Initiation of primary vascular elements in the stamens and carpel of Triticum aestivum L.

The primary vascular elements originate in the procambia of the single carpel and three stamens of the floret independently and in isolation from the vascular system of the spikelet. The initiation of protophloem elements is earlier in the stamens than in the carpel. The median strand of the carpel differentiates before the two lateral strands. The protoxylem elements are initiated after the protophloem elements are welt differentiated.
The differentiation of both protophloem and protoxylem elements is bidirectional in the stamens and in the three strands of the carpel, whereas their differentiation is acropetal in the funicular strand of the carpel.     

Cotyledon venation patterns in the Leguminosae: Gaesalpinioideae

Cotyledon venation patterns are described for 93 species representing all tribes of the Caesalpinioideae. Patterns are grouped into a series of levels of complexity according to the number of primary veins, nine, seven, six, five, three or one, but five- and three-vein patterns predominate. The number of petiolar vascular strands varies from one to eight but most species have two or four strands. It is proposed that all the patterns have been derived from one in which four strands and a protoxylem trace in the petiole branch and anastomose to form seven primary veins in the lamina. Venation patterns show correlations with cotyledon anatomy, size and shape and with taxonomic grouping at tribal, infratribal and generic levels. Each tribe is characterized by a particular set of related patterns, pattern frequencies and evolutionary trends.     

Cotyledon venation patterns in the Leguminosae: Mimosoideae

Cotyledon venation patterns are described for 131 species representing the four main tribes of the Mimosoideae. The range of variation in venation pattern falls within that described previously for the Caesalpinioideae and is consistent with the proposal that all patterns in the Leguminosae have been derived from a prototype with four vascular strands and a protoxylem trace in the petiole and seven primary veins in the lamina. Each tribe is characterized by a particular set of patterns, pattern frequencies and evolutionary trends. In the Mimoseae, correlations between venation pattern and cotyledon size and anatomy match those found in the Caesalpinioideae, but different correlations unique to these tribes occur in the Acacieae and Ingeae.     

CAULINE VASCULATURE AND LEAF TRACE PRODUCTION IN MEDULLOSAN PTERIDOSPERMS

Medullosa and Sutcliffia specimens from the Paleozoic of North America and Europe are examined to determine the architecture of the cauline vasculature and mode of leaf trace production. Emphasis is placed on the identification and characterization of protoxylem strands and their relationship to leaf trace production. Organization of the primary xylem varies from a single protostele to a dissected stele composed of two to many more or less independent bundles. In Medullosa the bundles of primary xylem are each surrounded by secondary xylem, forming separate segments of vascular tissue (‘steles’ of previous workers). These vascular segments may divide and fuse at different levels in the stem. A definite number of protoxylem strands occur near the periphery of the primary xylem. The protoxylem strands divide at intervals producing protoxylem to the departing leaf traces. Leaf traces thus formed arise from all the vascular segments in a coordinated and predictable way and pass outward through emission areas in the secondary xylem. This type of cauline vascular architecture is compared to that of other seed plants. The vascular system of Medullosa stems is interpreted as a dissected monostele. Sympodial vascular architecture has apparently evolved from a protostele separately within the medullosan pteridosperms.     

INITIAL VASCULARIZATION OF THE VEGETATIVE SHOOT OF ABIES CONCOLOR

Parke , Robert V. (Colorado State U., Fort Collins.) Initial vascularization of the vegetative shoot, of Abies concolor. Amer. Jour. Bot. 50(5): 464–469. Illus. 1963.—In the dormant winter bud, the future vascular system of the shoot exists as a rather ill-defined system of procambial strands, which extends acropetally from the scale traces through a plate of thick-walled, deeply staining cells, the crown, and into the axis and the numerous foliar primordia making up the telescoped shoot. Each foliar primordium receives a single procambial strand or leaf trace. The procambial strands differentiate acropetally. No differentiated vascular tissue was observed in the dormant shoot. As the shoot elongates in the spring, vascular differentiation progresses at a rapid rate. In the leaf traces, protophloem differentiates acropetally. The protoxylem, which appears first in the axial region of the trace, differentiates acropetally into the foliar primordium and basipetally into the stem. The first-formed phloem elements are short-lived. They are nucleate and without sieve areas. In the protoxylem, the first-formed tracheids are mostly of the annular or spiral-thickened type.     

PRIMARY XYLEM ELEMENTS AND ELEMENT ASSOCIATIONS OF ANGIOSPERMS

Observations based on a study of more than 1350 species distributed among 165 families of angiosperms are presented. The tracheary elements which mature later than the helical ones in the protoxylem-metaxylem transition are described in terms of a 2-phase wall deposition process. These elements have a helical framework (first-order secondary wall) between the gyres of which is deposited additional secondary wall material in the form of sheets or strands or both (second-order framework). This is indicated both by the sequence of mature elements throughout the primary xylem and by the ontogeny of later maturing elements. Elements in which the second-order system of the secondary wall is deposited more or less synchronously and in which the sheets or strands are restricted to cell edges, i.e., lines of intersection of adjacent cell faces, are interpreted as primitive. Elements in which the second-order sheets and strands appear nonsynchronously and with less regard for cell edges are interpreted as advanced. Alternate pitting results from the appearance of oblique second-order strands with subsequent wall deposition maintaining strand orientation such that pit axes are tilted. In certain elements second-order strands are deposited before, and wall deposition continues after, the cessation of cell elongation. This results in an alternate pit pattern and may explain certain irregular patterns. Branching of the first-order helix seems to be relatively insignificant in the development of more elaborate wall patterns. It is more significant in perforation plate elaboration. “Open pits” occur in a number of dicotyledons. These are pit-like openings which are extended laterally as the thin areas between the gyres of a helix or comparable openings in a reticulum. They constitute a conspicuous feature of the entire protoxylem-metaxylem transition in certain species. The simple perforation plate of only certain angiosperms seems to be the result of bar breakdown in a multipored plate. Reduction in pore number is also the result of fusions in the first-order framework lateral to a multipored plate. In dicots this trend rarely culminates in a simple perforation plate, but it frequently does so in monocots. This type of pore number reduction and enlargement frequently accompanies bar breakdown in dicots and certain monocots. The perforation plate is often simple as the result of a terminalization on the cell, in which case the pore does not intersect the first-order framework. This type of perforation plate occurs in species with and without more obliquely oriented simple perforation plates subject to a breakdown interpretation. Complex multiperforate plates are interpreted as falling into 3 categories : Plates in which a reticulum has resulted from introduction of second-order secondary wall strands at various orientations and with variable amounts of distortion following deposition; plates in which a reticulum has resulted from ramification and fusion of the strands of the first-order framework ; and plates which are multiperforate as the result of the presence of a number of separate loci of breakdown within a single pore membrane. Possible ontogenetic complexities in the development of perforation plates subject to breakdown interpretation are discussed. Protoxylem-metaxylem transitions are described in terms of the sequence of types of perforation plates. Most sequences with various types of plates support the concept of progressively earlier ontogenetic expression of specialized features with progressive elimination of primitive ones. The concept is contradicted by those species in which nearly all perforation plates are simple. Non-simple plates in these species are found from early protoxylem through mid-metaxylem but not in the earliest protoxylem. If non-simple plates are uncommon in a species, they may have a different ontogenetic history in terms of procambial divisions and apical cell growth. In the monocots with a variety of perforation plate types, the probability that a given element will be imperforate or perforate in one way or another will depend on its diameter, not on its position within the sequence. The occurrence of vessels of very limited extent is discussed. It has been calculated that vessels in the stem of Scleria average from 2 to nearly 50 cells in length depending on their diameter.     

STUDIES OF SPHENOPHYLLUM SHOOTS: SPECIES DELIMITATION WITHIN THE TAXON SPHENOPHYLLUM

Structurally preserved ultimate vegetative shoots and attached foliage of Sphenophyllum multirame and two additional taxa from Upper and Middle Pennsylvanian coal balls are described. Shoot axes of all taxa are delimited by small cube-shaped epidermal cells and contain three isolated strands of protoxylem tracheids separated by undifferentiated procambial cells. Metaxylem maturation is delayed for some distance below the apical meristem. Branches originate from a single protoxylem strand and are contiguous with the subjacent leaf trace for a short distance. Sphenophyllum multirame is considered a valid taxon. It is suggested that shoot and foliar anatomical characteristics may form valid criteria for species delimitation within the genus, and S. reedae is delimited by such characteristics. An additional form is tentatively suggested as a possible new species. Anatomical similarities between 5. reedae and Peltastrobus reedae suggest that these taxa represent vegetative and reproductive structures of the same plant.     

AUREALCAULIS CROSSII GEN. ET SP. NOV., AN ARBORESCENT,OSMUNDACEOUS TRUNK FROM THE FORT UNION FORMATION (PALEOCENE), WYOMING

Aurealcaulis crossii gen. et sp. nov., is based on permineralized trunks of an osmundaceous tree fern from the Paleocene Fort Union Formation from near Bitter Creek Station of southwestern Wyoming. This new species is characterized by centripetal (exarch) development of its xylem strands which form part of the leaf traces. Most of the leaf traces depart the stele as two segments that fuse into a single C-shaped petiole vascular strand outside of the outer cortex. Stipular expansions of the petiole bases of this species lack sclerenchyma, and roots arise from the lateral edges of leaf traces in the inner cortex. The family Osmundaceae and subfamily Osmundoideae are slightly emended to accept genera assignable to this family and subfamily with exarch protoxylem in their steles. Foliage similar to Osmunda greenlandica (Heer) Brown, which is possibly the leaf form of A. crossii, occurred next to an axis of this species which was in growth position. This axis was anchored in a lignite suggesting that this species grew under swampy conditions. Aurealcaulis crossii is the first arborescent member of the Osmundaceae of Tertiary age and the second arborescent form in this family reported from the Northern Hemisphere.     

PROCAMBIUM VS. CAMBIUM AND PROTOXYLEM VS. METAXYLEM IN POPULUS DELTOIDES SEEDLINGS

The concept of a procambium-cambium continuum was examined in Populus deltoides by following its development in serially sectioned bud and stem tissues. As in other species, the term cambium is used to refer to that part of the continuum associated with the formation of secondary vascular tissues; i.e., with secondary growth. However, that part of the continuum associated with the formation of primary vascular tissues is subdivided to facilitate interpretation of the consecutive stages of primary xylem differentiation. Thus, the procambium as envisioned by other authors is subdivided into procambium, initiating layer, and metacambium, all of which develop acropetally and in complete continuity. The procambium is derived from the residual meristem in the form of acropetally developing strands and traces. The initiating layer is represented by the first, tangentially separated, periclinal divisions that delineate the position of the prospective cambium. The metacambium is a later stage during which additional periclinally dividing cells unite the initiating layer into a tangentially continuous meristem within a trace bundle. After establishment of the initiating layer, the procambial trace is completely phloem dominated. Protoxylem differentiation begins in an originating center at the base of the leaf primordium and it progresses basipetally to form the protoxylem pole. Cells of the initiating layer do not contribute to the formation of either protoxylem or protophloem. However, those cells of the initiating layer directly opposite the protoxylem pole divide precociously and later differentiate to metaxylem, thus forming a radial file of protoxylem-metaxylem elements. Protoxylem elements of lateral traces are longitudinally continuous with the protoxylem of their parent traces, whereas those of a central trace are longitudinally continuous with the metaxylem of its parent trace. Metaxylem is formed later than protoxylem and it is derived from the metacambium. Metaxylem does not form a continuous system with protoxylem of the same trace because of the different temporal and spatial origins of the two kinds of xylem. Rather, metaxylem is longitudinally continuous with secondary xylem of older traces below. An attempt was made to determine the functional significance of the pattern of protoxylem and metaxylem differentiation in relation to primary and secondary plant development.     

The Control of Vascular Branching in Coleus 2. The Corner Traces

Corner trace connections are less well defined than those ofthe side bundle in Coleus, the locations of branch points, branchpartners, and number of connections made by a corner trace beingmore variable. The auxin balance between corner traces was alteredby leaf removal and by application of exogenous auxin. Branchingof new strands was shifted toward the pre-existing strand withthe lower auxin flux, but only within a narrow range of developmentalstages and with the imposition of a large auxin imbalance. Branchingoccurred only in nodal regions, as in control plants. Thus,auxin balance can be made to control xylem strand branching,but it does not account fully for the control of vascular branchingin intact plants. In the intact pattern, corner trace branchesappear to be directed toward the pre-existing strand with thehigher auxin flux. It is proposed that, in the vicinity of astrand with high flux, auxin is transported laterally withinthe nodal vascular cambium, facilitating vessel differentiationbetween strands in the derivatives of the vascular cambium.These vessels comprise the connections between traces. Coleus, vascular differentiation, vascular anatomy, vascular branching, vascular patterns, auxin, auxin balance, node     

Developmental features of the discontinuous stem vascular system in the rattan palm Calamus (Arecaceae-Calamoideae-Calamineae)

Calamus is a climbing palm marked by considerable internodal extension and limited stem-thickening growth, but with a surprisingly discontinuous axial vascular system. Stem bundles end blindly in a basipetal direction and are connected to each other only by narrow and late-developing transverse commissures. Vascular connection via leaf traces between stem and leaf is made over about nine plastochrons (P), but the dominant central system is completed by about P(7), with subsequent bundles forming the crowded fibrous peripheral system, which has reduced or no vascular tissues. The stem internode below a leaf completes its extension and maturation only by P(10) to P(11). Axial stem bundles originate as procambial strands that are discontinuous apically for up to 15 plastochrons before being "captured" by a developing leaf. Their distal unconnected ends arise by dedifferentiation of ground parenchyma cells. Protoxylem is initiated as short overlapping initials that differentiate progressively during extension growth, which ruptures all but the last-formed elements. Their form, with tapered ends, means that they mature as tracheids. Metaxylem appears only late in shoot development, shortly before internodal elongation ceases (P(8)) and always unconnected to the late-differentiating protoxylem. In each axial bundle protophloem differentiates as a single strand, subsequently and much later appearing as two separate metaphloem strands as the early initials, ruptured by extension growth, are replaced by fibers. It is suggested that the unique features of this stem can be ascribed to the absence of a "meristematic cap," which otherwise typifies palms of normal habit, and that discontinuity is causally related to the pronounced late stem extension growth.     

Occurrence of Intraxylary Phloem in Hevea brasiliensis (Willd. ex A. Juss.) Muell. Arg.

The occurrence of intraxylary phloem in Hevea brasiliensis isreported. The phloem elements were observed as strands associatedwith the protoxylem group in the pericentral region. The natureand importance of such tissue in this species are discussed. Hevea brasiliensis, intraxylary phloem, laticifers, tapping     

Proteome composition and codon usage in spirochaetes: species-specific and DNA strand-specific mutational biases

The genomes of the spirochaetes Borrelia burgdorferi and Treponema pallidum show strong strand-specific skews in nucleotide composition, with the leading strand in replication being richer in G and T than the lagging strand in both species. This mutation bias results in codon usage and amino acid composition patterns that are significantly different between genes encoded on the two strands, in both species. There are also substantial differences between the species, with T.pallidum having a much higher G+C content than B. burgdorferi. These changes in amino acid and codon compositions represent neutral sequence change that has been caused by strong strand- and species-specific mutation pressures. Genes that have been relocated between the leading and lagging strands since B. burgdorferi and T.pallidum diverged from a common ancestor now show codon and amino acid compositions typical of their current locations. There is no evidence that translational selection operates on codon usage in highly expressed genes in these species, and the primary influence on codon usage is whether a gene is transcribed in the same direction as replication, or opposite to it. The dnaA gene in both species has codon usage patterns distinctive of a lagging strand gene, indicating that the origin of replication lies downstream of this gene, possibly within dnaN. Our findings strongly suggest that gene-finding algorithms that ignore variability within the genome may be flawed.     

A New Species of Millerocaulis (Osmundaceae) from the Lower Cretaceous of California

A small permineralized osmundaceous stem has been collected from marine sediments of the Early Cretaceous (Aptian), Upper Chickabally Member of the Budden Canyon Formation near Ono, California. The specimen, 8.5 cm long and 5.4 cm wide, represents a stem surrounded by a mantle of stipular leaf bases and adventitious roots. A large number of sections were studied through the use of the cellulose acetate peel technique. The stem was erect, 11x13 mm in diameter, with a parenchymatous pith and two-layered cortex. The stele is an ectophloic siphonostele with 65-79 leaf traces in the stem per cross section. Leaf gaps are only produced in 13% of the departing traces. Most leaf traces have "delayed" gaps or completely lack leaf gaps. Leaf traces are C-shaped, endarch, with one protoxylem strand, and have sclerenchyma lining the adaxial concavity. Leaf bases have stipular wings with large patches of heterogeneous sclerenchyma and a few scattered strands outside of the heterogeneous sclerotic ring. Patches of sclerenchyma occur inside the ring and outside of the vascular tissues. Numerous diarch roots arise singly or doubly from the leaf traces as they depart the axis stele. Although the stem compares fairly closely to both Ashicaulis Tidwell and Millerocaulis Erasmus ex Tidwell emend. Tidwell, it is most similar to Millerocaulis. However, the combination of characters observed in our specimen differs from that of the seven known species of Millerocaulis. This stem is described as Millerocaulis embreei sp. nov. and is the youngest known species of the genus and the first to be found in the Northern Hemisphere.     

STENOKOLEOS BIFIDUS SP. N. IN THE UPPER DEVONIAN OF NEW YORK STATE

Stenokoleos is a genus for petrified axes from the Mississippian New Albany Shale to which an Upper Devonian occurrence in New York is added. Two orders of branching were known and the plant was thought to be related to coenopterid ferns. The new petrified axes from New York reveal three orders of branching. A pair of rachides emerges from one side of the stem at each node. Their position alternates at successive nodes (distichous). Each rachis bears alternately arranged pinnae. The shape of the xylem strand and the number of protoxylem areas are variable. Traces to the pairs of rachides arise either as two separate strands or as a single strand that is presumed to divide while still within the cortex of the stem. Traces to pinnae are ellipsoid or clepsydroid. Tracheids are scalariform and uni- or biseriate, circular-bordered pitted. Peripheral loops are present in all orders of branches. Protoxylem strands are numerous and maturation is mesarch. Cortex is parenchymatous where it is preserved but outer cortex is missing. Stenokoleos and Reimanniopsis are placed in a new family, Stenokoleaceae. This is classified as Incertae Sedis among Pterophytina in Tracheophyta. It is suggested that the plant is related more closely to the Mississippian pteridosperms Tristichia and Tetrastichia than to the coenopterid ferns.     

The Control of Vascular Branching in Coleus 1. The Side Bundle

Xylary branching at the proximal end of a differentiating sidebundle was modified by surgical alteration of the surroundingleaf traces and manipulation of their auxin fluxes. Incisionsthrough one corner trace, with the other pre-existing traceintact, resulting in xylem differentiation in the branch ofthe newly formed side bundle toward only the severed trace.Application of IAA to the cut trace allowed xylem branchingof the new strand in both directions. With sufficient auxinimbalance created by increasing the concentration of the appliedIAA, the new xylem strand branched away from the higher auxinsource. Auxin relations were thus able to regulate the courseof differentiation of vascular strands, but their role in regulatingbranching patterns in intact plants may be questioned. Xylembranched exclusively toward an incised trace only when the auxinflux of the incised trace was virtually eliminated. Phloem andprocambium of the differentiating strand were unaffected bythis treatment. Coleus, vascular differentiation, vascular anatomy, vascular branching, vascular patterns, auxin, auxin balance, node     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. II. INITIATION AND DEVELOPMENT OF SECONDARY ROOTS

In seedlings of Ipomoea purpurea secondary roots are initiated in the primary root pericycle opposite immature protoxylem. Cells derived from immature endodermis, pericycle, and incipient protoxylem and stelar parenchyma contribute to the primordium. The derivatives of the endodermis become a uniseriate covering over the tip and flanks of the primordium and emerged secondary root; the endodermal covering is sloughed off when the lateral root reaches 1–5 mm in length. A series of periclinal and anticlinal divisions in the pericycle and its derivatives gives rise to the main body of the secondary root. The initials for the vascular cylinder, cortex, and rootcap-epidermis complex are established very early during primordium enlargement. After emergence from the primary root, the cortical initials undergo significant structural modifications related to enlargement of the ground meristem and cortex, and the rootcapepidermal initials are partitioned into columellar initials and lateral rootcapepidermal initials. Procambium diameter increases by periclinal divisions in peripheral sectors. The mature vascular cylinder is comprised of several vascular patterns, ranging from diarch to pentarch, that are probably related ontogenetically. Cells derived from incipient protoxylem and stelar parenchyma cells of the primary root form the vascuar connection between primary and secondary roots.     

STEM ANATOMY AND ASPECTS OF DEVELOPMENT IN TOMATO

Aspects of anatomical development were correlated with internodal growth in tomato plants, variety ‘Yellow Plum,’ grown for more than 3 months. Internodal length was measured weekly in control plants and those harvested for anatomical study. Gross structure indicated progressive development with increasing age. Primary xylem and phloem first mature in distinct strands and the strands are joined laterally by procambium to form a continuous vascular cylinder. Primary phloem occurs on the outer periphery of the procambium between the early-formed vascular strands. Successive periclinal divisions in the procambium during internode elongation give rise to pronounced radial seriations of the cells. Procambial derivatives are included in the cylinder of thick-walled, lignified vascular cells that become prominent after elongation ceases. Secondary xylem is of greater radial width in the stem sectors which include protoxylem. During early secondary growth, vessels develop in the secondary xylem only in these sectors. Nucleate fibers and rays constitute the remainder of the secondary xylem. The rays exhibit an organization noted in other plants of reduced growth habit. Some of these interpretations do not agree with those described for tomato in earlier studies, and they are discussed in relation to pertinent aspects of development.