Palaeosmunda Emended and Two New Species

The genus Palaeosmunda was established by R. E. Gould in 1970 based upon some Late Permian Osmundaceous trunks with well-developed leaf gaps and rhomboidal sclerotic ring within petiolar base seen in cross section. As he thinks that the latter character is more important than the former, this genus could not be assigned to any subfamily of Osmundaceae. However, the leaf gap is one of the most important characters in the structure of the fern stem, so the author suggests that this genus should be assigned to subfamily Osmundoideae and its diagnosis must be emended as follows: The genus Palaeosmunda is represented by some rhizomes (or trunks), roots and leaf bases of ferns which structurally are preserved, resembling Osmundacaulis but which can’t be assigned to any group of this genus. Stem containing an ectophloic dictyoxylic siphonostele; if tracheids present in the pith, they being multiseriate scalariform pitted; pith or cortex sometimes contain ing groups of secretory cells or sclerenchyma; number of leaf traces seen in a tran sverse section of cortex more than 30; leaf traces adaxially curvature, rarely oblong shaped; petiolar bases with or without stipular expansion, containing a C-shaped vascular strand; root diarch. Type species——Palaeosmunda williamsii. According to this diagnosis some primitive osmundaceous species with the leaf gaps, which have already found in Upper Permian and Lower Triassic, could be assigned to this genus. Two of them are P. williamsii Gould and P. playfordii Gould, and Osmundacaulis beardmorensis, which was from Lower Triassic of Antarctica in 1978, should be assigned to the genus Palaeosmunda. In this paper two osmundaceous new species: P. primitiva and P. plenasioides were found in the coal balls of Upper Permian age from Wangjiazhai of Shuicheng of Guizhou Province, China. P. primitiva is represented by two trunks; stem about 4 cm in diameter; stele actophloic dictyoxylic siphonostele; pith cavity about 3—4 mm in diameter, contianing parenchyma and tracheids; xylem cylinder thin, less than 10 tracheids in radial thickness, dissected by leaf gaps. Inner cortex about 1.5 cm thick, mainly parenchymatous, but sometimes containing a few sclerenchymatous; number of leaf traces seen in a transverse section about 50—60; leaf traces departing at 35—45º,open C-shaped at point of departure, gradually becoming shallow C-shaped or V-shaped in different parts; protoxylem in base of leaf traces single, endarch; when leaf traces pass through inner cortex, protoxylem biturcating. Petiole bases without stipular expansion, probablyloosely embracing the stem; xylem strand of potiole trace shallow C-shaped, surrounded by selerenchyma; sclerotic ring round, connected with single sclerenchyma mass in the concavity of the petiole trace. Root arising singly from leaf trace, diarch, with inner and outer cortex. P. plenasioides is represented by a rhizome; stem more than 4 cm in diameter; stele actophloic dictyoxylic siphonostele; xylem cylider with about 20 tracheids in radial thickness, dissected by leaf gaps; xylem bundle U-, O-, or crosier- (i.e. query-) shaped; pith and inner cortex parenchymatous, with many groups of secretory cells; leaf trace C-shaped, its base containing two endarch protoxylem groups; root diareh,with inner and outer cortex, arising singly from leaf trace or its base.     

OSMUNDA WEHRII,A NEW SPECIES BASED ON PETRIFIED RHIZOMES FROM THE MIOCENE OF WASHINGTON

Silicified rhizomes from Miocene strata near Yakima, Washington represent a new species of Osmunda. The stems are 8–13 mm in diameter and are surrounded by a thick sheath of adherent leaf bases, each of which shows stipular expansions typical of the Osmundaceae. The new species has an ectophloic siphonostele in which the xylem cylinder is dissected by leaf gaps with 12–14 strands being visible in a given stem cross section. Such sections also show 12–16 leaf traces in the cortex. The xylem of each leaf trace diverges from the xylem cylinder of the stem as an adaxially concave strand with its protoxylem organized into a single medial adaxial cluster. Initial bifurcation of the leaf-trace protexylem occurs as the leaf trace passes through the outer cortex of the stem. In the basal part of the stipular region of the petiole base, thick-walled fibers form an arch on the abaxial side of the sclerenchyma ring around the petiolar bundle. This arch persists throughout most of the length of the stipular region, with the thick-walled fibers becoming reorganized into two lateral masses in the distal part of the stipular region. Similar thick-walled fibers form an elongate strip of tissue in each wing of the stipule along with several small clusters scattered near the sclerenchyma ring. The new species belongs to the subgenus Osmunda and shows that during the Neogene, the latter existed as a group of closely related species much as it does today. Furthermore, Osmunda wehrii combines features of the modern O. regalis, O. japonica, and O. lancea with those of O. claytoniana and thus supports the inclusion of the latter species in the subgenus Osmunda.     

Plenasium xiei sp. nov. from the Cretaceous of Northeast China: Additional evidence for the longevity of osmundaceous ferns

A new species of the Osmundaceae, Plenasium xiei sp. nov., is herein described from the Cretaceous of Northeast China. The specimens examined here represent the earliest unequivocal record of the extant genus Plenasium in Eurasia based on fossil rhizomes. The rhizome consists of a central stem with a mantle of petiole bases and adventitious roots. The stem contains an ectophloic‐dictyoxylic siphonostele and a two‐layered cortex. The C‐shaped leaf trace bears two protoxylem bundles at the point of separation from the stele. The pith is heterogeneous. The parenchymatous inner cortex is thinner than the sclerenchymatous outer cortex. Lobed sclerenchyma bands occur at the adaxial sides of the stem xylem strands, in the concavity of the leaf trace, and along the adaxial side of the vascular bundles of the petiole base. In distal petiole portions, the sclerenchyma band splits into several groups in the transverse view. Sclerenchyma rings are heterogeneous with an abaxial sclerenchymatous arc of thick‐walled fibers. Numerous sclerenchyma strands of thick‐walled fibers appear in the petiolar inner cortex and the stipular wing. These fossils provide unambiguous evidence for the existence of subgenus Plenasium of modern Plenasium by at least the Late Cretaceous, demonstrating the longevity of this extant subgenus. Altogether the leaf and rhizome fossil records of Plenasium indicate that this genus was widely distributed across North America and Eurasia from the Early Cretaceous to the Early Cenozoic, followed by a range restriction to Eurasia in the Late Cenozoic. Extant Plenasium species are only known from East and Southeast Asia.     

Ashicaulis woolfei n. sp.: additional evidence for the antiquity of osmundaceous ferns from the Triassic of Antarctica

Numerous small fern trunks and dispersed osmundaceous frond fragments occur within a Middle Triassic silicified peat near Fremouw Peak in the Transantarctic Mountains of Antarctica. These specimens form the basis of a new species of osmundaceous ferns that further helps to characterize the early Mesozoic vegetation of high latitude Gondwana. Ashicaulis woolfei n. sp. consists of small, upright trunks with a persistent armor of frond bases, adventitious roots, and vegetative frond parts. In cross section the trunks are ～2.5 cm in diameter and include up to 45 frond bases. Stems range from 5 to 8 mm in diameter with a xylem cylinder of 8-9 xylem segments separated by leaf gaps. Phyllotaxy is variable, approaching 2/5 or 3/8, with 10-12 frond traces in the cortex. Stipes have parenchymatous, stipular wings that are usually devoid of sclerenchyma; fronds are pinnate with alternate-subopposite pinnatifid pinnules. Although the absence of fertile pinnules and sporangia precludes assigning the fossils to a living genus, this species demonstrates that ferns with stelar architecture and histology similar to Osmunda subgenus Osmundastrum (Osmundaceae) were present in the Southern Hemisphere by the mid-Triassic.     

Todea from the Lower Cretaceous of western North America: implications for the phylogeny, systematics, and evolution of modern Osmundaceae

The first fossil evidence for the fern genus Todea has been recovered from the Lower Cretaceous of British Columbia, Canada, providing paleontological data to strengthen hypotheses regarding patterns of evolution and phylogeny within Osmundaceae. The fossil consists of a branching rhizome, adventitious roots, and leaf bases. The dictyoxylic stem has up to eight xylem bundles around a sclerenchymatous pith. Leaf traces diverge from cauline bundles in a typical osmundaceous pattern and leaf bases display a sheath of sclerenchyma around a C-shaped xylem trace with 2-8 protoxylem strands. Within the adaxial concavity of each leaf trace, a single sclerenchyma bundle becomes C-shaped as it enters the cortex. The sclerotic cortex is heterogeneous with an indistinct outer margin. The discovery of Todea tidwellii sp. nov. reveals that the genus Todea evolved by the Lower Cretaceous. A phylogenetic analysis combining morphological characters of living and extinct species with a previously published nucleotide sequence matrix confirms the taxonomic placement of T. tidwellii. Results also support the hypothesis that Osmunda s.l. represents a paraphyletic assemblage and that living species be segregated into two genera, Osmunda and Osmundastrum. Fossil evidence confirms that Osmundaceae originated in the Southern Hemisphere during the Permian, underwent rapid diversification, and species extended around the world during the Triassic. Crown group Osmundaceae originated by the Late Triassic, with living species appearing by the Late Cretaceous.     

Ontogeny of the vascular system ofBotrychium multifidum (S. G. Gmelin) Rupr.(Ophioglossaceae) and its bearing on stellar theories

Basipetal to the shoot apex, a procambial ring with parenchymatous gaps is present. The protoxylem poles are endarrh in both the ectophloic siphonostele and the collateral vascular bundle which comprises the leaf trace. Each leaf trace has an anastomosing system of protoxylem poles that decreases in number basipetally from five to three to two. Differentiation of the leaf trace procambium and protoxylem is bidirectional, that is the differentiation first occurs near the base of the leaf and acropetally in the leaf and basipetally in the stem. Then a fascicular cambium differentiates betweem the primary xylem and phloem in the leaf. This vascular cambium which is also present in the stem is unidirectional and only produces secondary xylem centripetally. Limited secondary growth also occurs in roots. Medullary tracheids when present are longitudinally continuous with the vascular system. The stele of the stem is interpretated as a sympodium of leaf traces and the pith is considered to be fundamental tissue enclosed by the anastomosing of leaf traces.     

菰适应湿地环境的解剖和屏障结构特征研究

菰(Zizania latifolia)是一种多年生挺水植物,为了探讨该植物根、茎和叶的解剖结构、组织化学及其质外体屏障的通透性生理。该文利用光学显微镜和荧光显微镜,对菰的根、茎、叶进行了解剖学和组织化学研究。结果表明:(1)菰不定根解剖结构由外而内分别为表皮、外皮层、单层细胞的厚壁机械组织层、皮层、内皮层和维管柱;茎结构由外而内分别为角质层、表皮、周缘厚壁机械组织层、皮层、具维管束的厚壁组织层和髓腔。叶鞘具有表皮和具维管束皮层,叶片具有表皮,叶肉和维管束。(2)不定根具有位于内侧的内皮层及其邻近栓质化细胞和外侧的外皮层组成的屏障结构;茎具内侧厚壁机械组织层,外侧的角质层和周缘厚壁机械组织层组成的屏障结构,屏障结构的细胞壁具凯氏带、木栓质和木质素沉积的组织化学特点,叶表面具有角质层。(3)菰通气组织包括根中通气组织,茎、叶皮层的通气组织和髓腔。(4)菰的屏障结构和解剖结构是其适应湿地环境的重要特征,但其茎周缘厚壁层和厚壁组织层较薄。由此推测,菰适应湿地环境,但在旱生环境中分布有一定的局限性。     

ITOPSIDEMA,A NEW GENUS OF THE OSMUNDACEAE FROM THE TRIASSIC OF ARIZONA

Daugherty , Lyman H. (San Jose State College, San Jose, Calif.) Itopsidema, a new genus of the Osmundaceae from the Triassic of Arizona. Amer. Jour. Bot 47(9): 771–777. Illus. 1960.—Itopsidema vancleavei, a new genus and species of the family Osmundaceae, is described. The specimen consists of several segments of an arborescent stem obtained from the Upper Triassic of the Petrified Forest National Monument near Holbrook, Arizona. The surfaces on 2 of the segments are covered by adventitious roots and the remaining segments are covered by leaf bases. The fronds are spirally arranged and have an 8/21 phyllotaxy. The cortex of the stem, which contains numerous leaf traces and adventitious roots, consists of parenchyma with cell walls of medium thickness. The leaf traces are oblong to crescent-shaped in the inner cortex and horseshoe-shaped in the outer cortex. The base of the petiole contains a single, large vascular bundle and is covered by multicelled, glandular spines. The adventitious roots originate on the abaxial side of the leaf traces in the region of the inner cortex. The center of the stem is occupied by an ectophloic, mesarch siphonostele without leaf gaps. The pith is composed of firm-walled parenchyma cells which have isolated tracheids with reticulate pitting scattered among them. These tracheids are so rare the pith cannot be considered a “mixed pith.” The relationship and morphological significance of Itopsidema with respect to other members of the Osmundaceae are briefly discussed.     

QUAESTORA AMPLECTA GEN. ET SP. N., A STRUCTURALLY SIMPLE MEDULLOSAN STEM FROM THE UPPER MISSISSIPPIAN OF ARKANSAS

A well preserved, permineralized seed fern stem is described from the Upper Mississippian Fayetteville Formation of north central Arkansas. Quaestora amplecta gen. et sp. n. is 41.6 cm long and exhibits six pairs of decussate, highly decurrent petiole bases. The stem has a cruciform, exarch protostele with prominent secondary xylem, vascular cambium and secondary phloem. Leaf traces are terete and occur as an outer ring with a small number of internal strands. The cauline vasculature, leaf-trace production, petiolar anatomy and several other features indicate that this specimen represents the most structurally simple and geologically ancient medullosan stem presently recognized.     

SHOOT APEX ORGANIZATION AND ORIGIN OF THE RHIZOME-BORNE ROOTS AND THEIR ASSOCIATED GAPS IN DENNSTAEDTIA CICUTARIA

The shoot apex of Dennstaedtia cicutaria consists of three zones—a zone of surface initials, a zone of subsurface initials, and a cup-shaped zone that is subdivided into a peripheral region and central region. A diffuse primary thickening meristem, which is continuous with the peripheral region of the cup-shaped zone, gives rise to a broad cortex. The roots occurring on the rhizomes are initiated very near the shoot apex in the outer derivatives of the primary thickening meristem. The roots that occur on the leaf bases also differentiate from cortical cells. Eventually, those cortical cells situated between the newly formed root apical cell and the rhizome procambium (or leaf trace) differentiate into the procambium of the root trace, thus establishing procambial continuity with that of the rhizome or leaf trace. Parenchymatous root gaps are formed in the rhizome stele and leaf traces when a few of their procambial cells located directly above the juncture of the root trace procambium differentiate into parenchyma. As the rhizome procambium or leaf trace continues to elongate, the parenchyma cells of the gap randomly divide and enlarge, thus extending the gap.     

爬树蕨的解剖学研究

本文对爬树蕨（Ａｒｔｈｒｏｐｔｅｒｉｓｏｂｌｉｔｅｒａｔａ（Ｒ．Ｂｒ．）Ｊ．Ｓｍ）孢子体各主要器官进行了解剖学研究及对孢子进行了电子显微镜扫描观察,研究结果表明;茎的中柱具有两个新月形的维管束;幼茎的中部有髓．在较老的茎,髓部及中柱周围的细胞均特化为厚壁细胞．根属二原型中柱．木质分化方式是外始式;在对正后生木质部的两侧的皮层有几层特化为厚壁细胞。叶的叶肉细胞不分化出栅栏组织和海绵组织,为等面对。孢子囊具有纵向环带,孢子的形状、外壁的纹饰和裂缝情况均与以前的研究有所不同。     

VASCULAR CONNECTION BETWEEN LATERAL ROOTS AND STEM IN THE OPHIOGLOSSACEAE

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots. Gradual root-stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them. The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root. Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems. The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.     

ONTOGENY AND PHYLLOTAXIS OF THE TERMINAL VEGETATIVE SHOOTS OF MICHELIA FUSCATA

Tucker Shirley C. (Northwestern U., Evanston, Ill.) Ontogeny and phyllotaxis of the terminal vegetative shoots of Michelia fuscata. Amer. Jour. Bot. 49(7): 722–737. Illus. 1962.—Two patterns of symmetry occur in Michelia fuscata In the lead shoots, leaves arise in a 2/5 spiral arrangement which may be either clockwise or counterclockwise. Other shoots are dorsiventrally organized; these shoots produce leaves in a modified ½ phyllotaxis in which the angle between the 2 files of leaves lies between 100° and 150°, according to the particular branch. Both types of shoot have a zonate apical meristem with a biseriate tunica a central initial zone, and a peripheral zone. The apical configuration of cells does not change appreciably during the plastochron. The flat to low-convex outline of the shoot apex is maintained by initiation of the leaves close to the summit of the apex; the diameter of the meristem diminishes greatly after such an initiation. Leaf inception in the subsurface tunica layer is followed by precocious activity of the marginal meristems which extend the stipular flanges completely around the base of the apical meristem. The stipular margins then fuse laterally and form a hood over the apex. A subapical initial meanwhile is active in the leaf blade, where it persists up to the time the leaf is 2 mm high. The most recent primordium is 300 μ high before another leaf is initiated. The vascular system of the stem is a cylindrical network of leaf traces, with 6–12 traces per leaf. The procambium develops acropetally from preexisting vascular strands in the stem below. Elements of the diverse sclereid system differ in shape in different tissues, according to the availability of intercellular space. Goebel's term “Pendelsymmetrie” is discussed with reference to apical activity in Michelia.     

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.     

CIBOTIUM OREGONENSE: AN EOCENE TREE-FERN STEM AND PETIOLES WITH INTERNAL STRUCTURE

Cibotium oregonense sp. nov. is described from the Upper Eocene of Medford, Oregon. The fossil comprises petiole bases and stem periphery of a large fern preserved via permineralization. A stele devoid of sclerenchymatous sheathing and a petiole vascularized with a large number of leaf traces in a cyatheacean pattern characterize the fossil. A distinctive U-shaped interior series of leaf traces allows assignment to the genus Cibotium. This fossil provides evidence that the genus Cibotium was represented by an arborescent species with large leaves in the Early Tertiary of western North America.     

Nodal and leaf anatomy of Xanthophyllum (Polygalaceae)

The nodal anatomy of Xanthophyllum is unilacunar with a single broad trace departing the cauline stele. The "stipular glands" or extra floral nectaries of some species are vascularized by bundles originating from the base of the leaf trace. Considerable variation exists among species in petiole vasculature with siphonosteles, steles with medullary bundles and simple, flat traces present. The lamina also shows variation in the presence or absence of a hypodermis, nature of vein sheathing, presence or absence of abaxial epidermal papillae, amount of intercellular spaces, and mature stomatal patterns which range from anisocytic and paracytic to those in which no subsidiary cells are discernible. Of nearly uniform occurrence throughout the genus are extraordinary tracheoid foliar idioblasts, which are confined to the veins in terminal or subterminal positions. The large amount of variation in leaf anatomy is shown to be taxonomically significant within the genus.     

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.     

Effects of Developing Leaves on Stelar Pattern Development in the Shoot Apex of Matteuccia struthiopteris

A developmental study of the normal shoot apex of Matteucciastruthiopteris suggested that patterned stelar differentiationis initiated immediately beneath the single layer of promeristemand occurs prior to the initiation of the youngest leaf primordium.A developmental study in which all leaf primordia were suppressed,with or without lateral isolation of the terminal meristem byvertical incisions, has confirmed this interpretation of stelardifferentiation. Experimentally-induced changes in the tissueimmediately below the promeristem were reflected in the resultingmature structure of the stele. Failure of leaf gap initialsto differentiate, if all leaf primordia were suppressed at theincipient stage, resulted in a mature stele without leaf gaps.Similarly the disappearance of pith mother cells after severalweeks of leaf removal was associated with the formation of astele without pith. Leaf influence was further assessed by allowingone primordium to develop while all others were suppressed.The developing leaf had a small promoting effect on caulinevascular tissue differentiation but its major impact on theexpansion of the parenchymatous tissues of the stele. Characteristicprotoxylem and protophloem failed to differentiate when allleaves were suppressed and, when leaf was allowed to develop,formed only in relation to the leaf.Copyright 1995, 1999 AcademicPress Leaf influence, vascular pattern formation, experimental surgery, shoot apex development, protoxylem, protophloem, Matteuccia struthiopteris     

LYSSOXYLON GRIGSBYI,A CYCAD TRUNK FROM THE UPPER TRIASSIC OF ARIZONA AND NEW MEXICO

Lyssoxylon grigsbyi Daugherty, a petrified stem with petiole bases, was originally described from the Upper Triassic Chinle Formation of Arizona and considered to be a member of the Williamsoniaceae. Investigation of additional material from a similar horizon in New Mexico, together with re-examination of preparations of the holotype, suggest that the plant, with its monoxylic stele, girdling leaf traces, and bicelled epidermal hairs is a true cycad. Cells of the New Mexico specimens contain structures interpreted as preserved nuclei.     

THE OCCURRENCE OF “ROOT GAPS” IN THE RHIZOME SOLENOSTELE AND LEAF TRACE OF DENNSTAEDTIA CICUTARIA

The vascular system of the rhizome axes of Dennstaedtia cicutaria consists of a solenostele with an amphicribral vascular bundle centrally located in the pith. Each leaf has a single trace which is an amphicribral vascular bundle. At each node of an axis there is a complex consisting of the main axis, leaf base, and a branch axis attached to the basiscopic margin of the leaf base. Numerous roots are present on the rhizomes and the abaxial side of the leaf bases. Parenchymatous gaps occur in the rhizome solenostele and the leaf trace directly above the departure of some of the root traces. These gaps are termed root gaps. In some instances the root gaps are confluent. However, not all of the root traces have an associated root gap. The leaf trace is inserted laterally on the main and branch axes at the node so that the acroscopic leaf trace margin anastomoses with the main axis of the vascular system and the basiscopic margin with that of the branch axis. Two leaf gaps are associated with each leaf trace, one occurring in the main axis solenostele and the other in the branch axis solenostele. The medullary bundle of each axis anastomoses with each leaf trace at its point of attachment to the rhizome solenostele. Thus, the medullary bundle forms a continuous vascular strand from leaf trace to leaf trace in any given rhizome axis.