a semophyletical scheme. Spiral polyandry, i.e. a multistaminate and spiral androecium, represents the plesiomorphic condition for all Magnoliophytina and is restricted to the polymerous group. Cyclization, induced by an arrangement of the perianth in trimerous whorls and a fractioning of the continuous plastochron, leads to polycycly, i.e. an arrangement of the stamens in numerous cycles; the outer stamens are usually inserted as pairs or in alternation with the inner perianth parts. From this configuration reduction series in different groups result in androecia with a lower number of stamen whorls (such as tetracycly, tricycly, dicycly and (ob)monocycly). The transition from trimery to pentamery induces a derived stamen configuration by the merging of two tepaline whorls and the loss of some stamens. für ther reductions accompanied evolution in trimerous flowers and led to conditions resembling diplostemony as observed in Caryophyllaceae, some Hamamelidaceae and Ranunculaceae. Secondary increases, as well as reductions of stamens within a whorl must be regarded as gradual variations of each character state. Different trends affecting the number and position of the stamens can globally be traced along different lines. Polymery is consistent with other floral characters, such as the nature of perianth, vasculature (axial and cortical systems) and merosity. The androecium of a number of families and their relationships are discussed.     

Floral development of Urospatha: merosity and phylogeny in the Lasioideae (Araceae)

In this paper we study merosity in the genus Urospatha within the framework of a resolved phylogeny of the Araceae. We analyse how a transition from dimerous or tetramerous merosity to pentamerous or hexamerous merosity can occur developmentally in the Lasioideae. In Urospatha, initiation of floral primordia along the inflorescence is acropetal, while development of flowers is basipetal. This indicates the presence of two distinct phases in the development of the Urospatha inflorescence. The first phase corresponds to initiation of flowers and establishment of the phyllotactic pattern, and the second phase to differentiation of floral organs. Urospatha is characterized by the presence of trimerous, tetramerous, pentamerous and rarely hexamerous flowers. In all types of flowers, the stamens are closely associated and opposite to the tepals. Pentamerous flowers are formed by addition of a sector comprising a stamen and tepal. Likewise, in the case of hexamerous flowers, two sectors are added. In the Lasioideae, the increase in the number of tepals and stamens is linked with two developmental processes that have appeared independently in the subfamily: (1) addition of one or two stamen?Cpetal sectors (Anaphyllopsis and Urospatha), and (2) independent increase in the number of tepals and stamens on whorls, more or less organized and inserted in alternate position (Dracontium). Tetramerous whorls as they occur in basal Lasioideae would be homologous to two dimerous whorls from an evolutionary point of view.     

Gunnerales are sister to other core eudicots: implications for the evolution of pentamery

Phylogenetic relationships among many lineages of angiosperms have been clarified via the analysis of large molecular data sets. However, with a data set of three genes (18S rDNA, rbcL, and atpB), relationships among lineages of core eudicots (Berberidopsidales, Caryophyllales, Gunnerales, Santalales, Saxifragales, asterids, rosids) remain essentially unresolved. We added 26S rDNA sequences to a three-gene matrix for 201 eudicots (8430 base pair aligned nucleotides per taxon). Parsimony analyses provided moderate (84%) jackknife support for Gunnerales, which comprise the two enigmatic families Gunneraceae and Myrothamnaceae, as sister to all other core eudicots. This position of Gunnerales has important implications for floral evolution. A dimerous or trimerous perianth is frequently encountered in early-diverging eudicots (e.g., Buxaceae, Proteales, Ranunculales, Trochodendraceae), whereas in core eudicots, pentamery predominates. Significantly, dimery is found in Gunneraceae and perhaps Myrothamnaceae (the merosity of the latter has also been interpreted as labile). Parsimony reconstructions of perianth merosity demonstrate lability among early-diverging eudicots and further indicate that a dimerous perianth could be the immediate precursor to the pentamerous condition characteristic of core eudicots. Thus, the developmental canalization that yielded the pentamerous condition of core eudicots occurred after the node leading to Gunnerales.     

Does there exist a correlation between the number of whorls of the finger tips and birth order of sibs? (author's transl)]

It was stated by Birdsong and Rashad (1972) that the child of birth order number six of Korean families has significantly more true whorls on the finger tips than older brothers and sisters. To verify this assertion we studied a sample of all members of 43 normal German families with 5 or more legitimate children. The findings of the epidermal finger pattern types and those of the toe patterns were analysed. Results indicate a different conclusion regarding the number of digital whorls which were not increased statistically with increasing number of sibs. The number of whorls were randomly distributed within the heritable range of variability of the particular family. It appears of importance that, due to our analyses, the value of information of the characteristic "number of whorls" is not reduced and does not require a specific consideration regarding birth order. Furthermore, it can be stated preliminarly that there is no hint pointing to an influence of the maternal age on the number of whorls in their offspring.     

Comparative ontogeny of perfect and pistillate florets in Senecio vernalis (Asteraceae)

We studied the inflorescence, and in particular ontogeny and development of the florets in Senecio vernalis as a representative member of Asteraceae, using epi-illumination microscopy. Initiation and subsequent development of florets on the highly convex inflorescence apex occur acropetally, except for pistillate ray florets, which show a lag in initiation. Receptacular bracts derive from the receptacular surface after development of all florets. The order of whorl initiation in both disc and ray florets include corolla, androecium and finally the pappus, together with the gynoecium. Development of corolla lobes from a ring meristem occurs in bidirectional order starting from the lateral side, whereas stamens incept unidirectionally from the abaxial side. Concurrently with the inception of two median carpel primordia, a ring meristem develops at the base of the corolla from which pappus bristles differentiate in later stages. Pistillate ray florets show significant differences from perfect disc florets as reflected by the zygomorphic shape of the floral apex and a shift of floral merosity from pentamery to tetramery. Loss of stamens in ray florets occurs due to abortion of primordia after initiation.     

Pseudodiplostemony, and its implications for the evolution of the androecium in the Caryophyllaceae

The androecium of the Caryophyllaceae is varied, ranging from a two-whorled condition to a single stamen. A number of species belonging to the three subfamilies, Caryophyl-loideae, Alsinoideae and Paronychioideae have been studied ontogenetically with the SEM to understand their peculiar androecial development in the broader context of the Caryophyllales alliance. Although patterns of initiation are highly variable among species, there are three ontogenetic modes of stamen initiation: all stamens simultaneous within a whorl, the antepetalous stamens simultaneous and the antesepalous sequentially with a reversed direction, or both whorls sequentially with or without a reversed direction. The most common floral (ontogenetic) sequence of the Caryophyllaceae runs as follows: five sepals (in a 2/5 sequence), the stamens in front of the three inner sepals successively, stamens opposite the two outermost sepals, five antepetalous stamens (simultaneously or in a reversed spiral superimposed on the spiral of the antesepalous stamens), five outer sterile (petaloid) organs arising before, simultaneously or after the antesepalous stamens, often by the division of common primordia. A comparison with the floral configurations of the Phytolaccaceae and Molluginaceae indicates that the outer petaline whorl of the Caryophyllaceae corresponds positionally to the alternisepalous stamens of somePhytolacca, such asP. dodecandra. The difference withP. dodecandra lies in the fact that an extra inner or outer whorl is formed in the Caryophyl-laceae, in alternation with the sepals. A comparable arrangement exists in the Molluginaceae, though the initiation of stamens is centrifugal. A comparison of floral ontogenies and the presence of reduction series in the Caryophyllaceae support the idea that the pentamerous arrangement is derived from a trimerous prototype. Petals correspond to sterillized stamens and are comparable to two stamen pairs opposite the outer sepals and a single stamen alternating with the third and fifth sepals. Petals are often in a state of reduction; they may be confused with staminodes and they often arise from common stamenpetal primordia. The antesepalous stamen whorl represents an amalgamation of two whorls: initiation is reversed with the stamens opposite the fourth and fifth formed sepals arising before the other, while the stamens opposite the first and second formed sepals are frequently reduced or lost. Reductive trends are correlated with the mode of initiation of the androecium, as well as changes in the number of carpels, and affect the antesepalous and antepetalous whorls in different proportions. It is concluded that the androecium of the Caryophyllaceae is pseudodiplos-temonous and is not comparable to diplostemonous forms in the Dilleniidae and Rosidae. The basic floral formula of Caryophyllaceae is as follows: sepals 5—petals 5 (sterile stamens)—antesepalous stamens 3+2—antepetalous stamens 5 gynoecium 5.     

Secretion of multilamellar whorls by Eimeria tenella zoites.

Multilamellar whorls were demonstrated by transmission electron microscopy to be associated with sporozoites and all generations of merozoites of Eimeria tenella, in chicken cecal tissue fixed without tannic acid or ruthenium red at room temperature. Whorls were found within the parasitophorous vacuoles of recently invaded cells at all stages of development, suggesting a role in the formation of the host parasite interface. Whorls were also associated with intraluminal third-generation merozoites prior to host cell invasion and appeared to be secreted directly through the pellicle. Membranous sheaths, shown by serial sectioning to be derived from intracellular whorl material, were observed enveloping some intraluminal merozoites. In many third-generation merozoites, whorl material was located within discrete novel organelles (here termed lamellosomes) located in the apical region. These densely staining spherical organelles were morphologically distinct from micronemes and rhoptries and were one-third the size of dense granules. These findings confirm that whorls are nonartifactual secretions whose lamellar organization is lost during normal fixation on ice without tannic acid. It is hypothesized that whorls secreted prior to invasion are involved in protection of the motile zoite, immune evasion, or some aspect of gliding motility.     

Floral ontogeny of Annonaceae: evidence for high variability in floral form

Background and Aims

Annonaceae are one of the largest families of Magnoliales. This study investigates the comparative floral development of 15 species to understand the basis for evolutionary changes in the perianth, androecium and carpels and to provide additional characters for phylogenetic investigation.

Methods

Floral ontogeny of 15 species from 12 genera is examined and described using scanning electron microscopy.

Key Results

Initiation of the three perianth whorls is either helical or unidirectional. Merism is mostly trimerous, occasionally tetramerous and the members of the inner perianth whorl may be missing or are in double position. The androecium and the gynoecium were found to be variable in organ numbers (from highly polymerous to a fixed number, six in the androecium and one or two in the gynoecium). Initiation of the androecium starts invariably with three pairs of stamen primordia along the sides of the hexagonal floral apex. Although inner staminodes were not observed, they were reported in other genera and other families of Magnoliales, except Magnoliaceae and Myristicaceae. Initiation of further organs is centripetal. Androecia with relatively low stamen numbers have a whorled phyllotaxis throughout, while phyllotaxis becomes irregular with higher stamen numbers. The limits between stamens and carpels are unstable and carpels continue the sequence of stamens with a similar variability.

Conclusions

It was found that merism of flowers is often variable in some species with fluctuations between trimery and tetramery. Doubling of inner perianth parts is caused by (unequal) splitting of primordia, contrary to the androecium, and is independent of changes of merism. Derived features, such as a variable merism, absence of the inner perianth and inner staminodes, fixed numbers of stamen and carpels, and capitate or elongate styles are distributed in different clades and evolved independently. The evolution of the androecium is discussed in the context of basal angiosperms: paired outer stamens are the consequence of the transition between the larger perianth parts and much smaller stamens, and not the result of splitting. An increase in stamen number is correlated with their smaller size at initiation, while limits between stamens and carpels are unclear with easy transitions of one organ type into another in some genera, or the complete replacement of carpels by stamens in unisexual flowers.     

Phenotypic relationships between hair whorl characteristics and spermatozoal attributes in Holstein bulls

The objective of this study was to characterize facial whorl characteristics in Holstein stud bulls (n = 485) and to investigate their association with measures of semen quantity and quality. Bulls had 1 (n = 454), 2 (n = 21) or no facial hair whorls (n = 5). Some 39% of bulls had whorls in the middle of their forehead, whereas 43% had whorls very low down on their forehead. Spiral whorls were present on 77% of bulls while 22% had elongated whorls. Abnormally shaped whorls were found more often on bulls with low whorls (27% versus 17%, P = 0.01) than on bulls with high or middle whorls. Semen production data were obtained from the AI center database to provide a representative sample of each sires most recent semen production characteristics. The number of semen collections per bull averaged 38 with a range of 1 (n = 9) to 272 (n = 1). The average number of days between collections was 5 but varied from 1 to 425 days. Average age of bulls at first recorded collection was 679 days and ranged from 307 to 4555 days. There were no significant differences between black and white bulls in semen quality or quantity. Semen quality over the summer months was reduced in comparison to the winter months. Age of the bull at time of collection had no effect on any of the semen quality traits. Older bulls had larger scrotal circumference and produced more semen (P > 0.01). Bulls with whorls located in the center of the forehead were not significantly different in semen quality or quantity for the 10 traits considered when compared to bulls with whorls located outside the center. In contrast to previous research findings with Angus cattle, semen attributes were not significantly different between bulls characterized with round (or spiral) epicenters and those with abnormally shaped whorls. The difference between the two studies may be due to the fact that Angus bulls have a higher percentage of abnormal elongated whorls compared to Holsteins.     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Evolution of erect helical colony form in the Bryozoa: phylogenetic, functional, and ecological factors

Erect helical colony forms have evolved at least six separate times within the Bryozoa, but only among those in which branches are composed of a single layer of feeding zooids. The four best known genera with helical colony forms evolved independently, and each occupied different benthic marine environments, achieved different growth habits, and utilized different aspects of an array of functional potentials resulting from the radially symmetrical colonies. Examination of the distribution of these four genera ( Archimedes , Bugula , Crisidmonea , and Retiflustra ) within a theoretical morphospace of hypothetical helical colony form reveals that each occupies its own characteristic region of morphospace, broadly overlapping in some dimensions but separated in others. The genera differ markedly in the branching densities within their filtration-sheet whorls, reflecting their phylogenetic legacies rather than constructional or functional constraints associated with helical growth. In contrast, all tend toward helices in which the radiating whorls of the unilaminate branches are held at an average of 50–60° to the central axis of the colony, and this may reflect a common functional optimum associated with the cilia-driven, auto-generated currents within the helix. The region of morphospace characterized by high values of surface area – i.e. helical geometries with branches orientated at very low angles to the central axis, and with very closely spaced whorls along the axis – is entirely empty of bryozoans, and these geometries apparently represent functionally unrealistic colony forms.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 235–260.     

THE DEVELOPMENTAL BASIS OF TRISTYLY IN EICHHORNIA PANICULATA (PONTEDERIACEAE)

Eichhornia paniculata is a tristylous, self-compatible, emergent aquatic. A given plant produces flowers with either long, mid or short styles and two levels of stamens equal in length to the styles not found in that flower. Flowers of each morph have two whorls of three tepals, six stamens and three fused carpels. The six stamens differentiate into two sets of three stamens each. A relatively short set, having either short- or mid-level stamens, occurs on the upper side of the flower, while a relatively long set, having either mid- or long-level stamens, occurs on the lower side. Stamen level depends on differences among stamens in filament length and position of insertion on the floral tube. Floral parts arise in whorls of three, but the two stamen whorls do not form the two sets of stamens found in each mature flower. Instead, stamens from both whorls make up a given set. Floral differences among morphs are not present at flower origin or floral organ initiation. Morphological differences arise first among stamen sets. The two sets within a flower differ prior to meiosis in the size, number, and timing of comparable developmental events in the sporogenous cells. After these initial differences arise, anther size diverges. In later developmental stages differences in filament and floral tube length, cell size, and cell number, as well as differences in the length, cell size, and cell number of styles, develop among morphs. This sequence of developmental events suggests that the genes controlling development in different morphs do not control flower and floral organ initiation but are first morphologically visible in sporogenous cell differentiation.     

Notes on the Evolution of Androecial Organisation in the Magnoliophytina (Angiosperms)

This paper aims to summarize briefly and to update our ideas about androecial architecture formulated in earlier publications. Ontogenetic evidence of stamen development, viz. the initiation, arrangement and relationship of stamens to other floral morphomes, can be translated into a semophyletic scheme reflecting the phylogeny of the androecium. The ancestral androecium is discussed in the light of recent theoriesabout angiosperm phylogeny. Two divergent androecial processes are proposed for the angiosperms starting from a spiral androecium with a moderate number of stamens. However, transitions exist between spiral polyandry, numerous stamens in whorls, and chaotic polyandry. From an androecium with several alternating whorls of paired and single stamens, outer stamen pairs are retained following the successive loss of inner stamen whorls. Single stamens instead of pairs occur at the very end of this line and represent a more advanced condition. This line is mostly present in tri- and dimerous flowers. From the same starting point diplostemony (with two alternating whorls of single stamens) originated, again giving rise to various states usually present in pentamerous or tetramerous flowers.     

HETEROGENEITY IN POLLEN ALLELE FREQUENCIES AMONG CONES,WHORLS, AND TREES OF TABLE MOUNTAIN PINE (PINUS PUNGENS)

Pinus pungens Lambert (Table Mountain pine) is characterized by serotinous cones that are borne several (two to five) per whorl. Sporophytic and gametophytic tissues from whorls collected in different canopy zones of eight trees were electrophoresed and surveyed for six allozyme loci. Pollen allele frequencies received by individual trees, whorls on trees, and cones in whorls were estimated and used to estimate outcrossing rates and the distribution of genetic diversity within and among each level. Significant differences in pollen allele frequencies were detected at all levels, indicating that the genetic composition of the pollen pool available to different female strobili is heterogeneous. The fine-scale genetic structure in the seed pool is a record of past reproductive events and is the precursor to genetic structure in the next generation.     

Character divergences and convergences in canopy-dwelling Loranthaceae

The Loranthaceae is the largest plant family with aerial branch parasites termed mistletoes. Three genera of Loranthaceae are terrestrial root parasites and the remaining 72 genera are aerial parasites. Several characters, including habit, haustorial type, germination pattern, pollen morphology, chromosome number, inflorescence morphology and flower merosity, fusion, symmetry and size, are considered to reflect evolutionary relationships within the family. Convergence is a common evolutionary pattern and can confound interpretations of evolution. We investigated character evolution by mapping character states onto a phylogenetic tree based on the nuclear ITS and chloroplast trnL–trnF regions. Convergences in form were found in several characters, including habit, haustorial type, flower symmetry and merosity. These convergences typically correspond to ecological parameters such as pollination syndrome or stresses associated with the canopy habit. Other characters such as chromosome number and germination pattern illustrate divergent evolution among clades.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 101–113.     

The distribution and systematic relevance of the androecial character oligomery

Localization of the stamens can be approached by a preliminary distinction between two characters, oligomery and polymery, occurring in two different groups of taxa, respectively the oligomerous complex and the polymerous complex. Oligomery is described by four character states standing in a close semophyletic relationship: diplostemony, obdiplostemony, haplostemony and obhaplostemony. Each character state is analysed for its distribution and systematic value. Diplostemony is the synapomorphic character state for the oligomerous line and has arisen once from a polymerous ancestor or in parallel in different lines. Obdiplostemony arises ontogenetically in three different ways. Loss of one whorl leads either to obhaplostemony, or haplostemony; both character states are believed to represent evolutionary steps of no-return. Secondary increases and reductions of the stamens within a whorl are seen as expressions of the intrinsic variability of the character states and should not be homologized with them. Stamen numbers can be increased by the building-up of complex primordia or by secondary receptacular growth. Reductions of stamens affect one or two whorls of stamens and are caused by lack of space, interactions with the gynoecium and zygomorphy. The distribution of the different character states of oligomery is presented on Dahlgrenograms and the androecia of a number of families and their relationships are discussed. The interactions between oligomery and polymery are analysed as guidelines for a global phylogeny of the Magnoliatae.     

SHOOT VASCULAR SYSTEM AND PHYLLOTAXIS OF CASUARINA (CASUARINACEAE)

In species of Casuarina with multileaved whorls, each stem vascular bundle divides radially into two at the site of a leaf trace separation, and the same two bundles rejoin acropetally to where the trace supplies a leaf. Such divisions are divisions of a single vascular bundle, and the rejoining of bundles forms a single bundle. Proposals that the extant primary vascular systems of dicotyledons may have been derived as in conifers are incorrect in so far as Casuarina is concerned, or the system has evolved beyond that so far proposed for dicotyledons. Reasons are offered, however, for considering that fernlike leaf gaps are not present. Leaf traces supply leaves at the first nodes distal to their origins. The ways by which an increase or decrease of stem bundles occur are described. Phyllotactic patterns range from helical (rare) to whorled. In the embryo, where leaves occur decussately, of certain species with multileaved whorls, and in the shoot apices of species with tetramerous whorls, slight differences in the levels of leaf attachments and the bending of leaf traces indicate the probable evolution of extant whorled phyllotaxies from one or more helical arrangements. Stages in the evolution are suggested. The leaves in most species with multileaved whorls are in true whorls. The original periderm of branchlets lies internally to the internodal traces and chlorenchyma, but is otherwise external to the vascular system. It is concluded that each leaf originates at its level of separation from the axis despite several structural features suggesting that the leaf bases have become congenitally adnate to the stem.     

HELICAL FLORAL ORGANOGENESIS IN GLEDITSIA,A PRIMITIVE CAESALPINIOID LEGUME

In order to determine the extent of floral ontogenetic differences among species of a genus, six species of Gleditsia were studied. Gledilsia is one of only two leguminous genera known in which there is completely helical succession of floral organs. Floral ontogeny was compared in three species (Gleditsia amorphoides, G. aquatica, and G. triacanthos), and late stages in six species (including the first three plus G. caspica, G. delavayi, and G. japonica). Other unusual primitive developmental features include the unequal-sized flower primordia which produce flowers of variable merosity. Order of floral development is also loosely controlled, so that flowers of different growth stages are intermixed in the inflorescence. Variable features include the occurrence of floral bracts, merosity of flowers, number of organs, and position of the first organ (sepal) initiated. The inflorescence type, while usually a raceme, often has lateral branches near the base, or fascicles of flowers at some points. A terminal flower often is present, although not in all species. Sex of flowers and inflorescences also varies, although floral initiation tends to include both stamens and carpel primordia. Suppression of one or the other may occur at different stages of development. Carpel orientation also varies; the cleft may be tilted or inverted occasionally. It is proposed that absence of subtending floral bracts influences development so as to favor radial symmetry and establishment of other “chaotic” characters seen in Gledilsia flowers.     

SYMMETRY IN EQUISETUM

Bierhorst , David W. (Cornell U., Ithaca, N. Y.) Symmetry in Equisetum. Amer. Jour. Bot. 46(3) : 170-179. Illus. 1959.—A total of 118 leaf whorls and corresponding nodes from a total of 9 species of Equisetum were studied in serial cross-section. The number of whorls having the same leaf number as the 2 adjacent ones was 67. The size of the arc measured in degrees of circumference occupied by each leaf, as well as its position relative to leaves of adjacent whorls were measured. The disposition of the internodal and trans-nodal vascular strands was determined for each of the nodes. The average per cent deviations from theoretical leaf size within whorls were: whorls not involved in change in leaf number, 3.9; those with fewer leaves than the one below, 9.5; those with more leaves than one above, 7.1; those with more leaves than one below, 9.4; those with different leaf number from the 2 adjacent ones, 12.5. In Equisetum, it is a general rule that (1) a leaf which falls directly above a leaf in the next whorl where the younger whorl possesses fewer leaves than the older one is usually the largest leaf within its whorl, (2) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses fewer leaves than the older one is usually the smallest leaf within its whorl, (3) a leaf which falls directly above a leaf in the next older whorl where the younger whorl possesses a greater number of leaves than the older one is usually the smallest leaf in its whorl, and (4) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses a greater number of leaves than the older one is usually the largest leaf within its whorl. The numerous variations in the disposition of vascular tissue associated with changes in leaf number are described. Double leaf traces which originate in various ways are common, as well as vertical strands traversing the nodes. The leaf trace system is considered to be determined by the number, position, and relative sizes of leaf primordia. The disposition of the trans-nodal protoxylem seems to be determined to a large extent by the proximity of the leaf traces above and below the node.