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PREMISE OF THE STUDY The female gametophyte of Welwitschia has long been viewed as highly divergent from other members of the Gnetales and, indeed, all other seed plants. However, the formation of female gametes and the process of fertilization have never been observed. METHODS Standard histological techniques were applied to study gametophyte development and the fertilization process in Welwitschia. KEY RESULTS In Welwitschia, fertilization events occur when pollen tubes with binucleate sperm cells grow down through the nucellus and encounter prothallial tubes, free nuclear tubular extensions of the micropylar end of the female gametophyte that grow up through the nucellus. Entry of a binucleate sperm cell into a vacuolate prothallial tube appears to stimulate the rapid coagulation of cytoplasm around a single female nucleus, which differentiates into an egg cell. One sperm nucleus enters the female gamete, while the second sperm nucleus remains outside and ultimately degenerates. Only a single fertilization event occurs per mating pair of pollen tube and prothallial tube. CONCLUSIONS Welwitschia lacks the gnetalean pattern of regular double fertilization, as found in Ephedra and Gnetum, involving sperm from a single pollen tube to yield two zygotes. Moreover, an analysis of character evolution indicates that the female gametophyte of Welwitschia is highly apomorphic both among seed plants, and specifically within Gnetales, but also shares several key synapomorphies with its sister taxon Gnetum. Finally, the biological role of prothallial tubes in Welwitschia is examined from the perspectives of gamete competition and kin conflict.

Summary Life histories of photosynthetic eukaryotes traditionally-termed algae exhibit a considerably greater degree of complexity than those of ‘higher cryptogams.’ Some algae have a so-called ‘obligate’alternation between spore-producing and gamete-producing phases, but the majority seem capable of following other pathways depending upon environmental conditions. In only four algal classes do life histories show a change in morphological and/or nuclear phases. The following basic life histories are recognized in the Chlorophyceae, Phaeophyceae and Rhodophyceae:(a) monophasic, a diploid or haploid phase, (b) two or more phases, most commonly an alternation of an isomorphic or heteromorphic haploid gametangial phase and a diploid sporangial phase, and (c) three phases (unique to florideophyte Rhodophyceae), with a diploid spore-producing phase (carposporophyte) developing on the gametangial phase, a diploid phase (tetrasporophyte if meiosis is sporic) and a haploid gametangial phase. Evidence from recent research indicates that in many algae there is an uncoupling of the morphological and nuclear phases. The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis. Free-living morphs in heteromorphic life histories may be morphologically so dissimilar that formerly they were attributed to different genera. Evolution of the carposporangial phase in red algae is speculated to be a means of achieving zygotic amplification to compensate for the infrequency of syngamy. Such amplification allows the production of a large number of dispersible products from a single fertilization. The direct development of a free-living tetrasporangial phase is considered another mechanism for achieving amplification. In freshwater red algae the growth of an upright phase from a perennial microscopic one is considered an adaptation for maintaining their upstream position. Life history pathways in algae are controlled by subtle environmental influences (e.g. photoperiodism, temperature, light quality, nutrients). Experimental evidence is lacking to support the contention that spatial and/or temporal partitioning of the environment is a mechanism favouring the maintenance of heteromorphy. Herbivory is known to be an important selective force suppressing some morphs and accentuating the seasonal dominance of others. Differential resistance of morphs to herbivory in environments where grazing intensity is predictable may lead to the selective maintenance of heteromorphy. Algal life history patterns are unexplored in terms of evolutionary processes. Various models for the evolution of biphasic or polyphasic life histories stress the importance of the capacity for both asexual dispersal of successful genotypes and for the generation of new genotypes via meiosis and syngamy. All evidence points to the fact that many life history processes operative in algae differ significantly from those described for other cryptogams.

The South African clawed frog, Xenopus laevis, is used extensively as a model system to study development. It is easy to maintain a breeding colony of frogs, females can be induced to lay eggs all year round and hundreds of embryos can be generated that grow synchronously from a single fertilization. Embryos are easily amenable to microsurgery at any stage of development and adult frogs survive surgeries performed under limited aseptic conditions. The ease of injecting oligonucleotides or messenger ribonucleic acid (mRNA) into Xenopus leavis embryos has contributed to the discovery of genes with key functions in development. Another singularity of Xenopus leavis is the ease to manipulate its oocytes before fertilization; this has been a powerful tool to identify the function of maternally deposited mRNAs in development. Transgenic procedures are being developed using the related species Xenopus tropicalis and this will broaden the contribution of Xenopus to the understanding of early development. Key concepts: Xenopus laevis is easy to raise and feed in large colonies in the laboratory. The female Xenopus can be induced to spawn repeatedly at any season, making it possible to obtain embryos all year round. Xenopus leavis eggs are quite large, and are usually laid in large numbers that are fertilized in vitro. Xenopus leavis oocytes can easily be obtained and manipulated before fertilization, making it possible to study the function of maternally deposited mRNAs. The Xenopus embryos develop outside the female and are amenable to manipulation at any stage of development. The mechanism of tissue formation has been elucidated from research carried out on Xenopus leavis embryos. Xenopus leavis is an excellent model for studying body axis formation. The early development of Xenopus is very fast, making it possible to study a full developmental programme in about three days. The sequencing of the diploid genome, the ease of obtaining and injecting large numbers of eggs and the relatively short lifespan makes Xenopus tropicalis a very promising model for transgenic studies of organogenesis. Eggs of Xenopus leavis provide a model of choice for studying the regulation of the cell cycle. Keywords: frog; embryology; tissue formation; gene function; cell cycle

EXTERNALLY NORMAL FRUITS which are empty or contain only shriveled seeds occur frequently in the cultivated peanut, Arachis hypogaea L. These seed failures are caused by an unfavorable environment (calcium deficiency) or by an unfavorable genetic constitution (interspecific hybridization). An explanation in developmental terms of the observed sterility in this geocarpic plant is of both agronomic and botanic interest. To provide a factual basis for such an interpretation it first was necessary to examine the reproductive mechanism of the species for possible irregularity and to determine whether empty fruits could arise by parthenocarpy. This paper presents the result, a description of the life history of Arachis hypogaea from ovule initiation to syngamy. MATERIAL AND METHODS.-Flower buds and very young fruits were collected from plants of the Virginia and Spanish varieties, grown under field conditions at Raleigh, N. C. There were no important differences between the varieties in the phases of development considered. Fertilization stages were obtained by tagging individual flowers at anthesis and collecting at 2-hr. intervals during the succeeding 40 hr. Karpechenko's fixative was used with an aspirator; sections were cut at 10-15 g and stained in Heidenhain's haematoxylin or in Delafield's and safranin. Best results followed bleaching in one per cent potassium permanganate (Johansen, 1940). Plasmolysis was troublesome only in mature embryo sacs. OBSERVATIONS.-The ovule. Early stages of carpel and ovule development (fig. 1-3) occur in buds 1-3-mm. long in which the anthers already have a well differentiated archesporium. The apical ovule arises first (fig. 2) from the ventral margin of the carpel, and appears before the carpel is completely closed. The basal ovule arises immediately below the apical one (fig. 3). The two ovules (occasionally three), which occur in the Virginia and Spanish varieties, are inserted alternately along the ventral suture (fig. 14). One-ovulate carpels are rare. The primary archesporial cell is probably hypodermal, but is not distinguishable from the adjacent meristematic cells of the nucellar apex. When it first appears, the single megasporocyte is in the subhypodermal cell-layer and is associated with one