Meristem growth dynamics and branching patterns in the Cladoniaceae

Branching patterns in the lichen family Cladoniaceae are varied and taxonomically important. Branching occurs on the podetium, the erect secondary thallus that characterizes most species in the Cladoniaceae, and is influenced by growth dynamics of the fungal meristem tissue at the apex of the podetium. Branching is primarily the result of meristem divisions, and branching patterns are modified by meristem enlargement, deformation, and torsion. Branching processes are conserved, and early branch ontogeny provides information from which to determine relationships in the Cladoniaceae. Branching is characterized by two major patterns. In one pattern, branches arise from the relatively late divisions of a large meristem (≥100 μm in diameter), whose shape changes during ontogeny. In a second pattern, branches arise from small meristems (<100 μm in diameter), which split early in ontogeny but whose shape does not change. The trend toward reduced meristems that split early in ontogeny is seen as an evolutionary advance in the Cladoniaceae. Some "small meristem" species retain aspects of the "large meristem" habit in early ontogeny, and this provides a clue to their relationships. Patterns of meristem growth dynamics provide a basis for interpreting phylogeny in mycobionts of the Cladoniaceae. Meristem activities in four genera of the Cladoniaceae were compared in order to determine trends in growth dynamics within the family.     

THALLUS ORGANIZATION AND DEVELOPMENT IN THE FRUTICOSE LICHENASPICILIA CALIFORNICA,WITH COMPARISONS TO OTHER TAXA

Thallus organization is examined inAspicilia californicaRosentreter, a fruticose lichen known from several localities in central and southern California. The sprawling, terete thallus branches possess a dense central medulla of thick-walled, longitudinally oriented fungal cells. This central tissue emerges at branch apices to form a darkly pigmented fungal tip. Thallus development involves the apical extension of the tip to produce a fungal tissue over which a cylindrical algal layer and cortex will eventually be formed. Apical branches are initiated by furcation entirely within the fungal tip. Lateral branches, emerging from the lichenized thallus, arise as a divergent bundle of elongate fungal cells originating in the medulla. The photobiont appears to play no direct role in initiation of apical or lateral branches. It is concluded that thallus development inA. californicaoccurs with a relatively low degree of synchrony between mycobiont and photobiont growth, similar to the pattern observed in crustose lichens with prothallic growth. A rather similar type of thallus organization is observed inA. hispida, although in that species mycobiont growth and branch initiation appear to be somewhat more closely associated with algal cell proliferation. A squamuloseAspiciliafrom central Spain produces rhizomorphs that may sometimes become invested with an algal layer and cortex, resembling the thallus axes ofA. californica.     

Lateral growth patterns in the Cladoniaceae

Lateral growth from the apex of vertical structures is widespread in cladoniiform lichens. In the family Cladoniaceae, it is accomplished through a developmental shift in the meristem, in which growth orientation changes from isotropous to anisotropous. In anisotropous development, the more or less constant relationship among the axial, radial, and circumferential planes of growth is altered during ontogeny. The result is pronounced lateral elongation of the apical meristem, a departure from the isotropous body plan of early ontogeny. Development that favors radial and circumferential growth over axial growth is an innovation that provides ontogenetic flexibility but which also entails the loss of control from a single centralized meristem to one or more meristems. A shift from the constraints of symmetry to the risks and potential of asymmetry and a subsequent diversity of heritable thallus forms reflect evolutionary processes in the Cladoniaceae. Similar morphogenetic activities, which are apparently highly conserved, are shared by species that are presumably only distantly related.     

Effect of gibberellin and kinetin on the regeneration ability ofFucus vesiculosus L.

The influence of gibberellic acid (GA³) and kinetin (6-furfurylaminopurine) on the regeneration ability of the basal and apical thallus fragments ofFucus vesiculosus L. was examined. The naturally occurring gibberellin and kinetin-like substances in these thallus fragments were also studied. It was found that exogenously applied GA³ markedly increased the number of adventitious branches formed on the cut surface of the thallus fragments taken from the apical parts of plants. The concentration of 0.001 mg GA³ I-1 proved to be the most effective. The growth promoting effect of GA³ was increased by simultaneous action with kinetin. In experiments in which the fragments of the basal parts of the thallus were treated with GA³, as a rule a slight growth inhibition was observed. The growth responses of the investigated plant tissues to gibberellin and kinetin varied according to season. Usually their susceptibility to the applied plant hormones was greater in spring than is summer. The shifts in growth reaction were related to the seasonal changes in the content of endogenous gibberellin and kinetin-like substances in the investigated parts of the thallus. It is suggested that growth regulators of the gibberellin and cytokinin type are involved in the regeneration processes inFucus.     

Comparative leaf development ofOsmunda lancea andO. japonica (osmundaceae): Heterochronic origin of rheophytic stenophylly

Comparative development of the narrow pinnules of rheophyticOsmunda lancea and of the broad pinnules of a related dryland species,O. japonica, was examined and the origin of rheophytic stenophylly was discussed. The mature leaves and their various parts ofO. lancea are smaller and narrower than those ofO. japonica. The young pinnules ofO. lancea at the initiation of cell expansion are smaller than those ofO. japonica. The growth pattern of the pinnules is fundamentally the same in the two species, but pinnule growth period is shorter inO. lancea than inO. japonica. While the largest growth rate in pinnule length is quite similar, inO. lancea the pinnules are less elongated and much less broadened during ontogeny. Cell expansion in the mesophyll and epidermis proceeds acropetally and toward the margin along the axes of costules and veins. Although the numbers of mesophyll and epidermal cells between two adjacent veinlets are almost the same inO. lancea andO. japonica, during the subsequent growth period inO. lancea, the cells expand to a smaller extent and the veinlets become more narrowly oblique to the costule. This oblique distortion of laminar segments framed by veins causes stenophylly, an allometric modification. The stenophylly ofO. lancea is believed to have arisen by heterochronic evolution, in particular, progenesis.     

ON THE LIFE HISTORY OF MONOSTROMA FUSCUM (POSTELS ET RUPRECHT) WITTROCK1

Results of the study of Monostroma fuscum from Son Juan Island, Washington, fully support the separation of the species M. fuscum from Monostroma sensu Wittrock into a taxon including M. obscurum. It would appear from this and previous studies that the M. obscunim-M. fuscum complex is clearly Ulvacean. The characteristics of the complex include (1) alternation of morphologically similar haploid and diploid generations; (2) production of biflagellate gametes and quadriflagellate zoospores; (3) a uniseriate form in early ontogeny; (4) thallus development progressing through a vesicular stage that, as a result of localized degenerative processes, opens at a size of less than I cm into a monostromatic blade; (5) cells elongated perpendicular to the plane of the thallus and with rotund-truncate ends: (6) a single elongate plastid, thickened at the opposite poles of the cell and thinner in the equatorial region of the cell; (7) a thallus composed of cells with relatively inelastic walls; and (8) discrete discharge pores. The chromosome number of Monostroma fuscum is n =9. The first division of the zoosporangium nucleus is meiosis I.     

North Aegean marine algae. III. Structure and development of the encrusting coralline Titanoderma cystoseirae (Rhodophyta, Lithophylloideae)

Vegetative and reproductive features of the encrusting marine alga Titanoderma cystoseirae are described on the basis of type specimens from the N. Adriatic Sea and new material from the N. Aegean Sea. Growth proceeds by means of two meristem types: a superficial meristem that occurs on terminal cells of hypothallial filaments, producing the hypothallium, and an internal meristem that is confined to cells just below the epithallium, producing the perithallium.
Subepithallial divisions occur in groups of cells that form distinct meristematic zones perpendicularly to the plane of radial thallus sections. These coordinated divisions originally appear on hypothallial cells close to the margin and subsequently spread centripetally, as opposed to the centrifugal growth of the hypothallial filaments. At least two subepithallial meristematic zones occur at a time on the thallus and both spread centripetally and in sequential pattern, each producing a new layer of perithallial cells that gradually covers the preceding (underlying) one. This elaborate process of perithallial ontogeny seems to be a distinctive characteristic for T. cystoseirae. Coordinated subephitallial divisions with different developmental patterns, however, are also observed in other coralline algae, particularly in members of the Lithophylloideae and the Amphiroideae. These observations support the hypothesis of the close phylogenetic relationship between these taxa but also emphasize the need for further studies before taxonomic implications can be proposed.     

Anatomical study on the shoot apex ofOsmunda japonica thunb

The ontogenetic and seasonal variations in the organization of the shoot apical meristem ofOsmunda japonica Thunb. were investigated. The meristem is composed of an apical segmentation zone (SZ), a mother cell zone of the stele (MS) and a periphereal zone (PZ). A single apical cell is mostly discernible in all sesons throughout the whole process of ontogeny observed in the present study. The paical cell is usually four-sided, nearly triangular, with a regular segmentation pattern in transverse view. However, it is sometimes acccurately three-sided with a highly regular segmentation pattern in the active season, while it is often four-sided, nearly trapezoid or five-or six-sided, with a less regular segmentation pattern in the inactive season. The size of the apical cell represented by its free surface are increases with the increase in size of the plant body in the young plants. However, in the adult plants, the size of the apical cell is smaller in the active season and larger in the dormant season. The organization pattern of the shoot apical meristen ofO. japonica does not show an intermediate type between the eusporangiate and the leptosporangiate ferns, but the leptosporangiate fern type.     

ONTOGENY OF THE OPEN DICHOTOMOUS VENATION IN THE PINNA OF THE FERN NEPHROLEPIS

Pray , Thomas r . (U. South. California, Los Angeles.) Ontogeny of the open dichotomous venation in the pinna of the fern Nephrolepis. Amer. Jour. Bot. 47(5) : 319—328. Illus. 1960.–The venation of the pinna of Nephrolepis consists of a midvein and 2 lateral series of dichotomizing veins all of which terminate freely near the margins. The development of the pinna is analyzed with particular attention to the nature of the marginal meristem and the organization of the embryonic pinna as it appears in paradermal section. The arrangement of cells in pinna wings during the period of marginal growth displays a pattern which foreshadows the pattern of the mature venation. In contrast with the development of the leaves of angiosperms, marginal growth continues into a relatively late phase of pinna ontogeny and apparently is active throughout the phases of ontogeny concerned with blocking-out the pathways of procambial differentiation. Thus the pattern of venation appears to be correlated with the manner of activity of the marginal meristem and subsequent orientation of its derivatives. The theoretical aspects of the result of this investigation are discussed in relation to other studies of foliar venation ontogeny.     

CHANGE IN EPILOBIUM PHYLLOTAXY DURING REPRODUCTIVE TRANSITION

The ontogeny of Epilobium hirsutum grown under natural summer photoperiod in a glasshouse was divided into vegetative, early transitional, transitional, and floral stages. Bijugate phyllotaxy, common to both the vegetative and early transitional stages, is transformed into spiral phyllotaxy during the transitional stage by an initial change in the divergence angle of a single primordium inserted at a unique level on the shoot. Leaf primordia subsequently are inserted in a spiral arrangement in the indeterminate floral shoot apex. The early transitional shoot apical meristem is about 1.5 times the volume of the vegetative meristem but expands at about two-thirds the relative plastochron rate of volume increment of the vegetative meristem. There are progressive decreases in the plastochron and relative plastochron rates of radial and vertical shoot growth through ontogeny. Relative chronological rates of shoot growth, however, are not altered during ontogeny. Spiral transformation results from changes in the relative points of insertion of leaf primordia on the shoot meristem. These changes are accompanied by an increased rate of primordia initiation on a more circular shoot meristem. The change in phyllotaxy during ontogeny is similar to that which was artificially induced by chemical modification of auxin concentration gradients in the shoot apex, with the additional feature that there is an initial increase in the volume of the shoot meristem prior to the natural spiral transformation. Size of the shoot apical meristem, however, appears to have little influence on Epilobium phyllotaxy; but the geometric shape of the meristem is well correlated with bijugate to spiral transformations. This suggests that geometric parameters of the shoot meristem should be considered in theoretical models of phyllotaxy.     

EARLY GAMETOPHYTE ONTOGENY OF GLEICHENIA BIFIDA (WILLD.) SPRENG.: PHYLOGENETIC AND ECOLOGICAL IMPLICATIONS

Past investigation of gametophyte ontogeny in Gleichenia bifida indicated that there was considerable plasticity in early developmental stages (spore germination to initiation of two-dimensional growth). Recent examination has shown that this early ontogeny is not so plastic and can be defined by a characteristic series of developmental stages. Cell divisions following germination result in a three-dimensional mass of cells which secondarily initiates the two-dimensional thallus. Early development as a three-dimensional mass correlates with other gametophyte and sporophyte features which indicate Gleichenia to be phylogenetically primitive. This pattern of early ontogeny may also be of adaptive significance in enabling the gametophytes to survive the environmental fluctuations of the exposed, bare-soil habitats which they colonize.     

INTRASPECIFIC INTERACTIONS AND PARASITISM IN AN ASSOCIATION OFRHIZOCARPON LECANORINUMandR. GEOGRAPHICUM

Intraspecific thallus interactions in mixed populations ofRhizocarpon lecanorinumR. geographicumin North Wales are described. In contrast to the most commonly occurring morphotype ofR. geographicumwhich forms mosaics comprised of sharply delimited individual thalli,R. lecanorinumthalli consistently merge with one another at intraspecific contacts, leaving no visible trace of their initial boundaries. On the analogy of mycelial interactions in populations of wood-rotting and other non-lichen-forming fungi, it is postulated that these differences are the outcome, inR. geographicumof somatic incompatibility between distinct genotypes, and inR. lecanorinumof somatic compatibility among genetically identical or very closely related thalli. Possible mechanisms for the evolution of clonal population structure inR. lecanorinumand links between somatic interactions and reproductive output in these species are discussed. Facultative parasitism ofR. lecanorinumbySchaereria fuscocinereaand resistance to such parasitism by co-occurringR. geographicumare also documented.     

Development of the floral shoot and pre‐anthesis cleistogamy in Hydrobryopsis sessilis (Podostemaceae)

The reproductive biology of Hydrobryopsis sessilis (Podostemaceae, subfamily Podostemoideae), a reduced, threatened, aquatic angiosperm endemic to the Western Ghats of India, was examined. This is the first report on the transition from the vegetative to the reproductive phase in this plant, describing floral ontogeny, pollination and the breeding system. The cytohistological zonation of the apical meristem of the reproductive thallus is identical to that of the apical meristem of the vegetative thallus. The floral shoots do not replace vegetative shoots (i.e. the vegetative shoots never bear flowers), but form at new sites at the tip of the flattened plant body. Each floral shoot meristem is tiny, deep‐seated and concave and arises endogenously following lysigeny. The floral shoot meristem gives rise to four to six bracts in a distichous manner. The development of spathe, stamens and carpels is described. The ab initio dorsiventrality of the carpels and the occurrence of endothelium in the ovules are reported. The mature stigmas and anthers lie close to each other. The pollen germinates within undehisced anthers and the pollen tubes enter the stigmas in the unopened floral bud, leading to pre‐anthesis, complete, constitutional cleistogamy under water. The seed set is 63.2%. A significant finding is the penetration of several pollen tubes into the filaments of stamens in 16% of the flower buds, indicating a trend towards cryptic self‐fertilization. The Indian Podostemoideae appear to show a shift from xenogamy or geitonogamy or autogamy in a chasmogamous flower to complete autogamy in a cleistogamous flower. The floral modifications leading to cleistogamy in H. sessilis have been identified. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 222–236.     

Testing the ontogenetic base for the transient model of inflorescence development

Backgrounds and Aims

Current research in plant science has concentrated on revealing ontogenetic processes of key attributes in plant evolution. One recently discussed model is the ‘transient model’ successful in explaining some types of inflorescence architectures based on two main principles: the decline of the so called ‘vegetativeness’ (veg) factor and the transient nature of apical meristems in developing inflorescences. This study examines whether both principles find a concrete ontogenetic correlate in inflorescence development.

Methods

To test the ontogenetic base of veg decline and the transient character of apical meristems the ontogeny of meristematic size in developing inflorescences was investigated under scanning electron microscopy. Early and late inflorescence meristems were measured and compared during inflorescence development in 13 eudicot species from 11 families.

Key Results

The initial size of the inflorescence meristem in closed inflorescences correlates with the number of nodes in the mature inflorescence. Conjunct compound inflorescences (panicles) show a constant decrease of meristematic size from early to late inflorescence meristems, while disjunct compound inflorescences present an enlargement by merging from early inflorescence meristems to late inflorescence meristems, implying a qualitative change of the apical meristems during ontogeny.

Conclusions

Partial confirmation was found for the transient model for inflorescence architecture in the ontogeny: the initial size of the apical meristem in closed inflorescences is consistent with the postulated veg decline mechanism regulating the size of the inflorescence. However, the observed biphasic kinetics of the development of the apical meristem in compound racemes offers the primary explanation for their disjunct morphology, contrary to the putative exclusive transient mechanism in lateral axes as expected by the model.     

Developmental anatomy of the fifth shoot-borne root in young sporophytes of<Emphasis Type="Italic"> Ceratopteris richardii</Emphasis>

Young sporophytes of the homosporous fern Ceratopteris richardii produce a single shoot-borne root below each leaf. The developmental anatomy of the fifth sporophyte root is described using scanning electron microscopy and histological techniques. Three merophyte orthostichies in the body of the root originate from three proximal division faces of a tetrahedral root apical cell. Eight or nine divisions occur in a relatively regular sequence within each merophyte and produce a characteristic radial anatomical pattern in the root. The exact number of early divisions within a merophyte depends on the merophytes position within the root as a whole. Predictable inter-merophyte differences arise because a 2-fold (diarch) anatomical symmetry that is characteristic of mature roots is superimposed on a 3-fold radial symmetry that originates behind the apical cell. Before early formative divisions within a merophyte are completed, additional proliferative divisions begin to increase the number of cells within previously established tissue zones. The cellular parameters of early fifth root development in C. richardii are relatively invariant, and are reminiscent of patterns previously described for the heterosporous fern Azolla. Young sporophytes of C. richardii provide a useful model to further investigate the genetic regulation of root development in a non-seed plant, where the anatomy of meristem organization differs from that seen in flowering plant species.Abbreviations SEM Scanning electron microscopy - RAC Root apical cell     

Development of the cranium ofNeoceratodus forsteri,with a discussion of the suspensorium and the opercular apparatus in Dipnoi

Summary The ontogeny of the cranium of Dipnoi is restudied. The investigation especially refers to the basic components of the dipnoan cranium and several functional and developmental aspects of the structure of the larval skull ofNeoceratodus. There are fundamental differences even in the early development and composition of the chondrocranium ofNeoceratodus and Lepidosirenidae. This result, and comparison with several osteichthyans and Tetrapoda, requires a reinterpretation of the components of the dipnoan skull base. The pterygoid processes are not reduced, but incorporated into the cranial base early in ontogeny. The characteristic elongate trabecular rods, which in Gnathostomata usually bridge the ethmoidal plate and the orbito-temporal base of the chondrocranium, are much delayed in development inNeoceratodus, or even seem absent inLepidosiren andProtopterus. Accordingly, in Dipnoi no typical basitrabecular junction is formed in early ontogeny. Instead, the pars quadrata is fused to the mesodermal basis cranii posteriorly. InNeoceratodus a mesially directed basal process of the palatoquadrate is recognizable, which topographically corresponds to the basal process of Urodela and the pseudobasal process of anuran larvae. The ethmosphenoid region of the dipnoan skull also develops quite differently. In Lepidosirenidae, the palatoquadrates are interconnected anteriorly by a distinct commissura palatoquadrati, whereas inNeoceratodus a continuous planum ethmoidale (trabecular plate) is formed. The primary embryonic quadrato-trabecular connection persists as a commissura quadratocranialis anterior below the foramen opticum, at the root of the ectethmoid process. The formation of the skull base in living Amphibia appears to provide the best model for comparison, though it is difficult to propose any undisputable shared derived character states of the cranium of Dipnoi and Tetrapoda beyond this similarity. A similar difficulty presents the phylogenetic interpretation of the hyoid arch. In contrast to the absence of any dorsal hyoid arch elements inLepidosiren, the small hyomandibula ofNeoceratodus is surprisingly complete. In larvae it consists of a laterohyale, an epihyal part, and a processus symplecticus. A stylohyal cartilage is present, which forms rather late in ontogeny. The major chondral components of the hyoid arch are thus comparable to those of living Actinopterygii, except that a distinct symplecticum is not separated off, the components are relatively smaller, and they do not ossify. In view of the early-immobilized palatoquadrate, the hyomandibula ofNeoceratodus has no suspensorial function, but represents part of an opercular hinge and opening mechanism. The hamuloquadrate knob at the posterior face of the quadrate body is comparable to the processus hyoideus in some Urodela. It provides a pivoting joint for the ceratohyale, and therefore functions in buccal expansion. The closed spiracular canals include mechanoreceptive lateral line organs, which probably represent proprioreceptive organs for adjustment of mandibular, hyoid, and opercular movements. It is concluded that considerable differences between the skull architecture of Dipnoi and other Osteognathostomata (Teleostomi) can be assigned to the fact that palatoquadrate and trabecular anlagen fail to separate, resulting in a dramatic and highly adaptive change of palatoquadrate development in early ontogeny. Though these differences include several characters that suggest a plagiostomate condition of the jaw apparatus, this can be explained as a secondary acquisition. The multitude of retained plesiomorphies observed in the cranium of Dipnoi do not exclude a sister group-relationship to Tetrapoda. However, the ancestral osteognathostome suspensorial pattern still presents a problem of interpretation, for we lack a detailed survey of the development and significance of different quadrato-neurocranial connections.     

THE GAMETOPHYTE OF ANEMIA COLIMENSIS

Anemia colimensis, the most primitive known species of the genus, has been investigated with respect to gametophyte development and morphology. In the early stages the thalli are in general similar to those of species of Anemia which are charcterized by the development of an initially lateral marginal meristem. One significant difference was noted in the frequent appearance of meristematic zones on both sides of the young plate. One of these becomes the marginal meristem; the other eventually ceases activity. The mature gametophyte is a spherical cushion of complex organization, the result of extreme ruffling of the wings of the thallus and the remarkable abundance of filamentous outgrowths and deep lobing of the wings. This form of gametophyte is unique among the investigated anemias. It is proposed that the formation of a lateral meristem, the “Ceratopteris type” of prothallial development, is primitive in Anemia and that the “Adiantum type,” at least in the Schizaeaceae, is derived. This suggestion is discussed in relation to other possible interpretations.     

Monthly fluctuations in radial growth of individual lobes of the lichenParmelia conspersa (Erhr. ex Ach.) Ach.

The radial growth (RG) of 120 lobes from 35 thalli of the foliose lichenParmelia conspersa (Ehrh. ex Ach.) Ach. was studied monthly over 22 months in south Gwynedd, Wales, UK. Autocorrelation analysis of each lobe identified three patterns of fluctuation: I) random fluctuations (58% of lobes), 2) a cyclic pattern of growth (23% of lobes), and 3) fluctuating growth interrupted by longer periods of very low or zero growth (19% of lobes). In 80% of thalli, two or three patterns of fluctuation were present within the same thallus. Growth fluctuations were correlated with climatic variables in 31% of lobes, most commonly with either total rainfall or number of rain days per month. Lobes correlated with climate were not associated with a particular type of growth fluctuation. RG of a lobe was positively correlated with the degree of bifurcation of the lobe tip. It is hypothesised that lobes ofP. conspersa exhibit a cyclic pattern of growth due in part to lobe division. The effects of climate, periods of zero growth, and microvariationsin the environment of a lobe are superimposed on this cyclic pattern resulting in the random growth of many lobes. Random growth fluctuations may contribute to the maintenance of thallus symmetry inP. conspersa.     

A REVISION OF THE GENUS GYMNODERMA

Gymnoderma, which has been considered a monotypic genus, is composed of three species. G. insulare is described from Japan and differs from G. coccocarpum of southeastern Asia in being smaller and having a thicker cortex with didymic acid present. G. lineare from the Southern Appalachians is transferred from Cladonia because the podetia lack symbiotic algae, are solid, and are located on the marginal lower side of the thallus. Gymnoderma, including three species and possibly representing an ancestral form of Cladonia, is disjunctively distributed in eastern Asia and the Southern Appalachians. G. coccocarpum is reported from Thailand and Borneo for the first time.