Allometric allocation in fruit and seed packaging conditions the conflict among selective pressures on seed size

Selective pressures on seed size could vary among the different stages of plant life cycles, so no simple relation could explain a priori its evolution. Here, we determined the relationships between seed size and two fitness components—seed dispersal and survival from predation—in a bird-dispersed tree, Crataegus monogyna. We interpret these relationships in relation to the patterns of mass allocation to fruit and seed components. Selection patterns were assessed at two levels (1) selection pressures on the parent tree; comparing seed dispersal efficiency among individual plants and (2) selection pressures at the individual seed level; comparing seed size variation (i) before and after dispersal, and (ii) before and after postdispersal seed predation. Dispersal efficiency (percentage of seed crop dispersed) was positively correlated with fruit mass and fruit width. Differences in crop size did not offset this effect, and larger seeds were overrepresented in the seed rain relative to the seed pool before dispersal. However, the advantage of larger seeds during the dispersal stage was cancelled later by an opposite selection pressure exerted by seed predators. As a result, smaller seeds had a higher probability of surviving postdispersal seed predation, establishing an evolutionary conflict imposed by the need for dispersal and the danger of being predated. Birds and rodents preferentially selected highly profitable fruits and seeds in terms of the relative proportion of their components. Larger fruits had a higher pulp to seed proportion than smaller ones, and all seeds had the same proportion of coat relative to the embryo-plus-endosperm fraction. Hence, although predator pressures were stronger than disperser ones, larger seeds invested proportionally less in structural defense than in dispersal.     

Cache site selection by chipmunks (Tamias spp.) and its influence on the effectiveness of seed dispersal in Jeffrey pine (Pinus jeffreyi)

The effectiveness of Jeffrey pine (Pinus jeffreyi) seed dispersal performed by seed-caching yellow pine chipmunks (Tamias amoenus) and lodgepole chipmunks (Tamias speciosus) was compared to that of wind dispersal in the Sierra Nevada of western Nevada. Wind-dispersed seeds typically fall under or near the parent tree. Chipmunks removed 90 and 97% of 1064 radioactive seeds from each of two simulated wind-dispersed seed shadows in less than 24 h. Wind-dispersed seeds were deployed within 12 m of the two source trees, but chipmunk caches were found from 2–69 m from the trees. Chipmunks carried nearly all seeds away from source trees, greatly reducing the density of seeds under and near source trees. Caches contained from 1–35 seeds and most were buried 7–21 mm deep. Chipmunks cached in open bitterbrush shrubland with mineral soils much more than expected and cached in closed-canopy Jeffrey pine and lodgepole pine forests with thick needle litter much less than expected. Many Jeffrey pine seedlings and saplings grow in the bitterbrush habitat and few grow in the pine forests. Ten and 20% of the original caches survived until April, the time of seed germination, at the two sites. The movement of wind-dispersed seeds is random relative to environmental variables important in seedling survival, and the wind in coniferous forests cannot quickly bury seeds. The quality of seed dispersal rendered by chipmunks was superior to that provided by the wind because the chipmunks quickly harvested seeds on the ground, moved them away from source trees, and buried them in the ground in habitats and microhabitats where they were more likely to establish new seedlings. The increased quality of seed dispersal provided by animals relative to the wind may help explain why over twenty species of pines have evolved seeds and cones that are adapted for dispersal by seed-caching animals.     

Experimental evaluation of the foliar flag hypothesis using fruits of Rhus glabra (L.)

I tested experimentally whether the presence of colorful plastic ovals (simulating foliar flags) attached to infructescences of Rhus glabra increase fruit removal by birds in a forest-oldfield border. I used a factorial experimental design testing for the effect of size (small or large) and color (yellow or red) of the flags. There was also a control, without flags. Large red flags increased the percent of fruits removed from the panicles, but yellow and small flags had no effect. My results give partial support to Stiles (1982) hypothesis that early color change of leaves close to the fruits in some plants may serve as visual signals that attract frugivorous birds and enhances seed dispersal.     

Ecological correlates of seed mass variation in Phoradendron juniperinum,a xylem-tapping mistletoe

Summary We investigated several ecological correlates of seed mass variation in the hemiparasitic, xylemtapping mistletoe, Phoradendron juniperinum. Mean seed mass varied two-fold among plants between the ages of 4 and 14 years old and was positively correlated with parental plant age. Both the standard deviation and the coefficient of variation in mean seed mass decreased with increasing plant age demonstrating that, on average, younger plants produced seed with more variable mass. Nitrogen concentrations (mg nitrogen per gram of seed) of both the seed and fruit (pericarp) were not correlated with mass or the age of the parent plant from which the seed was taken. However, the nitrogen content per seed (mg nitrogen per seed) was positively correlated with the mean seed dry mass and the age of the seed parent, suggesting that the carbon to nitrogen ratio of individual seeds remained relatively constant as seed mass increased and plants grew older. Seed germination ranged between 20% and 86% and was positively correlated with mass and parental plant age. Heavier seeds (seeds from older plants) also had the highest root radicle growth rates. Furthermore, the final root radicle length after 76 d of growth was positively correlated with seed dry mass. When grown on a medium containing an extract prepared from the host plant foliage, all seeds showed lower germination, grew more slowly and had shorter overall root radicles, but had significantly greater development of the haustorial disks (the holdfast which forms the host-parasite junction in Phoradendron) than seeds grown on a control medium. Our results suggest that, on average, seeds of greater mass produced by older plants have a greater total resource pool per propagule (fruit + seed). This resource pool may be important in conferring a greater potential for dispersal (fruit), survival, colonization, and establishment.     

Community seed rain patterns and a comparison to adult community structure in a West African tropical forest

We examined the seed rain throughout a twelve month period in a lowlandtropical forest in Cameroon, West Africa, 1996–97. Traps (0.5m², n = 216) were erected throughout a 25km² area in 24 randomly placed clusters of nine trapseach. Fruits and seeds that landed in traps were collected every 7–10daysand classified by species and dispersal type. More than 32,000 seeds fromapproximately 200 species fell into the traps, an average of 297 seedsm^–2 yr^–1 Thirty species represent 82%of the total seed rain while an additional 175 species comprise the remaining18%. When we compared the adult community to the seed rain community within thesame plots, we found no apparent correlation between seed rain patterns andadult community structure for this year of study. Furthermore, only 49% of theadult tree community produced and dispersed seed into traps in this year. Morethan 100 species (52%) found in the seed rain represented long-distanceimmigrant seed rain. Seed rain was highly variable at several scales, bothspatially and seasonally, although seeds arrived in traps during eachcollectionperiod. Cluster analyses showed that traps within plots were seldom moresimilarto one another than traps between plots. While 82% of the tree species in thecommunity are thought to be animal dispersed, only 28% of all seeds that fellinto traps had been obviously handled (bitten, chewed, or passed)by animals. Tests for fruit and seed removal by predators or dispersers found5%or less removal rate from traps.     

Seed size selection by olive baboons

Seed size is an important plant fitness trait that can influence several steps between fruiting and the establishment of a plant’s offspring. Seed size varies considerably within many plant species, yet the relevance of the trait for intra-specific fruit choice by primates has received little attention. Primates may select certain seed sizes within a species for a number of reasons, e.g. to decrease indigestible seed load or increase pulp intake per fruit. Olive baboons (Papio anubis, Cercopithecidae) are known to select seed size in unripe and mature pods of Parkia biglobosa (Mimosaceae) differentially, so that pods with small seeds, and an intermediate seed number, contribute most to dispersal by baboons. We tested whether olive baboons likewise select for smaller ripe seeds within each of nine additional fruit species whose fruit pulp baboons commonly consume, and for larger seeds in one species in which baboons feed on the seeds. Species differed in fruit type and seed number per fruit. For five of these species, baboons dispersed seeds that were significantly smaller than seeds extracted manually from randomly collected fresh fruits. In contrast, for three species, baboons swallowed seeds that were significantly longer and/or wider than seeds from fresh fruits. In two species, sizes of ingested seeds and seeds from fresh fruits did not differ significantly. Baboons frequently spat out seeds of Drypetes floribunda (Euphorbiaceae) but not those of other plant species having seeds of equal size. Oral processing of D. floribunda seeds depended on seed size: seeds that were spat out were significantly larger and swallowed seeds smaller, than seeds from randomly collected fresh fruits. We argue that seed size selection in baboons is influenced, among other traits, by the amount of pulp rewarded per fruit relative to seed load, which is likely to vary with fruit and seed shape.     

Fruit Fate, Seed Germination and Growth of an Invasive Vine – an Experimental Test of ‘sit and Wait’ Strategy

Oriental bittersweet (Celastrus orbiculatus Thunb.) is a non-indigenous, invasive woody vine in North America that proliferates in disturbed open sites. Unlike most invasive species, C. orbiculatus exhibits a sit and wait strategy by establishing and persisting indefinitely in undisturbed, closed canopy forest and responding to canopy disturbance with rapid growth, often overtopping trees. We compared fruit fates of C. orbiculatus and native American holly (Ilex opaca). We also explored mechanisms for this sit and wait invasion strategy by testing the effect of C. orbiculatus fruit crop density on removal rates and by examining the influence of seed treatment and light intensity on seed germination and seedling growth. More C. orbiculatus than I. opaca fruits became damaged, and damage occurred earlier. More fruit fell from C. orbiculatus than I. opaca, but removal rates by frugivores did not differ (76.0 ± 4.2% vs 87.5 ± 3.7%, respectively). Density (number of fruits in a patch) of C. orbiculatus did not influence removal rates. Scarification (bird-ingestion) of C. orbiculatus seed delayed germination but seeds germinated in similar proportion to manually defleshed seeds (sown either singly or all seeds from a fruit). Germination of seeds within intact fruits was inhibited and delayed compared to other treatments. Seed treatment did not affect seedling growth. The proportion of seeds germinating and time until germination was similar among five light intensity levels, ranging from full sun to closed-canopy. Seedlings in >70% photosynthetically active radiation (PAR) had more leaves, heavier shoots, and longer, heavier roots than seedlings at lower PAR levels. Results show that most (>75%) C. orbiculatus seeds are dispersed, seedlings can establish in dense shade, and plants grow rapidly when exposed to high light conditions. Control strategies for this highly invasive species should likely focus on minimizing seed dispersal by vertebrates.     

Spatial dynamics of <Emphasis Type="Italic">Ilex aquifolium</Emphasis> populations seed dispersal and seed bank: understanding the first steps of regeneration

The objective of this study was to analyse quantitatively the spatial distribution of holly (Ilex aquifolium L.) seed rain and seed bank, and to detect the relationships between these consecutive processes. We measured seed dispersal by birds and fallen fruits, and also density and viability of seed bank in two Ilex populations in central Spain. Analysis was made distinguishing the following microhabitats: holly woodland, edge of holly woodland, open grassland 10 m and 100 m from the woodland, fleshy fruit shrubs, dry fruit shrubs, and adjacent non-holly woodland. Spatial distribution of dispersed and in-soil seeds was measured by the clumping index. Seed rain and seed bank under holly woodlands were significantly higher than in the other microhabitats. Forest edges and fleshy fruit shrubs were the next microhabitats with the highest seed rain and seed bank density. Interannual and interlocality variations were not significant. The relative importance of the different dispersal methods varied between microhabitats, with a similar support of bird dispersed seeds and fallen fruits within the woodland and a greater influence of cattle dispersal in open areas. Seed spatial aggregation was significant in both dispersed seeds and soil seeds from holly woodlands and the edge of the forest. Aggregation under shrubs, grasslands and the adjacent forests evidenced a general random distribution of holly seeds (only in some cases clumping index was significant). Quantitative differences between seed rain and seed bank are important. Post-dispersal seed predation did not modify seed rain distribution, which was mirrored in the seed bank pattern. These two phases of holly regeneration had a heavy spatial influence, determined by the landscape structure and activity of the dispersal agents, that reflects a differential recruitment potential. Comparisons between both populations suggest that in the southern locality (Robregordo) holly has a weaker capacity to colonize open areas, and a stronger recruitment limitation due to propagule availability.     

Estimation of the retention times and distances of seed dispersed by two monkey species, Alouatta seniculus and Lagothrix lagotricha, in a Colombian forest

Isolated-bout method to estimate the retention times and dispersal distances was applied to the seed dispersal by red howler monkeys (Alouatta seniculus) and Humboldts woolly monkeys (Lagothrix lagotricha) in a lowland tropical forest at La Macarena, on the border of the Macarena and Tinigua National Parks, the Department of Meta, Colombia. Continuous observations were made on the feeding and ranging behavior of well-habituated troops of howler monkeys and woolly monkeys as well as continuous collection of monkeys feces. We selected out the isolated-bout as a feeding bout on the specific species that was only once recorded within 48 h before the seeds of that species appeared in the feces of monkeys. In that case, the seeds were strongly suggested to come from that isolated bout. Then retention times, route seed dispersal distances and direct seed dispersal distances were estimated. Howler monkeys, which are regarded as generalist herbivores, showed longer retention times and dispersal distances along monkeys route than did woolly monkeys, which are specialist frugivores. However, the direct distances that seeds were carried from the mother tree were not significantly greater for howler monkeys than for woolly monkeys. This shows that both retention time and movement patterns by the monkeys, especially the total ranging area, influence the direct distance that seeds are carried from the mother tree.     

Ant seed removal in a non-myrmecochorous Neotropical shrub: Implications for seed dispersal

This study investigated ant seed removal of Piper sancti-felicis, an early successional Neotropical shrub. Neotropical Piper are a classic example of bat-dispersed plants, but we suggest that ants are underappreciated dispersal agents. We identified eleven ant species from the genera Aphaenogaster, Ectatomma, Paratrechina, Pheidole, Trachymyrmex, and Wasmannia recruiting to and harvesting P. sancti-felicis seeds in forest edge and secondary forest sites at La Selva, Costa Rica. We also tested for differences in ant recruitment to five states in which ants can commonly encounter seeds: unripe fruit, ripe fruit, overripe fruit, bat feces, and cleaned seeds. Overall, ants harvested more seeds from ripe and overripe fruits than other states, but this varied among species. To better understand the mechanisms behind ant preferences for ripe/overripe fruit, we also studied how alkenylphenols, secondary metabolites found in high concentrations in P. sancti-felicis fruits, affected foraging behavior in one genus of potential ant dispersers, Ectatomma. We found no effects of alkenylphenols on recruitment of Ectatomma to fruits, and thus, these compounds are unlikely to explain differences in ant recruitment among fruits of different maturity. Considering that P. sancti-felicis seeds have no apparent adaptations for ant dispersal, and few ants removed seeds that were cleaned of pulp, we hypothesize that most ants are harvesting its seeds for the nutritional rewards in the attached pulp. This study emphasizes the importance of ants as important additional dispersers of P. sancti-felicis and suggests that other non-myrmecochorous, vertebrate-dispersed plants may similarly benefit from the recruitment to fruit by ants.     

Specialized and generalized dispersal systems: where does ‘the paradigm’ stand?

I explore the specialization versus generalization paradigm in frugivory and seed dispersal. This view predicts that some tropical trees produce nutritious fruits adapted for use by a small coterie of specialized frugivores that provide reliable seed dissemination. Other tree species are expected to offer superabundant fruits of lower nutritional reward, relying instead on common opportunistic frugivores that are individually less reliable, but collectively disperse seeds effectively. Though widely referenced, many aspects of the paradigm are untested with tropical trees and avian frugivores, primarily because plant ecologists rarely determine whether specialist or generalist foragers are responsible for different patterns of seed distribution, while students of foraging behavior rarely determine the effects seed dispersal by different animals for survival of seeds or seedlings of specialist or generalist trees.Ecological paradigms provide alternative hypotheses, without evolutionary arguments. Keystone species have ecological effects disproportionate to their abundance; it is important for management considerations to know whether fruiting trees or frugivores serve as keystone mutualists in tropical forests. Alternatively, the extent to which vertebrate seed dispersers influence density-dependent seed, seedling, sapling, or adult mortality may have important consequences for spatial dispersion and population dynamics of tree species in tropical forests.     

Effect of pollination intensity on fruit and seed set in cacao (Theobroma cacao L.)

We studied the functional relationship between pollination intensity and fruit survival as well as the number of seeds per pod in the tropical tree Theobroma cacao L. on a Forastero Upper-Amazon clone (UPA 409) in Ivory Coast. Cutting the style 24 h after pollination allowed for counting the number of pollen grains deposited on a stigma without affecting fruit set and seed development. Forty-three pollen grains were necessary to reach 50% of maximum fruit set 28 days after pollination. Above 115 pollen grains, the proportion of developing ovaries reached a maximum of 88% 28 days after pollination and 75% at maturity. With fewer than 238 pollen grains per stigma, there was a close relationship between pollination intensity and number of seeds per pod; the pollenseed ratio increased from 1.61 to 3.81 for PI increasing from 30 to 238 pollen grains. For higher pollination intensities, the average number of seeds per pod reached a maximum of 58. The relationship between pollination intensity and seed content was modelled. Results are consistent with the hypothesis that ovules attracted pollen tubes in a similar way regardless of whether or not they had already been reached by another pollen tube.     

Seed removal in a tropical North American desert: an evaluation of pre‐ and post‐dispersal seed removal in Stenocereus stellatus

• To determine seed removal influence on seed populations, we need to quantify pre‐ and post‐dispersal seed removal. Several studies have quantified seed removal in temperate American deserts, but few studies have been performed in tropical deserts. These studies have only quantified pre‐ or post‐dispersal seed removal, thus underestimating the influence of seed removal. We evaluated pre‐ and post‐dispersal seed removal in the columnar cactus Stenocereus stellatus in a Mexican tropical desert.
• We performed selective exclosure experiments to estimate percentage of seeds removed by ants, birds and rodents during the pre‐ and post‐dispersal phases. We also conducted field samplings to estimate abundance of the most common seed removers.
• Birds (10–28%) removed a higher percentage of seeds than ants (2%) and rodents (1–4%) during pre‐dispersal seed removal. Melanerpes hypopolius was probably the main bird removing seeds from fruits. Ants (62–64%) removed a higher percentage of seeds than birds (34–38%) and rodents (16–30%) during post‐dispersal seed removal. Pogonomyrmex barbatus was probably the main ant removing seeds from soil.
• Birds and ants are the main pre‐ and post‐dispersal seed removers in S. stellatus, respectively. Further studies in other S. stellatus populations and plants with different life forms and fruit types will contribute to evaluate seed removal in tropical American deserts.

     

Experimental studies of seed predation in old-fields

Summary In a pair of experiments conducted in old-field habitats in southwestern Michigan (USA), we examined rates of seed loss to post-dispersal predators (ants and rodents). Seeds from 4–6 species of biennial plants were tested over a range of seed densities and habitat types. We found that seed removal was significantly higher in vegetated habitats than in areas of disturbed soil (both simulated small-animal diggings and a plowed field). In the undisturbed vegetation, seed losses ranged from 1–20% of seeds removed/day.An exclosure experiment demonstrated that ants and rodents foraged selectively for seeds of the six plant species tested. Rodents (Peromyscus maniculatus) fed preferentially on species producing large seeds (predominantly Tragopogon dubius). Ants (Myrmica lobicornus) foraged on smaller seeds, although their foraging preferences were not based strictly on seed size.Seed density had only a minor effect on predation rate over the range of densities tested. Predators, instead appeared to treat each experimental group of seeds as a single prey patch. Consequently, predation intensity was quite variable over distances of <20 m within a relatively homogeneous section of habitat.These field experiments provide initial estimates of seed losses to post-dispersal predators in old-field habitats. Rates of seed loss were generally less than those reported from desert or semi-arid habitats. However, for some old-field species, seed losses averaged an appreciable 10–20% day. The selective nature of the seed predators, plusthe relative patchiness of predation intensity in space, suggest that postdispersal seed predation can play a role in determining the distribution and/or abundance of old-field herbs.     

Fecundity,fruiting pattern,and seed dispersal in Piper amalago (Piperaceae), a bat-dispersed tropical shrub

Summary This paper describes the nightly and seasonal production of ripe fruit by Piper amalago (Piperaceae), a patchily distributed, bat-dispersed forest shrub, at Parque Nacional Santa Rosa, Costa Rica. Phenological observations over several years indicate that individuals produce a low (usually 1–3) and variable number of ripe fruit each night for 3–4 wks in the early wet season (June and July). Observations of the disappearance rates of marked fruits and fruit manipulation experiments indicate that fruit removal probabilities are high (often nearly 1.0) and independent of nightly and seasonal ripe fruit crop size. Data from previous feeding and foraging studies of the bat Carollia perspicillata (Phyllostomidae) are used to estimate the mobility of P. amalago's seeds. Most seeds (>90%) are deposited 50 m from parent plants under night feeding roosts. Relatively few seeds move >300 m, and movements this long are more likely to occur early and late in the fruiting season when bats change feeding sites more frequently. Seed experiments indicate that P. amalago seedling establishment probabilities are higher in light gaps than under forest canopy. The dispersal quality (sensu McKey 1975) of P. amalago's chiropteran seed dispersers is directly proportional to the number of seeds they excrete in actual or incipient light gaps.     

Effects of seed distribution and competitors on seed harvesting efficiency in heteromyid rodents

Summary The foraging strategies of four naturally co-existing heteromyid rodent species were investigated: Dipodomys deserti (100 g), D. merriami (38 g), Microdipodops pallidus (13 g), and Perognathus longimembris (7 g). In 208 over-night laboratory foraging trials animals were provided with millet seed distributed in clumped and scattered patterns. Net removal of seeds from the foraging arena and amounts of seeds in cheek pouches and in caches were determined. When alone in an areana none of these species specialized extensively on either clumped or scattered seeds, although each tended to take more clumped than scattered seeds. When placed together with other individuals, animals once again tended to cache more clumped than scattered seeds in all but one paired combination of species: P. longimembris cached more scattered than clumped seeds when opposed by D. deserti. This suggests that the smaller species obtained a less preferred distribution of seeds in the face of competition. The two smaller species showed a great reduction in general foraging success in the presence of either of the two larger species. In general, a species cached less seeds when faced by larger opponent species.     

An evaluation of vertebrate seed dispersal syndromes in four species of black nightshade (Solanum sect. Solanum)

We examined the ecological relevance of bird versus mammal dispersal syndromes in four species of Solanum, S. americanum Type A, S. americanum Type B, S. ptychanthum, and S. sarrachoides. These plants were selected because their morphological characteristics, such as fruit color, mass, and persistence, resembled those typically associated with classically-defined bird and mammal dispersal syndromes. We monitored persistence of tagged fruits, compared physical and chemical chaacteristics, performed fruit preference trials with northern bobwhite quail (Colinus virginianus), deer mice (Peromyscus maniculatus), and American robins (Turdus migratorius), and assessed differences in use of olfactory cues by foraging deer mice. We predicted that principal components analysis of physical and chemical characteristics would place fruits of these species along a clear gradient from bird to mammal dispersal syndromes (S. americanum Type A, S. americanum Type B, S. ptychanthum, and S. sarrachoides). However, physical and chemical characteristics did not consistently follow the gradient. Also, contrary to expectations, both birds and mammals demonstrated a preference for S. americanum Types A and B, both bird fruits. Deer mice and bobwhite quail showed much less discrimination among fruit types than did American robins. While the relatively strong odor of the green-fruited S. sarrachoides suggested a mammalian attractant, deer mice discovered the relatively odorless S. americanum Type A significantly more quickly. We conclude that in Solanum, suites of morphological characteristics resembling bird and mammal dispersal syndromes are not good predictors of fruit choice by birds and mammals. We note, however, that this conclusion is based on a sample of three animal species. Alternative explanations for fruit character suites should be considered. For example, the adaptive significance for an association of green coloration of ripe fruit with impersistence (dropping when ripe), as in S. sarrachoides and some S. ptychanthum, may relate more to photosynthesis and carbon balance in detached fruits than to disperser attraction.     

Order of fertilization within the ovary in Phaseolus coccineus L. (Leguminosae)

Summary Phaseolus coccineus typically has six linearly arranged ovules per ovary. The three ovules near the stylar end of the fruit (positions one, two, and three) are more likely to produce mature seeds, to produce heavier seeds, and to produce more vigorous progeny than the ovules in positions near the peduncular/basal end of the fruit (ovule positions four, five, and six). We conducted a series of field experiments designed to supplement our understanding of the mechanisms determining these position effects. We found that approximately 98% of the ovules in 752 fruits were fertilized — about 0.6% of the stylar ovules were not fertilized, whereas 3.2% of the basal ovules were unfertilized. Moreover, we found that only about 49% of the ovules in these 752 fruits produced mature seeds. Over 60% of the stylar ovules produced mature seeds, whereas only 37% of the basal ovules produced mature seeds. Consequently, the proportion of fertilized ovules cannot explain the differences in seed maturation among the ovule positions. We found that after 6.5 h most of the fertilized ovules were located in the stylar ovule positions, and that there were no fertilized ovules in ovule positions five and six, indicating that the stylar ovules are fertilized first. When only the fastest growing pollen tubes were permitted to enter the ovary (due to exision of the style), only the ovules at the stylar end were fertilized, indicating that the ovule positions that are fertilized first are indeed fertilized by the fastest growing pollen tubes.On leave from the Escuela de Biologia, Universidad de Costa Rica, Cuidad Universitaria Rodrigo Facio, San Jose, Costa Rica, Central America     

Seed dispersal curve of a Mediterranean myrmecochore: Influence of ant size and the distance to nests

Euphorbia characias is a Mediterranean spurge with a diplochorous dispersal system: after a ballistic dispersal that scatters the seeds, some ant species find and retrieve the seeds to their nest (myrmecochorous dispersal). The seed dispersal curve generated by ants in an abandoned field was described and partitioned according to ant size and to the distance to nest entrance from where seeds fell after ballistic dispersal. Both variables (ant size, distance to nest) affected dispersal distance. The seed dispersal curve showed a peak at short distance (median=1m) and a tail extending to 9.4m. The peak and the tail are explained differently. Short distances are usually generated by small ants (Pheidole pallidula and Tapinoma nigerrimum; 0.56±0.41m [n=48]) both from the nearest or farther nest entrances. The tail of the curve is generated disproportionately by big ants (Aphaenogaster senilis and Messor barbarus; 2.09±1.71m [n=61]) from farther nests. Seeds have a much greater probability (P=0.734) of being transported to nests which are not the nearest. This effect is largely due to transportation by big ants.     

Distinctions betweenHordeum bulbosum andH. murinum s.l. in seed and herbarium collections

It is sometimes necessary to identify eitherH. bulbosum orH. murinum on the basis of the inflorescence or seeds alone. The majority of taxonomic keys use the presence of swollen basal culms for the former against the annual habit for the latter. Confusion is due to similarities in inflorescences and spikelet morphology. Lodicules which always persist and are present beside the fruit in a mature caryopsis, and other characters such as the awns of the lemmas of the lateral spikelets enable conclusive distinction.