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Premise Hybridization is a key process in plant speciation. Despite its importance, there is no detailed study of hybridization rates in fern populations. A proper estimate of hybridization rates is needed to understand factors regulating hybridization. Methods We studied hybridization in the European Dryopteris carthusiana group, represented by one diploid and two tetraploid species and their hybrids. We sampled ~100 individuals per population in 40 mixed populations of the D. carthusiana group across Europe. All plants were identified by measuring genome size (DAPI staining) using flow cytometry. To determine the maternal parentage of hybrids, we sequenced the chloroplast region trnL-trnF of all taxa involved. Results We found hybrids in 85% of populations. Triploid D. ×ambroseae occurred in every population that included both parent species and is most abundant when the parent species are equally abundant. By contrast, tetraploid D. ×deweveri was rare (15 individuals total) and triploid D. ×sarvelae was absent. The parentage of hybrid taxa is asymmetric. Despite expectations from previous studies, tetraploid D. dilatata is the predominant male parent of its triploid hybrid. Conclusions This is a thorough investigation of hybridization rates in natural populations of ferns. Hybridization rates differ greatly even among closely related fern taxa. In contrast to angiosperms, our data suggest that hybridization rates are highest in balanced parent populations and support the notion that some ferns possess very weak barriers to hybridization. Our results from sequencing cpDNA challenge established notions about the correlation of ploidy level and mating tendencies.

Our goal was to infer the phylogenetic relationships and historical biogeography of the genusWe obtained sequence data for five chloroplast markers from 214 species ofWe found that 22 of 27 accessions of sub-Saharan AfricanThe majority of sub-Saharan African

• Premise of the study: The full impact of climate change on ecosystems and the humans that depend on them is uncertain. Anthropogenic climate change is resulting in winters with less snow than is historically typical. This deficit may have an impact on wintergreen ferns whose fronds lie prostrate under the snowpack and are thereby protected from frost.• Methods: Frost damage and ecophysiological traits were quantified for three species of wintergreen fern (Dryopteris intermedia, Dryopteris marginalis, and Polystichum acrostichoides) near Delhi, NY following the winters of 2012 (which had very little snowfall) and 2013 (which had typical snowfall).• Key results: Dryopteris intermedia was the most common species and had the highest percentage of frost-damaged fronds and the highest percentage of its cover damaged in 2012. Frost damage was significantly less in 2013 for all species. Polystichum acrostichoides had the highest vernal photosynthetic rate in undamaged fronds, and all three species had a negative net photosynthetic rate in frost-damaged fronds. The wintergreen fern community lost 36.69 ± 2.80% of its productive surface area to frost damage in 2012. Dryopteris intermedia had the thinnest leaves and this trait may have made it the most susceptible to frost damage.• Conclusions: These results demonstrate that repeated winters of little snow may have a significant impact on the structure and functioning of the wintergreen fern community, and species will respond to a reduced snowpack on an individual basis.

 Premise of the study: Dryopteris is a large, cosmopolitan fern genus ideal for addressing questions about diversifi cation, biogeography, hybridization, and polyploidy, which have historically been understudied in ferns. We constructed a highly resolved, well-supported phylogeny for New World Dryopteris and used it to investigate biogeographic patterns and divergence times.  Methods: We analyzed relationships among 97 species of Dryopteris , including taxa from all major biogeographic regions, with analyses based on 5699 aligned nucleotides from seven plastid loci. Phylogenetic analyses used maximum parsimony, maximum likelihood, and Bayesian inference. We conducted divergence time analyses using BEAST and biogeographic analyses using maximum parsimony, maximum likelihood, Bayesian, and S-DIVA approaches. We explored the monophyly of subgenera and sections in the most recent generic classifi cation and of geographic groups of taxa using Templeton tests.  Key results: The genus Dryopteris arose ca. 42 million years ago (Ma). Most of the Central and South American species form a well-supported clade which arose 32 Ma, but the remaining New World species are the result of multiple, independent dispersal and vicariance events involving Asia, Europe, and Africa over the last 15 Myr. We identifi ed six long-distance dispersal events and three vicariance events in the immediate ancestry of New World species; reconstructions for another four lineages were ambiguous.  Conclusions: New World Dryopteris are not monophyletic; vicariance has dominated the history of the North American species, while long-distance dispersal prevails in the Central and South American species, a pattern not previously seen in plants.

Debates on speciation processes in pteridophytes have revived. In order to study the evolutionary origin of an apomictic fern species, we investigated the genetic variation in the strictly agamosporous Dryopteris remota. We determined the genotypes of 22 individuals from many different locations within the species' European distribution and of 20 individuals from a Swiss population. A previous study on isozyme variation showed no intraspecific genetic variation in a similar sample set (Schneller and Holderegger, 1994, American Fern Journal 84: 94-98). In contrast to this, four out of 12 random amplified polymorphic DNA (RAPD) primers tested revealed low genetic diversity among individuals of D. remota from different locations. Intrapopulational genetic variation was also very low, but in the single population studied, a unique multiband genotype could be detected. The geographic distribution of genetic variation found in D. remota was best explained by the assumption of a single origin, the accumulation of somatic mutations during spread, and occasional, but effective, events of dispersal over large distances. The present study thus stresses the importance of long-distance dispersal in evolutionary processes and biogeography of ferns.

Studies on genetic diversity help us to unveil the evolutionary processes of species and populations and can explain several traits of diploid-polyploid complexes such as their distributions, their breeding systems, and the origin of polyploids. We examined the allozyme variation of Dryopteris aemula and D. oreades, diploid ferns with highly fragmented habitats, and the allotetraploid D. corleyi to (1) analyze the putative relationship between both diploids and the tetraploid, (2) compare the levels of genetic variation among species and determine their causes, and (3) assess the breeding system of these taxa. The allozymic pattern of D. corleyi confirms that it derived from D. aemula and D. oreades. The lack of genetic diversity in D. aemula, a species of lowland habitats, may be due to genetic drift associated with the contraction of populations in the last glaciation. By contrast, the alpine D. oreades had moderate intrapopulation genetic variation, which may derive from the expansion of populations during the last glaciation. In the latter species, low interpopulational variation suggested effective gene flow (spore exchange), and genotype frequencies in Hardy-Weinberg equilibrium indicated cross-fertilization of gametophytes. Evolutionary history appears to be an essential element in the interpretation of genetic variation of highly fragmented populations.

 Premise of the study: Discovering missing ancestors is essential to understanding the evolutionary history of biodiversity on Earth. Evidence from extinct species can provide links for reconstructing intricate patterns of reticulate relationships among extant descendents. When fossils are unavailable and other evidence yields competing hypotheses to explain species ancestry, data from proteins and DNA can help resolve confl icts and generate novel perspectives. The identity of a parent shared by two tetraploid species in the cosmopolitan fern genus Dryopteris has remained elusive for more than 50 years. Based on available data, four hypotheses were developed previously, each providing a different resolution to this uncertainty.  Methods: New molecular evidence from studies of isozymes and restriction site analysis of chloroplast DNA tested the competing hypotheses about the diploid ancestors of these two extant Dryopteris polyploids.  Key results: The results falsify two of the hypotheses, resolve the uncertainty in the third, and support the fourth.  Conclusions: Our data validate the prior existence of Dryopteris “ semicristata, ” which was proposed 38 years ago as a diploid progenitor of the allotetraploids D. cristata and D. carthusiana but has never been collected. After developing a phylogeny using the new molecular data, we describe a plausible morphology for D. “ semicristata ” by extrapolating likely character states from related extant species.

Levels and distribution of genetic variation were analyzed in 15 populations of the homosporous fern Gymnocarpium dryopteris ssp. disjunctum to evaluate the mating system and the genetic structure of this taxon. Estimated rates of intragametophytic selfing were 0.000 in all populations, signifying that all sporophytes examined arose via intergametophytic matings. Furthermore, values of F, the fixation index, ranged from -0.171 to 0.038, indicating that all populations approach Hardy-Weinberg equilibrium. Therefore, not only is intragametophytic selfing rare in G. dryopteris ssp. disjunctum, but intergametophytic selfing is also uncommon. Factors promoting outcrossing in G. dryopteris ssp. disjunctum include an antheridiogen system, an ontogenetic sequence that favors intergametophytic matings, and high levels of genetic load. Analysis of F-statistics indicated statistically significant genetic differentiation among populations despite a mean genetic identity of 0.973 and high levels of interpopulational gene flow (Nm = 3.41 to 4.09). These patterns may reflect levels of gene flow that prevent significant interpopulational divergence while permitting slight genetic differentiation among populations.

The methods described here make it possible to use data on sporophytic genotype frequencies to estimate the frequency of gametophytic self-fertilization in populations of homosporous plants. Bootstrap bias reduction is effective in reducing or eliminating the bias of the maximum likelihood estimate of the gametophytic selfing rate. The bias-corrected percentile method provides the most reliable confidence intervals for allele frequencies. The percentile method gives the most reliable confidence intervals for the gametophytic selfing rate when selfing is common. The maximum likelihood intervals, the percentile intervals, the bias-corrected percentile intervals, and the bootstrap t intervals are all overly conservative in their construction ofconfidence intervals for the gametophytic selfing rate when self-fertilization is rare. Application of the recommended methods indicates that gametophytic self-fertilization is quite rare in two sexually reproducing populations of Pellaea andromedifolia studied by Gastony and Gottlieb (1985). DATA ON THE GENETIC structure of populations of vascular cryptogams is gradually beginning to accumulate. Levin and Crepet (1973) published electrophoretic data on populations of Lycopodium lucidulum, although they did not determine the genetic basis of the banding patterns observed. Haufler and Soltis (1984) used electrophoretic data from natural populations of Bommeria hispida to confirm the prediction from laboratory experiments that it is an obligate outcrosser. Gastony and Gottlieb (1985) reported the results of an electrophoretic survey of five polymorphic allozyme loci in nine sexually reproducing populations of the facultatively apogamous fern Pellaea andromedifolia. McCauley, Whittier, and Reilly (1985) recently provided estimates of the gametophytic selfing rate in Botrychium dissectum. Soltis and Soltis (1986 and personal communication) have conducted large population surveys of species in the fern genera Botrychiurn, Polystichum, and Dryopteris. Part of the interest of these studies is that 'Received for publication 15 April 1986; revision accepted 5 December 1986. 2 Current address is Department of Ecology and Evolutionary Biology, U-43, University of Connecticut, 75 North Eagleville Road, Storrs, CT 06268. I am indebted to Gerald Gastony for piquing my interest in this topic and encouraging me to pursue it. Marc Feldman, Doug Soltis, and Pam Soltis provided helpful comments on an earlier draft of this paper. A listing (in Pascal) of a program that performs the procedures described here is available upon request. A compiled version is available for IBM PCs and PC-compatibles. This research was supported in part by NIH grants GM28016 and GM 10452. they allow us to determine how frequently selffertilization of gametophytes occurs in natural populations of homosporous plants. In particular, it may be possible to evaluate one aspect of the hypothesis that gametophytes of homosporous ferns frequently self-fertilize in nature and that they compensate for the consequent complete homozygosity by storing and releasing heterozygosity via duplicated loci on homoeologous polyploid chromosome sets (Klekowski and Baker, 1966; Klekowski, 1973; Chapman, Klekowski, and Selander, 1979). [Genetic evidence already indicates that many presumed polyploids among the homosporous ferns are genetically diploid (Haufler and Soltis, 1986).] Since gametophytic self-fertilization will result in complete homozygosity at all loci, Gastony and Gottlieb (1985) were able to demonstrate that a minimum of 81.3% of the sporophytes they examined were the result of outcrossing events. This estimate is clearly a lower bound, however, since outcrossed matings could occur between different individuals that have identical genotypes at the loci they sampled. A variety of statistical techniques for estimating rates of self-fertilization has been developed in seed plants. (See Clegg, 1980, or Ritland, 1983, for recent reviews and references.) These techniques are all based on the analysis of progeny arrays, i.e., the analysis of genotype frequencies within a set of halfor full-sib progeny arrays drawn from different mothers. Ferns and other vascular cryptogams are not susceptible to such an analytical tech

A B S T R A C T Analyses of chromosome pairing behavior in fern hybrids, as evidenced by the degree of bivalent and/or univalent formation at meiotic prophase, have frequently been employed in studies of evolutionary relationships within polyploid fern complexes. Pairing is often seen to involve genomic numbers of chromosomes. Many examples exist, however, that indicate pairing on subgenomic levels. If this type of behavior is not recognized in analyses of F1 hybrids and their polyploid derivatives, interpretations of evolutionary relationships based upon pairing behavior may be misleading. Contrary to some views, sterile triploid hybrids possessing less than three distinct genomes may play a significant role in the formation of reticulate polyploid complexes. This possibility must be considered in interpretations of these complexes. With these factors in mind, the Appalachian Dryopreris complex has been reinterpreted. The reinterpretation provides explanations for several unexplained inconsistencies in previous interpretations.

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 intemodes 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. THE STRUCTURE AND FUNCTION of lateral buds in seed plants have been extensively investigated, and literature on the origin and development of buds and the physiology of bud expansion has been amply summarized by Esau (1965) and Phillips (1969). However, our knowledge of bud morphogenesis in lower plant groups is far more rudimentary. The ferns, in particular, exhibit a marked diversity in bud insertion and assumed origin. For instance, examples of dichotomous or pseudodichotomous (Dryopteris), extra-axillary (Polypodium), axillary (Trichomanes), and phyllogenous (Dennstaedtia) branching have been described. Because of this variation, and especially because the axillary type which predominates in seed plants also occurs in this group, an opportunity is available for morphological comparisons to be made between the modes of fern branch origin and their ontogenetic patterns of development with the inception and early growth of lateral buds in seed plants. Accounts of branching by Bower (1923), Goebel (1928), and Troll (1937) emphasize the high degree of variability found in the ferns. Although each author presents a scheme of bud origin and provides a generic example to illustrate each branching type, the modes of branch origin typi1 Received for publication 15 January 1975. The author wishes to thank Dr. Donald Kaplan for his advice throughout the course of this study and for his critical evaluation of the manuscript. I also thank Dr. Ernest Gifford, Dr. Russell Jones, and Dr. Rudolf Schmid for their reading of the manuscript and helpful suggestions and Ms. Charlotte Mentges for her assistance in preparation of the illustrations. cally were deduced from the mature disposition of buds rather than from detailed developmental investigation. While it is clear from macroscopic observation of shoots of different fern species that buds can occupy a variety of positions, the critical evidence for distinct and different modes of bud inception, as these authors suggest, is wanting. An especially interesting and problematic type of branch insertion is seen in the epiphytic fern Davallia. The dorsiventral creeping shoots bear highly divided deltoid leaves in two ranks on the dorsal surface of the main axis with buds appearing near, but slightly ahead of, each leaf base. Although this positioning suggests axillary origin, Bower (1923) suggested three alternative interpretations to explain their unique position of insertion: (1) the bud arises acrogenously (from the apical meristem) and the associated leaf is its first appendage, (2) the bud arises phyllogenously (from leaf tissue) and is secondarily displaced to an axillary position, or (3) both buds and leaves are cauline in origin and their ultimate disposition is due to the rhythmic inception of these primordia and the correlative longitudinal growth of the axis. The present investigation of the anatomical features of leaf and bud inception was initiated to discriminate the actual mode of origin from these possible explanations. The origin of buds in the ferns is of fundamental morphological importance, but of equal interest is the continued growth and expansion of these buds which ultimately will reflect the branching pattern and overall form of the plant. Early bud morphogenesis previously has been characterized in the

WALKER, S. (Liverpool U., England.) Cytogenetic studies in the Dryopteris spinulosa complex. I. Amer. Jour. Bot. 48(7): 607-614. Illus. 1961.-A comparison of the D. spinulosa complex, as represented in North America, is made with that in Europe. Although in North America, other Dryopteris species are known to hybridize with members of the complex, the report has been limited to species which compare directly with those in Europe. Three tetraploid species and 2 diploids, the latter representing at least 1 ancestral genome in the complex, are discussed together with the interspecific hybrids. Synthetic hybrids have been produced, involving taxa from both sides of the Atlantic. Evidence from these hybrids suggests that D. intermedia, of North America, and D. maderensis, of Madeira, are conspecific. CYTOGENETICAL investigation of the genus Dryopteris has revealed a reticulate pattern of evolution. The results of studies in the Dryopteris spinulosa complex in Europe were published in an earlier paper (Walker, 1955). Similar work on this complex, as represented in North America, has led to a much wider investigation involving many other associated Dryopteris species and hybrids (briefly reported, Walker, 1959). It has become impossible to present all the data obtained in a single paper and, to facilitate a direct comparison with the earlier work, the present report covers only those representative species and hybrids found in North America which are comparable with the members of the complex in Europe. The relationships between the various taxa on both sides of the Atlantic have been investigated by the synthesis of a number of hybrids between them. Details of the wider investigation into the genus Dryopteris in North America will be published separately. The D. spinulosa complex in Europe includes 3 allotetraploid species, D. cristata (L.) Gray, D. spinulosa (O. F. Muell.) Watt and D. dilatata (Hoffm.) Gray, which require 4 ancestral diploids to account for their origin. One of these ancestral diploids, the genome of which is common to both D. spinilosa and D. dilatata, is now represented by 2 distinct species (previously regarded as diploid forms of "D. dilatata"); one, D. assimilis S. Walker,2 is found in Britain and on the continent of Europe, the other, D. maderensis Alston,3 on the island of Madeira. These species are very different in their pinnation and spore size, but cytogenetic evidence indicates they are descendants from the same ancestral diploid (Walker, 1955). From crosses between D. assimilis and D. maderensis, 2 1 Received for publication January 24, 1961. 2 Dryopteris assimilis, sp. nov., S. Walker. Based on Lastrea dilatata var. alpina. Moore, Handbook Brit. Ferns, 3rd ed., 126 (1857). Dryopteris dilatata var. alpina Moore, Brit. Ferns, I, 225 (1859). 3 Dryopteris maderensis Alston, published erroneously as D. maderensis (Milde) Alston (Alston, 1956), but Milde used 'forma maderensis' as a descriptive phase, not as a taxonomic forma (Milde, 1867). plants were produced showing normal meiotic pairing but approx. 40% abortive spores which suggests gene imbalance (Walker, unpublished). More hybrids are necessary to establish this. Wild hybrids occur within the complex both at the tetraploid and triploid levels. Chromosome counts for some members of the complex in North America have been reported previously (Britton, 1953; Manton and Walker, 1953), and detailed spore studies have been carried out (Crane, 1953). More material has been sampled by me and additional counts made. This complex has caused confusion to taxonomists; hence R. C. Benedict, with a long-standing interest (1924) in the spinulose dryopterids, referred to them (1954) as "one of the most disputed of our native fern groups." Separation of the various taxa into distinct species or varieties still remains a problem to most authors; this is particularly exemplified in the new Britton and Brown Flora (Gleason, 1952) and Gray's Manual of Botany, 8th ed. (Fernald, 1950). The presentation of the latter was criticized by Wherry (1952). Nomenclature for the various taxa is at present under consideration. The use of the species epithet, C"austriaca,"' in place of "dilatata,." as in the Flora of the British Isles (Clapham, Tutin and Warburg, 1952), and in the new Britton and Brown Flora (Gleason, 1952) has already been criticized (Walker, 1955; cf. Morton, 1958). The name. D. spinulosa (0. F. Muell.) Watt, is also invalid but is retained in this paper since, so far, no satisfactory replacement has been published. Neither D. lanceolata-cristata (Hoffm.) Alston (Alston, 1957) nor D. carthusiana (Villars) H. P. Fuchs (Fuchs, 1958) appear to be satisfactory substitutes. The following is a list of the taxa discussed in this paper: D. cristata (L.) Gray; D. spinulosa (0. F. Muell.) Watt; D. campyloptera (Kunze) Clarkson; (syn. D. spinulosa var. americana [Fisch.] Fernald),; D. intermedia (Muhl.) Gray (syn. D. spinulosa var. intermedia [Muhl.] Underw.); "D. dilatata" (Hoffm.) Gray; D. X uliginosa (A.

1926. Meristeme. Linsbauer's Handb. Pflanzenanatomie. IV. Berlin. SCHWARZ, W. 1927. Die Entwicklung des Blattes bei Plectranthus fruticosus und Ligustrum vulgare und die Theorie der Periklinalchimaren. Planta 3: 499-526. SIFTON, H. B. 1944. Developmental morphology of vascular plants. The New Phytol. 43: 87-129. SNOW, MARY, AND R. SNOW. 1947. On the determination of leaves. The New Phytol. 46: 5-19. SONTAG, P. 1887. Ueber Dauer des Scheitelwachsthums und Entwicklungsgeschichte des Blattes. Jahr. Wiss. Bot. 18: 236-262. SPROTTE, K. 1940. Untersuchungen fiber Wachstum und Nervatur der Fruchtblatter. Bot. Arch. 40: 463-506. THIELKE, CHARLOTTE. 1948a. Beitrage zur Entwicklungsgeschichte und zur Physiologie panaschierter Blatter. Planta 36: 2-33. 1948b. Beitrage zur Entwicklungsgeschichte unifazialer Blatter. Planta 36: 154-177. TROLL, W. 1939. Vergleichende morphologie der h6heren Pflanzen. Bd. I T. II. Berlin. WARDLAW, C. W. 1947. Tensile stress as a factor in fern phyllotaxis. Nature 160: 678. . 1948a. Experimental and analytical studies of pteridophytes. XIII. On the shoot apex in a tree fern, Cyatheca Manniana Hooker. Ann. Bot. N. S. 12: 371384. . 1948b. Experimental and analytical studies of pteridophytes. XI. Preliminary observation on tensile stress as a factor in fern phyllotaxis. Ann. Bot. N. S. 11: 97. . 1949. Experimental and analytical studies of Pteridophytes. XIV. Leaf formation and phyllotaxis in Dryopteris aristata. Druce. Ann. Bot. N. S. 13: 163-198. WEIDT, E. 1935. Die Entwicklung der Blatter der Melastomataceen Heterotrichum macrodon Planch. und Clidemia hirta Don. Beitr. Biol. Pflanz. 23: 252-281.

A B S T R A C T Two types of pteridophylls are very common in the Mesozoic era: the Polypodium and the Dipteridaceae. The Polypodium-type formed by pendulous overtopping is represented in the Mesozoic era by the form-genus Cladophlebis, in the Paleozoic era by the form-genus Pecopteris, and today by Dryopteris. Overtopping differs in these three pteridophylls. Nearly all Paleozoic pteridophylls show a catadromic structure. Today pteridophylls with anadromic and combined catadromic/anadromic structures are the dominant ones. The Mesozoic genera demonstrate a transitional structure: catadromic structures are still frequent, but equally frequent are combined catadromic/anadromic structures. Pteridophylls solely with anadromic structures are unknown in the Mesozoic era. The Dipteridaceae-type is almost exclusively Mesozoic. It is primitive in the dichotomy of the rachis, but advanced in that it is net-veined. Thus the Mesozoic pteridophylls represent connecting links between Paleozoic and recent leaf-types. This nature of Mesozoic pteridophylls can be understood only on the basis of the telome theory. THE MESOZOIC PTERIDOPHYLLS represent morphologically and in time sequence a link between the Paleozoic pteridophylls and those of today. In the Mesozoic they play a role similar to that of the earlier Paleozoic pteridophylls (Zimmermann, 1966). These Paleozoic pteridophy]ls are a link between the early Devonian land plants (Rhyniales) which are still thalloid and the more recent pteridophylls. Thus during the last three billion (109) years a continuous semiphyletic line of plants leads to the development of recent ferns

'WALKER, S. (U. Liverpool, England.) The problem of Dryopteris leedsii. Amer. Jour. Bot. 49(9): 971-974. Illus. 1962.-Investigation of 12 plants of 'Dryopteris leedsii' from the type locality in Harford County, Maryland, has revealed 3 cytological entities, diploid, triploid and t*etraploid. The diploid and triploid types exhibit hybrid characters and are sterile, while the tetraploid is a fertile species. The relationship between the 3 entities remains a problem, but the possibilities are discussed.

A SURVEY of botanical literature shows that the origin and the early stages in the development of axillary buds have been investigated a number of times, yet the later phases of development have, in contrast, received comparatively little attention. The works of Koch (1893), Schmidt (1924), and Louis (1935) are especially noteworthy for their detailed accounts of the initial stages in bud formation. Recently several other papers (Majumdar on Heracleum, 1942; Majumdar and Datta on Heracleum and Leonurus, 1946; Reeve on Garrya, 1943; Wardlaw on Matteuccia, Onoclea, and Dryopteris, 1943a, 1943b, 1946; Sterling on Sequoia, 1945a, 1945b, and on Pseudotsuga, 1947; Miller and Wetmore on Phlox, 1946) have further added to our knowledge on this subject, though on widely separated groups of plants. With few references to the later stages of development, especially vasculation, it has not been possible to make generalizations or to formulate a clear concept of bud ontogeny. The differentiation of xylem and phloem in axillary buds is considered in only one paper (Miller and Wetmore, 1946). Further, there has existed a marked divergence of opinion on some of the basic concepts of bud formation; the differences concern such fundamental problems as the origin of bud primordia, the origin of bud traces, and the differentiation of the procambium. With incomplete information on certain developmental stages on the one hand, and with contrasting descriptions and interpretations on the other hand, it seemed desirable that a comprehensive study of the main phases of bud development be made. In this first paper of a series which is concerned with axillary buds of plants with different phyllotactic patterns, different nodal connections, and different bud arrangements, attention is centered upon the origin, the development of general form, and the vasculation of axillary buds of Syringa vulgaris. Consideration is given to the timing and the orderly succession of the progressive developmental stages and to the nature of the vascular strands connecting axillary buds with the vascular tissue of the main axis. MATERIAL AND METHODS.-Syringa vulgaris (fig. 1) was selected as a species for this first study because the plant has opposite leaves, a condition favorable for sectioning and examination of nodal structure; and because the leaf traces are unilacunar, the least complex type of vascular connection. Collections were made every 3 or 4 days or weekly during March and April, 1946, at the time when the buds were opening and during the early period of shoot elongation. Additional material was 1Received for publication September 8, 1948. The writer wishes to express her sincere appreciation and gratitude to Professor Ralph H. Wetmore for his kindly interest, his guidance, and constructive criticism throughout the course of this investigation. obtained at intervals throughout the late spring, summer, and fall months of 1945 and during the spring and summer of 1946. Buds and young twigs were fixed in Craf solution, dehydrated in a series of n-butyl alcohol (Zirkle, 1930), and embedded in rubberized paraffin. Serial sections, transverse and longitudinal, were cut 8 and 10 , thick. Two staining procedures were used: Heidenhain's iron-alum haematoxylin with safranin as the counterstain (Jeffrey, 1917), ......~~~~~~.. ........ . . , J......