APOGAMY AND SOMATIC RESTITUTION IN THE FERN CERATOPTERIS

A mutant stock of the fern Ceratopteris has been derived from an inbreeding study following an interspecific hybridization between two diploid species. The mutant is characterized by gametophytes that produce non-functional spermatozoids and are incapable of selfing. Sporophytes develop apogamously from the mutant gametophytes and, although they are initially haploid and sterile, portions of the fronds later become doubled somatically and behave like tissues of sexually derived homozygous sporophytes. The mutant segregates from sporophytes in a 1:1 ratio when crosses are made with wild type gametophytes. Certain aspects of the behavior are similar to those seen in some naturally occurring apomictic ferns.     

NONHOMOLOGOUS CHROMOSOME PAIRING IN THE FERN CERATOPTERIS

The homosporous fern Ceratopteris thalictroides forms completely homozygous offspring sporophytes in one generation of selfing. Such homozygotes do not breed true and can exhibit structural heterozygosity for chromosomal mutations in some meiocytes. This is due to occasional nonhomologous (presumably homoeologous) chromosome pairing at meiosis in these polyploid ferns.     

HOMOEOLOGOUS CHROMOSOME PAIRING AND RESTRICTED SEGREGATION IN THE FERN CERATOPTERIS

The segregation of a marker characterized by pale green gametophytes was monitored within an inbreeding study of the polyploid fern Ceratopteris. Although all of the sporophytes showing segregation were derived from the self-fertilization of haploid gametophytes, a low overall frequency of 2.5% pale gametophytes was observed in the F₃–F₅ generations. A model based upon a duplicated locus and homoeologous chromosome pairing can explain the segregational behavior within the study. The overall level of homoeologous pairing was determined to be 10%. Occasionally, green gametophytes that were presumably heterozygous for the marker contained pale sectors. This behavior may involve mitotic crossing-over between homoeologous chromosomes.     

FERN GAMETOPHYTE DEVELOPMENT: CONTROLS OF DIMORPHISM IN CERATOPTERIS THALICTROIDES

Multispore cultures of Ceratopteris thalictroides (L.) Brongn. gametophytes contain plants of two distinct morphologies, hermaphrodite and male (Fig. 3). The male gametophyte has previously been shown to be induced by an antheridogen system. In the absence of the antheridogen all gametophytes become hermaphroditic. It is shown that the hermaphroditic gametophytes are first to develop in multispore cultures and are also the only source of antheridogen. Thus, since males are only produced in the presence of antheridogen, an explanation of the occurrence of two morphologies is evident. That is, the first gametophytes to develop do so in the absence of antheridogen, become hermaphrodites and produce antheridogen to which later developing gametophytes respond by becoming males. This does not explain why there is a range in development times for the gametophytes. Three possible controlling factors of development are investigated in multispore cultures: spore size, time of germination, and gametophyte growth. Data collected on these three factors are statistically analyzed. Analyses were done to determine the relative importance (singly or in combination) of these factors in predicting the developmental potential of a gametophyte. It was found that spore size most accurately indicated (ca 75% of the time) future gametophyte development.     

ON THE CYTOLOGY OF ANTHERIDIUM FORMATION IN THE FERN SPECIES ONOCLEA SENSIBILIS. I. CYTOLOGY OF CELL PLATE AND CELL WALL FORMATION

In the four-celled antheridium of the fern species Onoclea sensibilis a central spermatogenous cell is enveloped by a jacket of three cells. Starting from the base, the jacket comprises the cup-shaped basal cell, the ring cell—both of which encircle the spermatogenous cell—and the cap cell. The lower wall of the spermatogenous cell has the configuration of a funnel; its upper wall is dome shaped. The choice of whole antheridia for study instead of sectioned ones has, for the first time, made it possible to study the formation of the uniquely shaped antheridial cell plates step by step. The cell plate antecedent of the funnel wall has the configuration of a funnel. This conclusion conflicts with Davie's contention that this cell wall is oriented transversely at first and acquires funnel-shape secondarily. The present studies further show that the funnel cell plate forms from base to rim. This finding contrasts with a report that in another fern species this cell plate begins to form on one side of the initial and then proceeds circularly around it. The base of the funnel cell plate attaches to the basal wall of the antheridium initial in a separate event. The genesis of the dome-shaped upper wall of the spermatogenous cell is described for the first time.     

A "CELL-CONTACT TEMPERATURE-SENSITIVE" MUTANT OF THE CELLULAR SLIME MOLD DICTYOSTELIUM MUCOROIDES

A mutant (TS-2) that is temperature-sensitive with respect to cell contact was isolated from the cellular slime mold, Dictyostelium mucoroides. The TS-2 were able to grow and develop normally at 21°C, but unable to grow at 31.5°C. When TS-2 were allowed to develop until the aggregation stage at 21°C and then shifted to 31.5°C, they instantly lost cell-to-cell contact, resulting in disintegration of the aggregation stream and flattening of the aggregation center. Although a slug transferred to 31.5°C retained its original shape, loss of cell-to-cell contact within the cell mass was evidenced by several facts. The TS-2 interphase amebas, at 31.5°C, also lost cell-to-substratum contact, and the loss of contact was followed by the production of cell-wall substance on their surface. The production of the same substance at 31.5°C was also observed in cells at aggregation and migration stages, but not in those at the vegetative stage. When TS-2 cells at various developmental stages were kept at 31.5°C for various periods of time and returned to 21°C they lost morphogenetic capacity in proportion to the production of the cell-wall substance.     

蕨类植物细胞学概说

<正> 现代蕨类植物细胞学是1950年Irene Manton教授创始的,在她的创造性著作《蕨类植物的细胞学与进化问题》(Problems of Cytology and Evolution in the Pteridophyta.1950)中,提出了主要是产于欧洲的27属和90种蕨类植物的细胞学证据。这本经典著作是相当大的、相当综合的和相当细致的,它激发了这一研究领域中的高潮。现在这     

CYTOLOGY AND MORPHOGENESIS IN THE RHIZOMORPH OF ARMILLARIA MELLEA

Aided by the techniques of thin sectioning and electron microscopy, the apical region of the rhizomorph of Armillaria mellea has been examined. This region is composed of concentric zones of morphologically distinct tissues derived from a subapical meristematic zone designated the apical center. Meristematic activity is of two types: (1) primary, localized in the apical center, in which new hyphal elements are formed from apical initials, and (2) secondary, localized in the lateral regions of the apex, in which elaboration of the hyphal elements by means of elongation and secondary crosswall formation takes place. From these meristematic zones the tissues of the mature rhizomorph are derived and include: (a) peripheral hyphae, (b) cortex, (c) subcortex, and (d) primary and secondary medulla. The manner of differentiation of an apical initial appears unique and involves synchronous nuclear divisions accompanied by segmentation in many planes. The result of this activity is the formation of multinucleate hyphae. Apical initials are usually highly cytoplasmic and possess peculiar non-membrane-bound fibrous bundles, but in all other respects they resemble the hyphae of most Basidiomycetes thus far examined with the electron microscope.     

INCOMPATIBILITY OF MERISTEMATIC AND FILAMENTOUS GROWTH IN THE FERN GAMETOPHYTE

Gametophytes of Pteridium aquilinum can be maintained in red light as either 1- or 2-dimensional structures. The mode of growth realized in red light is dependent upon the activity of the meristem. An active meristem in a 2-dimensional structure will permit a continued development of that structure. A breakdown in meristematic activity results in filament formation. It is suggested that a group of actively dividing cells in some manner inhibits cell elongation and thus prevents filament formation in red light.     

CHROMOSOME COUNTS IN CHEILANTHES AND ASPIDOTIS WITH A CONSPECTUS OF THE CYTOLOGY OF THE SINOPTERIDACEAE

Confirmatory, corrective and new chromosome counts are listed for species in the genus Cheilanthes, and a new chromosome count is given for a member of the genus Aspidotis. An analysis of five collections of C. castanea revealed no significant morphological differences despite the different chromosome numbers. The ploidy level of all known species in genera of the Sinopteridaceae is summarized, revealing directions in which future research might proceed to complete our cytological knowledge of this family.     

INBREEDING AND THE RATE OF SELF-FERTILIZATION IN A GRAPE FERN,BOTRYCHIUM DISSECTUM

Botrychium dissectum is a homosporous fern with bisexual, subterranean gametophytes. Because of these features, B. dissectum would be suspected of displaying a very high frequency of self-fertilization. Sporophytes collected from three populations of this species were assayed for heterozygosity by determining the electrophoretic mobility patterns displayed by two polymorphic enzymes. Extreme deviations from Hardy-Weinberg expectations were observed in each population and analyzed by means of F-statistics. The average inbreeding coefficient was found to be 0.951. A population genetic model is derived that demonstrates that the rate of intragametophytic self-fertilization in homosporous ferns is equal to the inbreeding coefficient calculated from deviations from Hardy-Weinberg expectations. It is therefore concluded that B. dissectum outcrosses about 5% of the time.     

MORPHOGENESIS OF THE STAMENLESS-2 MUTANT IN TOMATO. I. COMPARATIVE DESCRIPTION OF THE FLOWERS AND ONTOGENY OF STAMENS IN THE NORMAL AND MUTANT PLANTS

The single gene recessive mutant stamenless-2 (sl₂/sl₂) differs phenotypically from the normal (+/+) only in the stamen structure. Stamens of the mutant plants were laterally free, twisted, shorter, paler in color, possessed abnormal pollen, and bore naked external ovules (E.O.) on the adaxial surface near the junction of anther and filament. Mutant plants grown in the field during summer produced flowers in which a number of carpel-like organs (‘carpelloid stamens‘) with few or no E.O. replaced the stamens. On the other hand, plants grown in the greenhouse during winter possessed flowers with greater number of yellow and pubescent stamens and many E.O. Study of stamen ontogeny revealed that at initiation (up to 100 μ in length) stamen primordia of normal and mutant plants resembled each other. Thereafter the development of stamens in the two genotypes could be distinguished.     

ORIGIN AND EARLY MORPHOGENESIS OF LATERAL BUDS IN THE FERN DAVALLIA

The general organography, vascular organization, and leaf and bud development in Davallia solida and D. trichomanoides are described. These epiphytic species have creeping shoots with dorsally-borne leaves in a distichous phyllotaxis and the buds occur near each leaf base. Roots are borne on the ventral and flanking surfaces of the rhizome, but only at bud positions. The vascular pattern of these species is a perforated solenostele. Leaf and bud traces have distinctly different origins. While the proximity of buds to leaves has suggested that bud origin is axillary, observations show that the origin of buds is cauline and that their position is extra-axillary from inception. The stages of structural morphogenesis in Davallia buds differ significantly from the scheme proposed by Wardlaw. The principal difference is the absence of a resting period occurring between the origin and continued development of buds in Davallia. The elongated internodes which separate leaf-bud pairs from one another, the topographically distinct and predictable positions of leaves and buds, the structural equivalence of unexpanded buds, and vascular differences in leaves and buds make Davallia an useful species for physiological studies of differential bud expansion.