Castilla Elastica Research Articles

Unraveling the Olmec rubber balls from El Manatí, Mexico: a technological and compositional analysis

This work presents an analysis towards a description of the manufacturing technique of the Olmec rubber balls found at the offerings at El Manatí. The results present this procedure from the extraction of the Mesoamerican rubber from the Castilla elastica tree, discussing its composition and origin of the rubber balls analyzed towards the production of strips that are rolled to make the ball round form. This characterization was achieved through a series of imaging techniques (radiography imaging, UV-induced fluorescence imaging, and optical microscopy) and portable non-destructive and non-invasive analyses (XRF and FTIR) performed on the collection of archaeological rubber balls and compared with contemporary made rubber balls in the region near El Manatí. The methodology was complemented with laboratory chemical analytical techniques (13C NMR-MAS, FTIR, CEA, and GC-MS) applied to selected Olmec rubber microsamples. The new physical and chemical data obtained was also interpreted considering conservation science, to help understand the alterations and transformation processes that the balls have undergone since their recovery in the 1980s.

Structure and Composition of Woody Species in the Kpatawee Tropical Rainforest in Liberia

Studying ecosystem structure and species composition is pivotal, offering critical insights into environmental health and biodiversity. It facilitates the evaluation of environmental changes, like deforestation and climate shifts, and their influence on species dynamics within ecosystems. Our study aimed to assess the woody species composition and structure within Liberia's Kpatawee tropical rainforest. We conducted transect walks, crossing the forest south-north and east-west, establishing six 20 m x 20 m sampling plots. Using the PlantNet plant species identifier tool, we cataloged surviving trees with a DBH ≥ 5 cm, including their scientific names. Results unveiled 71 tree species across 42 families, with Leguminosae displaying the highest diversity. Prominent species included Quararibea asterolepis, Hasseltia floribunda, and Castilla elastica. An intriguing inverted J-shaped pattern in diameter distribution linked to stem-stand shrubs, small-sized trees, and young large trees characterized the first class. The tree layer, mainly species like Garcinia benthamiana, Iramyan therasagotiana, and Eschweilera decolarans, contributed significantly to the total DBH (65.45%). Total basal area (BA) for trees DBH ≥ 5 cm was 16.39 m² ha-1. Notably, eight species accounted for 93% of the basal area, reflecting a concentration of individuals in lower DBH classes in the first category. This trend suggests a gradual decline in numbers in higher DBH classes. Consequently, we recommend implementing sustainable forest management to mitigate the impacts of selective cutting on forest recruitment and regeneration processes. The inverted J-shaped pattern, indicating prolific regeneration but limited recruitment, possibly due to selective cutting, underscores the imperative for prudent forest management.

PLANTS OF THE POPOL VUH, THE SACRED BOOK OF THE MAYA

A compilation of the plants mentioned in the sacred book of the Mayans Popol Vuh, recognized as the framework of their cosmogony that was written in the K’iche’ area in Guatemala around the year 1550 was made. Thirty- two different species were identified, from 21 plant families, all native to Mesoamerica. The largest number of species were in Fabaceae with four species, Moraceae and Solanaceae with three species each, in addition to Bromeliaceae, Cucurbitaceae, Malvaceae and Poaceae with two species each. The five most important and frequently named species are maize (Zea mays), jícaro (Crescentia cujete), copal (Protium copal), rubber (Castilla elastica) and cocoa (Theobroma cacao). Eleven species were identified, such as pito wood (Erythrina berteroana) and zibak (Cyperus canus), considered of cosmogonic significance and food plants that are integrated into five groups: fruit trees [zapote (Manilkara zapota), nance (Byrsonima crassifolia), jocote (Spondias mombin), anona (Annona reticulata), matasano (Casimiroa edulis)], grains and seeds [maize, beans (Phaseolus lunatus), cocoa, pataxte (Theobroma bicolor)], vegetables [squash (Cucurbita moschata), tomato (Solanum lycopersicum), chilacayote (Cucurbita ficifolia)], flavourings [chili (Capsicum annuum)], and beverages [maguey (Agave americana)]. Seven species were domesticated: maize, squash, chilacayote, tobacco (Nicotiana tabacum), beans, tomato and cocoa and others for medicinal purposes, fuel and instruments. The description of the creation of man asserts the close relationship of the Mayan culture with plant biodiversity; moreover, the famous milpa, a multi-species Mesoamerican agroecosystem is mentioned that was practiced by their gods.

Longhorned Beetles (Coleoptera: Cerambycidae and Disteniidae) Collected in the Canopy of a Dry Tropical Lowland Forest in Panama

This study characterizes the abundance and richness of longhorned beetles (Coleoptera: Cerambycidae and Disteniidae) collected in the canopy of a dry tropical lowland forest in Panama. The nine tree species sampled included Enterolobium cyclocarpum (Jacq.) Griseb. (Fabaceae), Pseudobombax septenatum (Jacq.) Dugand (Malvaceae), Luehea seemannii Triana & Planch. (Malvaceae), Cordia alliodora (Ruiz. & Pav.) Oken (Boraginaceae), Castilla elastica Sessé ex Cerv. (Moraceae), Anacardium excelsum (Bertero & Balb. ex Kunth) Skeels (Anacardiaceae), Cecropia peltata L. (Urticaceae), Zuelania guidonia (Sw.) Britton & Millsp. (Flacourtiaceae) and Ficus insipida Willd. (Moraceae). A total of 227 specimens of Cerambycidae and Disteniidae were collected. The most abundant species was Trestonia pulcherrima Dillon and Dillon, 1946 (23%), followed by Compsibidion vanum (Thomson, 1867) (16%). The greatest abundance and richness of beetles were recorded in the canopy of L. seemannii (90 individuals = 39%). The presence of these beetles in the forest canopy is associated with their general preference for large trees, highly complex foliar canopy architecture, and the availability of resources for the development and feeding of larvae and adults. Synchronization with plant phenology may explain the greater abundance and richness observed in L. seemannii.

Germination success of large‐seeded plant species ingested by howler monkeys in tropical rain forest fragments

Primates are important seed dispersers, especially for large-seeded (>1 cm long) tropical species in continuous and fragmented rainforests. In three forest fragments within the Montes Azules Biosphere Reserve, southern Mexico, we investigated the effect of seed passage through the gut of howler monkeys (Alouatta pigra) on the germination rate and maximum germination (%) of native, large-seeded species. One group of howler monkeys, per fragment, was followed and fresh feces collected. Large seeds were removed to compare their germination success with non-ingested seeds of the same species collected underneath parent plants. Feces contained large seeds from seven tree species (Ampelocera hottlei, Castilla elastica, Dialium guianense, Garcinia intermedia, Pourouma bicolor, Spondias mombin, Trophis racemosa) and one liana species (Abuta panamensis). Except for G. intermedia, ingested seeds germinated significantly faster than non-ingested seeds, which had negligible germination. Ingested seeds of D. guianense, P. bicolor, S. mombin, T. racemosa, and A. panamensis had significantly greater germination, while G. intermedia had significantly lower germination and seed ingestion had no apparent effect for A. hottlei and C. elastica. In general, seed ingestion by howler monkeys confers faster germination compared with non-ingested seeds. Faster germination reduces predation probabilities and increases seedling establishment in forest fragments. Primate dispersal services contribute to germination heterogeneity within plant populations of old-growth forest species and to their persistence in forest fragments.

Кастилла эластичная (Castilla elastica Cerv.) – почти забытый каучуконос

When writing this botanical and historical review, it was necessary to get acquainted with the works of different authors from the XVII to the XXI century, written in Latin, old Spanish, old French, Spanish, French, German, Dutch, English and Russian. It shows the development of knowledge about growing on the American continents rubber-bearing trees, including their species definition and a botanical description. The main attention is paid to the Castilla elastica Cerv. from the Moraceae family, which served as a source of raw materials for the manufacture of an analogue of modern vulcanized rubber for ancient peoples of Mesoamerica and was then for many years the main rubber-bearing plant, whereas the priority have been given to Hevea brasiliensis (Wild. ex A. Juss.) Muell.-Arg. only after the beginning of plantation cultivation of this species in the end of XIX century. However, interest in Castilla did not completely disappear, and in the coming millennium, attempts were made to resume plantations. Sequencing of individual fragments of the nuclear and chloroplast genomes of Castilla species has also begun. The history of the appearance of the second erroneous name of this genus as Castilloa and the long process of returning the original name Castilla are described.

What makes a fig: insights from a comparative analysis of inflorescence morphogenesis in Moraceae.

Moraceae, the family of mulberry and fig trees, displays small homogeneous flowers but extremely diverse inflorescences ranging from simple and branched to complex and condensed. Inflorescences also vary in flower organization in the receptacle, in the degree of flower condensation and in receptacle shape. Thus, the objective of the present study was to compare the inflorescence morphogenesis of Moraceae species, to investigate whether clades with a similar pollination mode share the same patterns of inflorescence development and the developmental stages at which we observe the key changes resulting in the diversified inflorescence architecture that culminates in the Ficus syconium. Inflorescences at different developmental stages were sampled from Brosimum gaudichaudii, Castilla elastica, Clarisia ilicifolia, Ficus pertusa, Maclura tinctoria and Morus nigra and processed for surface and anatomical analyses. The inflorescence morphogenesis of the studied species is highly variable. The shape of the inflorescence meristem (bulging, hemispheric or elongated), the initiation order and arrangement of flowers along the receptacle and the occurrence of bracts vary between related species. This diversity originates early during inflorescence development. Brosimum gaudichaudii, C. elastica and F. pertusa have flowers enclosed or immersed within the receptacle, although inflorescences begin their development as flat and open structures, as occurs in the other three study species. Comparison of the inflorescence morphogenesis in Moraceae species allows us to infer that evolutionary ontogenetic changes driven by pollinators culminate in the enclosure of flowers inside the receptacle, as occurs in the Ficus syconium.

Floral development of Moraceae species with emphasis on the perianth and androecium

The mulberry and fig family (Moraceae) has tiny flowers with a single whorled or absent perianth and inner whorls with a reduced number of organs. Relevant questions about this reduced floral structure still remain unresolved. Thus, we studied the ontogenic processes that culminate in such a reduced flower, as well as the close relationship between the floral envelope and the androecium in Brosimum gaudichaudii, Castilla elastica, Clarisia ilicifolia, Maclura tinctoria and Morus nigra. Flower buds and flowers were processed for surface (scanning electron microscopy) and anatomical (light microscopy) analyses. The perianth consists of sepals, except the flowers of B. gaudichaudii and C. elastica (staminate), in which the perianth is lacking. The staminate flower of B. gaudichaudii exhibits a bracteole enveloping the stamens. In monochlamydeous flowers the order of sepal initiation is helical or asynchronous. No petals are initiated. The calyx displays different degrees of synorganization being united only basally or along its entire extension. The sepals can have tector and glandular trichomes or only tector trichomes. Laticifers, phenolic and crystalliferous idioblasts occur in the sepal mesophyll. Stamens initiate concomitant with the pistillode in B. gaudichaudii and after initiation of the pistillode in M. tinctoria and M. nigra. In C. ilicifolia there is no pistillode and the stamens initiate after the sepals. The stamens show variation in filament shape and connective structure. The filament is inflexed in M. tinctoria and M. nigra and straight in the other species, although these structures are anatomically similar. The connective is expanded (except in M. nigra and C. ilicifolia) and has phenolic idioblasts. We conclude that the tiny flowers of these species result of two distinct ontogenetic pathways: absence of organs from the inception and by abortion, mechanisms that act to decrease the number of floral whorls as well as the number of organs in each whorl.

Traits and Resource Use of Co-Occurring Introduced and Native Trees in a Tropical Novel Forest

Novel forests are naturally regenerating forests that have established on degraded lands and have a species composition strongly influenced by introduced species. We studied ecophysiological traits of an introduced species (Castilla elastica Sessé) and several native species growing side by side in novel forests dominated by C. elastica in Puerto Rico. We hypothesized that C. elastica has higher photosynthetic capacity and makes more efficient use of resources than co-occurring native species. Using light response curves, we found that the photosynthetic capacity of C. elastica is similar to that of native species, and that different parameters of the curves reflected mostly sun light variation across the forest strata. However, photosynthetic nitrogen use-efficiency as well as leaf area/mass ratios were higher for C. elastica, and both the amount of C and N per unit area were lower, highlighting the different ecological strategies of the introduced and native plants. Presumably, those traits support C. elastica’s dominance over native plants in the study area. We provide empirical data on the ecophysiology of co-occurring plants in a novel forest, and show evidence that different resource-investment strategies co-occur in this type of ecosystem.

Dynamics of novel forests of Castilla elastica in Puerto Rico: from species to ecosystems.

Novel forests (NFs)—forests that contain a combination of introduced and native species—are a consequence of intense anthropogenic disturbances and the natural resilience of disturbed ecosystems. The extent to which NFs have similar forest function as comparable native secondary forests is a matter of debate in the scientific community. Little is known about the performance of individual species in those forests. This study focuses on the functional attributes of Castilla elastica NFs in Puerto Rico and on the differences between introduced and native species growing side by side in these forests. Rates of processes measured here were later compared with data from literature about NSFs. I hypothesize that juvenile plants of C. elastica in NFs have higher survival rate than those of native species and that C. elastica trees have faster biomass fluxes than native trees. To test the hypotheses, I measured survival rates of juvenile plants and tree growth and characterized the aboveground litter fluxes and storage. Although juvenile plants of native species displayed higher survival rates than those of C. elastica (53% vs. 28%), the latter was dominant in the understory (96%). Stand biomass growth rate was 2.0 ± 0.4 (average ± one standard deviation) Mg·ha−1·year−1 for the whole forest, and Guarea guidonia, a native species, exhibited the highest tree growth. Total litter fall was 9.6 ± 0.5 Mg·ha−1·year−1, and mean litter standing stock was 4.4 ± 0.1 Mg·ha−1. Castilla elastica litter fall decomposed twice as fast as that of native species (5.8 ± 1.1 vs. 3.03 ± 1 k·year−1). Literature comparisons show that the present NFs differ in some rates of processes from NSFs. This study brings unique and detailed supporting data about the ecological dynamics under mature novel forest stands. Further comprehensive studies about NFs are important to strengthen the body of knowledge about the wide range of variation of emerging tropical ecosystems. Due to the large increase in the area covered by NFs, greater attention is needed to understand their functioning, delivery of ecological services and management requirements.

Linking fruit traits to variation in predispersal vertebrate seed predation, insect seed predation, and pathogen attack

The importance of vertebrates, invertebrates, and pathogens for plant communities has long been recognized, but their absolute and relative importance in early recruitment of multiple coexisting tropical plant species has not been quantified. Further, little is known about the relationship of fruit traits to seed mortality due to natural enemies in tropical plants. To investigate the influences of vertebrates, invertebrates, and pathogens on reproduction of seven canopy plant species varying in fruit traits, we quantified reductions in fruit development and seed germination due to vertebrates, invertebrates, and fungal pathogens through experimental removal of these enemies using canopy exclosures, insecticide, and fungicide, respectively. We also measured morphological fruit traits hypothesized to mediate interactions of plants with natural enemies of seeds. Vertebrates, invertebrates, and fungi differentially affected predispersal seed mortality depending on the plant species. Fruit morphology explained some variation among species; species with larger fruit and less physical protection surrounding seeds exhibited greater negative effects of fungi on fruit development and germination and experienced reduced seed survival integrated over fruit development and germination in response to vertebrates. Within species, variation in seed size also contributed to variation in natural enemy effects on seed viability. Further, seedling growth was higher for seeds that developed in vertebrate exclosures for Anacardium excelsum and under the fungicide treatment for Castilla elastica, suggesting that predispersal effects of natural enemies may carry through to the seedling stage. This is the first experimental test of the relative effects of vertebrates, invertebrates, and pathogens on seed survival in the canopy. This study motivates further investigation to determine the generality of our results for plant communities. If there is strong variation in natural enemy attack among species related to differences in fruit morphology, then quantification of fruit traits will aid in predicting the outcomes of interactions between plants and their natural enemies. This is particularly important in tropical forests, where high species diversity makes it logistically impossible to study every plant life history stage of every species.

Studies on the seed biology of 100 native species of trees in a seasonal moist tropical forest, Panama, Central America

This study quantified various aspects of the seed biology of 100 tree species native to the seasonal moist tropical forest in the Panama Canal Watershed. Species were selected for study based on their potential for use in reforestation projects in Panama. Seeds of 32, 29, and 33 species were dispersed during the dry (DS, January–March), early rainy (ERS, April–July), and late rainy (LRS, August–December) seasons, respectively. Seed mass was correlated with moisture content (MC) but not with dispersal time, and MC was lowest for seeds dispersed in the DS. Germination of nontreated seeds ranged from 0% (6 species) to 99% and was ≥50% for 46 species. Seeds of Beilschmiedia pendula, Castilla elastica, Diphysa robinioides, Genipa americana, Hura crepitans, Inga spectabilis, Jacaranda copaia, Protium tenuifolium, Pseudobambox septenatum, and Trattinnickia aspera germinated ≥85%. Fresh seeds of 49 (52.1%) of the 94 species that germinated were nondormant and 45 (47.9%) were dormant. Only 12 species had a median length of germination time (MLG) > 120 days. MLG for seeds of species dispersed in the LRS was higher than that of those dispersed in the ERS or DS. Forty-eight species had uniform germination (standard deviation of germination time ≤ 14 days). Seed longevity ranged from 0.5 to 36 months, and long-lived (L-L) seeds tended to be larger than short-lived (S-L) or very short-lived (very S-L) ones, but not significantly so. MC increased significantly from L-L to very S-L seeds. The highest proportion of L-L seeds was dispersed in the DS and the lowest in the LRS. Results of this study are used to make recommendations for nursery production of tree seedlings to use in forest restoration projects.

Thrips pollination of androdioeciousCastilla elastica(Moraceae) in a seasonal tropical forest

Androdioecy is a rare sexual system in nature, as predicted theoretically. Among the androecious species reported so far, Castilla elastica (Moraceae) is unique in that flowers are unisexual and staminate and pistillate flowers on cosexual plants are produced on different inflorescences. In addition, inflorescence structure of staminate inflorescences on males and staminate and pistillate inflorescences on cosexes is markedly different. Staminate inflorescences on males are bivalvate, while staminate inflorescences on cosexes are "fig-like" and urceolate. Pistillate inflorescences are discoidal. The difference may reflect different roles and requirements of the three inflorescences in pollination and protection from herbivores. This study reports thrips pollination of C. elastica, demonstrated by a pollinator introduction experiment. Thrips pollination of the species may be an example of mutualism originating from plant-herbivore interactions. In the Moraceae, shifts from simple herbivores on flowers to pollinators might have occurred many times, evolving into diverse pollination systems including the fig-fig wasp mutualism. The family, of which little is known about pollination systems, provides interesting and unique opportunities to study evolution of pollination systems and roles of nonpollinating associates of inflorescences.

Growth responses of seedlings of early and late successional tropical forest trees to elevated atmospheric CO2

Seedlings of nine tropical forest tree species were grown outdoors in open-top chambers at ambient and elevated (two times ambient) CO2 concentrations at the Smithsonian Tropical Research Institute, Republic of Panama. Plants were kept individually in large pots and received non-limiting supplies of water and mineral nutrients. The enhancement of biomass accumulation at elevated CO2 increased with increasing relative growth rates (RGR) of species at ambient CO2. Early successional pioneer trees (Cecropia longipes, Ficus insipida, Castilla elastica, Antirrhoea trichantha, Luehea seemannii, Pseudobombax septenatum) grew rapidly, and, at the end of 25 to 39 day treatments, dry matter accumulation was 74 +/- 26% higher at elevated than at ambient CO2 (mean increase +/- SD, n = 6). By contrast, seedlings of three late successional tree species (Calophyllum longifolium, Tetragastris panamensis, Virola surinamensis) grew slowly and, over an 84 day period, elevated CO2 led to either no (C. longifolium, T. panamensis) or only small enhancement in dry matter accumulation (V. surinamensis). These different responses to CO2 enrichment are a result of differences in growth kinetics between early and late successional species at ambient CO2 rather than a result of intrinsic physiological differences in CO2 responsiveness between plants from these two functional groups.

LIGHT GRADIENT PARTITIONING AMONG TROPICAL TREE SPECIES THROUGH DIFFERENTIAL SEEDLING MORTALITY AND GROWTH

To characterize species differences in light-dependent mortality and growth, I transplanted seedlings of four related tree species into a range of light environments (<1% to 85% full sun) on alluvial soils at La Selva Biological Station, Costa Rica. With maximum likelihood techniques, I fit species-specific, nonlinear regression models of seedling probability of mortality and radial growth as functions of light availability for Trophis racemosa, Castilla elastica, Pourouma aspera, and Cecropia obtusifolia (Family Moraceae, latter two also classified as Cecropiaceae). Models characterizing mortality in the first year of seedling life showed significant differences among the species, both in functional forms and in model parameter estimates. All species decreased in mortality with increases in light to 20% full sun. Above 20% full sun, the mortality of T. racemosa and Castilla elastica continued to decrease, while that of Cecropia obtusifolia remained constant and that of P. aspera increased. At <10% full sun, there was a fivefold difference in mortality probability; T. racemosa had the highest survivorship, followed by C. elastica, P. aspera, and C. obtusifolia. Mortality models for the initial 4 mo of the experiment revealed significant size effects for all species except P. aspera and increased mortality under high light for P. aspera and T. racemosa. In contrast, radial growth of all species increased with light availability. The radial growth models, based upon only variation in light availability and tree size, accounted for 60–86% of the variation in growth. Cecropia obtusifolia occupied the lowest rank in radial growth at <50% full sun and the highest radial growth at light levels >75% full sun. When considered together, the mortality and growth models revealed that differences in species performance could lead to light gradient partitioning. At distinct light levels, each species showed the highest growth relative to other surviving species. Neither growth nor mortality considered alone revealed this specialization. Light gradient partitioning provides an explanation for successional dynamics and suggests that tropical tree species coexistence and diversity is at least partly maintained through niche differentiation.

Increased xanthophyll cycle activity and reduced D1 protein inactivation related to photoinhibition in two plant systems acclimated to excess light

Young and mature canopy leaves of tropical forest trees ( Anacardium excelsum, Castilla elastica) and cold- and non-acclimated leaves of spinach ( Spinacia oleracea) grown in temperate climate were used as systems of differing susceptibility to high light stress to assess the role of xanthophyll cycle activity and D1 protein turnover in photoinhibition of photosynthesis. Previous investigations with spinach revealed two distinct stages of photoinhibition (indicated by a decline in the ratio of variable to maximum chlorophyll fluorescence) which were reflected in different kinetics of recovery in low light. An initial fast recovery phase of unknown biochemical mechanism (about 1 h) was followed by a slow phase (several hours), which may be based upon resynthesis of the D1 protein in the photosystem II reaction center. All leaves studied exhibited these two phases. However, the kinetics and relative amplitude of the two phases varied strongly and were dependent on leaf age and light acclimation. Recovery was rapid in young canopy sun leaves and cold-acclimated leaves of spinach due to a pronounced initial phase. On the other hand, the slow, probably D1 related phase dominated in mature canopy leaves and non-acclimated leaves of spinach. The fast phase of recovery and epoxidation of zeaxanthin via the xanthophyll cycle were closely correlated in all leaves studied. In addition, recovery following photoinhibition in the presence of dithiothreitol, which prevented formation of zeaxanthin, occurred only in a slow phase. On the other hand, leaf incubation prior to photoinhibition with streptomycin, an inhibitor of chloroplast-encoded protein synthesis, eliminated slow recovery by preventing resynthesis of the D1 protein. The young tropical sun leaves and the cold-acclimated spinach leaves exhibited a higher pool of xanthophyll cycle pigments per chlorophyll and in response to strong light converted a higher percentage of violaxanthin to zeaxanthin than the mature and the non-acclimated leaves, respectively. Quantification of D1 protein in photoinhibited spinach leaves by means of electrophoresis and Western blotting revealed a strongly diminished D1 protein degradation in the cold-acclimated compared to the non-acclimated state. High xanthophyll cycle activity and pool sizes observed in response to high light with young canopy and cold-acclimated spinach leaves may serve to protect these leaves against inactivation of the D1 protein.

Relations Between Mineral Nutrient Availability and Fine Root Biomass in Two Costa Rican Tropical Wet Forests: A Hypothesis

Relations Between Mineral Nutrient Availability and Fine Root Biomass in Two Costa Rican Tropical Wet Forests: A Hypothesis

Chemistry and Structure of Natural Rubbers

Abstract Natural rubbers are synthesized in thousands of species covering most plant families; moreover, rubber has been obtained commercially from over a dozen of them. A later report has suggested that polyisoprene formation is limited to dicotyledonous angiosperms. In recent years there has been renewed interest in developing alternative sources of natural rubber, different from Hevea brasiliensis. In particular there has been considerable development of guayule as a source of natural rubber. As a result there is a considerable body of recent results, using new techniques and extending older work concluded in the 1950's, which give new insight into the production of rubber in plants. This paper will mainly deal with guayule because it can ideally serve as a background for understanding the production of rubbers in other plants, including hevea. First, it is necessary to describe the physical nature of the rubber from guayule and where in the plant it is formed. Figure 1 shows a cross-section of a stem of guayule. The individual cells (parenchyma cells) are the site of rubber manufacture. The rubber can finally be seen as the small latex droplets inside a single cell (Figure 2). Thus the study of the natural rubber in guayule involves the isolation of the latex particles from the cell. It is extremely easy to separate an industrial rubber from other parts of the plant, including resin, by either laboratory or pilot plant methods. However, it is more difficult to get fresh latex in which no additional particle coagulation has occurred. The description of the average latex particle size and the distribution of sizes has been made and is needed to discuss the biosynthesis of rubber. It will not be discussed further. The question addressed in this paper is the chemical structure of the rubber in the plants—the micro-structure of the rubber, the molecular weight distribution, and the branching or gel content. Guayule natural rubber has the remarkable feature of containing no gel when taken directly from a fresh plant and carefully handled in the laboratory. Even the material from the pilot plant extraction process contains only 1–3 per cent gel. This paper describes extensively the chemical structure of natural rubber extracted from fresh guayule plants. Less extensive measurements on chicle, gutta-percha, and castilla elastica are also reported. Finally, all of the data are analyzed to determine if a model of rubber production can be formulated to encompass all of the above plants. Earlier, a model was proposed for guayule only. In this model the polyisoprene chain is polymerized on the polymerase enzyme site by recurring attachment and intermittent polymerization until the rubber droplet (containing a single polymer chain) can no longer be stabilized or further polymerized by the enzyme.

X-Ray Diffraction Patterns of Hevea, Manihot and Other Rubbers

Abstract When rubber from Hevea brasiliensis is stretched quickly and exposed to a beam of x-rays, a crystal fiber diffraction pattern is obtained.” During previous work, done jointly by the National Bureau of Standards and the University of Illinois on sol and gel fractions of this rubber, no pattern was produced by the stretched sol fraction. With the stretched gel fraction, however, the pattern was sharp and intense. Rubber from Manihot glaziovii also may be separated into sol and gel fractions, and in recent work they, too, have been stretched and examined by x-rays. With the stretched Manihot sol, a few interferences were obtained, but many more resulted with the stretched gel. When crystallized by “freezing” at low temperatures for 24 hours, the interferences from the gel again outnumbered those from the sol. The x-ray measurements of Manihot rubber agree with those of Hevea rubber and indicate that the same structure exists in each. Rubbers obtained from latices of Funtumia elastica, Cryptostegia grandiflora and Castilla elastica were also examined. Their structures, too, were like that of Hevea rubber, and the similarity confirms earlier observations made when x-ray technique was less highly developed. In crystallized specimens, the crystals melted within the range of temperatures previously observed with Hevea rubber. In earlier work a large spacing was observed in a specimen of unstretched Hevea gel rubber. This spacing has been observed also in a new specimen of stretched Hevea gel rubber, together with a second spacing approximately twice as large.