International Journal of Plant Sciences

Premise of research. As a shoot of Carnegiea gigantea (saguaro, columnar cactus) grows longer, its shoot apical meristem (SAM) occasionally adds new ribs, presumably because the SAM is increasing in size, giving it a larger basal circumference (the leaf primordium initiation zone). A new rib, which is a phyllotactic orthostichy, can be initiated when the SAM basal circumference has increased by the width of a leaf primordium. Rib initiation sites are markers that indicate when and where an SAM has enlarged. If SAMs grow uniformly, steadily, and symmetrically, the initiation sites of new ribs should occur at uniform intervals upward along the shoot and should also be symmetrical around the shoot’s circumference.

Methodology. In the field, 18 mature plants were examined, and the height and circumferential position of the start point of every new rib were mapped.

Pivotal results. Two to five new ribs would be initiated in rapid succession longitudinally, followed by a considerable period of shoot growth during which no new ribs would be initiated. Circumferentially, new ribs tended to be located laterally close to a previous new rib rather than being located symmetrically or randomly.

Conclusions. SAMs of C. gigantea grow episodically and asymmetrically.

Important floristic changes took place during the Early Cretaceous (145.0–100.5 Ma). They are notably marked by a peak in conifer diversity, especially within Pinaceae. This diversification is attested to by the numerous ovulate cone taxa found in the western European and North American fossil records. Exceptional deposits in the Wealden facies (Barremian-Albian, 125.0–100.5 Ma) of western Belgium (La Louvière, Houdeng-Aimeries) have yielded hundreds of exceptionally well-preserved pinaceous ovulate cones, yet most of these remain unstudied. These are presently revised. We investigate the morphological variation of the most abundant species, Pityostrobus andraei (Coemans) Seward , with a morphometric approach. This work applies statistical tests to a large sample (n = 132 fossil cones) for the first time, using 10 measurements to characterize the morphology of the cones. Our analyses recover eight distinct morphotypes included in the original specific concept of P. andraei, and most of the specimens belong to two main morphotypes that represent 50% and 25% of the total specimens analyzed. Our results suggest that the specific diversity of early Pinaceae is underestimated, as it is based on older species concepts. Modern quantitative approaches have the potential to better characterize the breadth and tempo of pinaceous diversification.

Premise of research. Winteraceae, a family within the Canellales, is composed of tropical trees and shrubs broadly distributed in the Southern Hemisphere. The family is found today in eastern Australia, New Zealand, Malesia, Oceania, Madagascar, and the Neotropics across a range of dry to wet tropical to temperate climate regions. The fossil record of woods related to the Winteraceae in the Southern Hemisphere is limited to the Late Cretaceous of the Antarctic Peninsula. Here, we present a detailed anatomical description of the secondary xylem of a well-preserved trunk from the early Eocene Laguna del Hunco site, Huitrera Formation, Patagonia (Chubut Province, Argentina), that is referable to a new species of the genus Winteroxylon (Gottwald) Poole and Francis.

Methodology. The wood is preserved as a siliceous permineralization; it was sectioned using standard petrographic techniques and observed under both light and scanning electron microscopy. The anatomy was compared with that of extant and fossil species of Winteraceae.

Pivotal results. The diagnostic anatomical features of Winteraceae preserved in the fossil include an absence of growth rings, a lack of vessels, tracheids that are rectangular in cross section with circular bordered pits, diffuse axial parenchyma, rays showing two distinct size ranges (uniseriate-biseriate or multiseriate, 3–15 cells wide), and the presence of heterocellular rays containing sclerotic nests, cells with dark contents, and oil cells. The new fossil species most resembles extant genera within the Zygogynum s.l. clade from Australasian and Malesian rain forests; its anatomy is very similar to that of the extant genus Bubbia. The new Patagonian Winteraceae fossil wood is characterized by the presence of sclerotic nests and oil cells in the rays, which differ from those of previously described species of Winteroxylon.

Conclusions. On the basis of the distinctive characters preserved, we erect Winteroxylon oleiferum sp. nov. The new fossil is the first reliable macrofossil record of Winteraceae from South America, supporting the abundant palynological record of the family from the continent, and it is the oldest record of the Zygogynum s.l. clade, adding to the long list of southern biogeographic connections between South America and Australasia via Antarctica during the warm early Cenozoic.

Premise of research. Heterospory, or the production of different spore size classes, is a key aspect of advanced vascular plant reproductive strategies. Heterospory is widespread across plant lineages, but early stages in its evolution have rarely been observed. Here we describe a new Devonian plant, Omniastrobus dawsonii, which may show an intermediate step in the emergence of heterospory.

Methodology. We examined Early Devonian (early Emsian) specimens from the Campbellton Formation of New Brunswick containing impressions of strobili with numerous carbonized spore masses. We used standard paleobotanical maceration and microscopy techniques to isolate spores and characterize their size distribution.

Pivotal results. Strobili in Omniastrobus are similar to those of the enigmatic order Barinophytales and consist of two rows of lateral sporangia. Unlike in other Barinophytales, however, sporangia within the strobili show a continuous gradient in average spore size, from those producing smaller spores (∼70–150 μm) to those producing larger spores (>200 μm), as well as some with a bimodal mix of sizes.

Conclusions. Omniastrobus is coeval with the earliest previously known heterosporous plant, and it represents the oldest lycophyte that is not strictly homosporous. The spore size distribution of Omniastrobus is unlike that of nearly all other known plants, although similar patterns of high spore size variation are present in a few unrelated Paleozoic taxa, suggesting that this may be a common early step in the evolution of heterospory.

Premise of research. Plant silica content depends on the phylogenetic position of a taxon; however, biological or ecological factors may also affect it. In this work, we analyzed data on silicophytolith content from 105 species of South America, examining, in a phylogenetic context, its relationship with anatomy and ecological features such as life cycle, growth form, plant origin, and environmental preferences.

Methodology. Data on the silicophytolith content and bioecological features of the species were obtained from published and unpublished sources. The relations between systematics, silica content, and bioecological variables were analyzed through measurements of phylogenetic signal and phylogenetic generalized least squares regressions.

Pivotal results. Eighty-six percent of the species produced between 0.38% and 19% of silicophytolith dry weight in leaves. Silica content was variable between and within clades. Values of λ and K indicate a low phylogenetic signal for the variable silica content. Dicotyledons accumulated silica in typical epidermal cells, and a few families also stored it in cystoliths. Most of the monocot families showed high silicophytolith contents and a high diversity of silicified cells. Plant origin affected the silica contents: exotic species accumulated more than native ones. On the other hand, no statistical relationship was found between silica content and the other ecological variables.

Conclusions. Silicophytolith accumulation is a common feature in most of the species studied. The low phylogenetic signal of silica content is explained by inter- and intraclade variability, which in turn supports the hypothesis that silicophytolith accumulation is a homoplasic character among plants. On the basis of the overall analysis of silicophytolith content and tissue distribution, high content could be related to the specific accumulation mechanisms and roles of silica. The origin of the plant was the only bioecological variable that influenced plant silica content. This finding may indicate some ecological role for silica in exotic plants involving their success in novel environments.

Premise of research. Conservation of imperiled plant species can benefit from ex situ collections, including the living collections of botanic gardens and arboreta. Such living collections should strive to capture and maintain the genetic diversity found in the remaining wild populations. Ex situ collections may also harbor genetic variability that has been lost from wild populations. Quercus hinckleyi is an extremely rare (International Union for Conservation of Nature and Natural Resources [IUCN] Critically Endangered) oak known from a few sites in Presidio County, Texas. It is the only native US oak protected under the Endangered Species Act. Here we evaluate the genetic variation of the Q. hinckleyi ex situ metacollection relative to that of the remnant in situ population.

Methodology. We apply microsatellite genotyping to Q. hinckleyi sampled from plants growing in ex situ collections (N=22, from nine gardens) and compare the results with genetic studies of the in situ population. We compare allelic diversity, genotypic diversity, and population structure.

Pivotal results. The ex situ metacollection is small but has high allelic diversity and captures about 57% of the allelic diversity of the wild population. Ex situ plants contain 22 new alleles that contribute 13% of the species’ total allelic diversity. All ex situ individuals have unique genotypes, none of which were found in situ, and thus they comprise 15% of the species’ genotypic variation. The ex situ plants align with only one of three genetic clusters identified in situ, demonstrating an important gap in the ex situ metacollection.

Conclusions. The ex situ collection of Q. hinckleyi, while small, is genetically diverse and provides additional allelic and genotypic diversity not currently existing in situ. However, a significant portion of genetic diversity is lacking from ex situ plants, with the metacollection likely derived from only one of the in situ clusters. Collecting seed from the other clusters is a high priority.

Premise of research. Worldwide, fire-sensitive shrubs have an important but underresearched life history, and in Australia they are threatened by anthropogenic changes to fire regimes and habitat loss. Typically, adults are killed by fire, but they can also senesce and die if interfire intervals are prolonged. Populations can regenerate from seed banks displaying fire-stimulated germination; however, populations are ephemeral, and true population sizes and connectedness are difficult to estimate. Persoonia hirsuta ssp. evoluta is an endangered, inconspicuous, fire-sensitive shrub with a soil-stored seed bank currently known only from a small number of locations within the fire-prone Sydney Basin. Although it appears highly fragmented and populations are typically small, we predicted that its seed bank would buffer populations against loss of genetic diversity and population differentiation.

Methodology. We used microsatellite markers to assess genetic variation within three aboveground populations of P. hirsuta separated by up to 25 km. The largest and most isolated population, at Appin, New South Wales, Australia, occurs on a mine site and may be subject to disturbance. We compared levels of genetic diversity and estimated the mating systems and genetic connectedness of plants within the three sites.

Pivotal results. As predicted, all populations displayed similar genetic diversity, as judged by expected heterozygosity and allelic richness, and displayed little differentiation. All populations appear predominantly outcrossed. However, STRUCTURE and principal coordinates analyses showed that Appin individuals were distinct from those at the other locations.

Conclusions. Our data imply that even for fire-sensitive species with few aboveground populations, genetic diversity can be maintained by the buffering effect of persistent seed banks with diversity reflecting historically greater interpopulation gene flow. While all P. hirsuta sites support comparable genetic diversity, the preservation of known populations is critical. Conservation efforts should include searches for additional populations, fire-stimulated germination of seed banks, and collection of seed for propagation in a nursery or direct sowing.

Premise of research. In hermaphroditic plants, pollen production can enhance fitness through siring success by attracting pollen-seeking pollinators. However, pollen production can also decrease female fitness if ovules penetrated by self-pollen tubes are usurped, thereby limiting opportunities for seed production.

Methodology. We investigated the hermaphroditic aloe Aloe pruinosa, which is pollinated by birds and native bees. We performed manual crossed and selfed hand-pollinations and examined pollen tube growth rates to locate the self-incompatibility (SI) barrier. To quantify the effects of pollen production on pollinator visitation and fecundity, we compared experimentally emasculated plants with intact controls.

Pivotal results. Pollen tubes grew into the ovaries and penetrated the ovules of both crossed and selfed flowers, indicating a putative late-acting SI system. Experimental emasculation resulted in reduced bee visitation but had no effect on bird visitation. Emasculation led to reduced pollen deposition on stigmas and slightly reduced fruit production but also led to marked increases in the number of seeds per fruit and overall seed production.

Conclusions. These results demonstrate that hermaphroditism can involve trade-offs between the quantity of visiting pollinators and the quality of pollen transfer between plants, thus highlighting the roles of sexual conflict and fitness compromise in flowering plants.