Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

Characters of seedlings from ten species of the genus Bursera were studied, in order to find additional taxonomic evidence. All species showed phanero-epigeal germination, developing a primary root system (axonomorphous root). Species included in section Bursera have a root swelling at basal level near the collar, and a slim hypocotyl. The cotyledonary leaves were multilobate in the ‘cuajiotes’ species group and three-lobed in species of B. simaruba complex. The species of Bursera sect. Bullockia showed a not swollen primary root system with a slim collar; a striate, lenticelled and swollen hypocotyl, and three-lobed cotyledonary leaves. There are four characters that can be considered important for phylogenetic analyses: (1) protophyll margin type, (2) protophyll venation, (3) form of cotyledonary leaves, and (4) the swolliness of the radicle. Groups like section Bullockia, the ‘cuajiotes’ and ‘mulatos’ are supported upon the distribution of characters.

With methods of light microscopy, histochemical staining and scanning electron microscopy, it was found that the ovule in the seed of Tigridia pavonia (Iridaceae) is anatropous, bitegmic, and crassinucellate. During development, the exotegmen is crushed and the endotegmen persists with tannins in the lumens and in the walls, which also react positively for lignin. The exotesta contains tannins and its outer walls are convex, thickened, and cuticularized. The mesotesta has multiple layers, accumulates abundant lipids, and forms a bulge in the chalaza. The cell walls of the endotesta collapse and accumulate tannins. In the chalaza, a hypostasal cushion contains tannins in the lumens and in the walls, which also react positively for lignin. At the micropylar end of the seed there is an operculum which consists of: a) a slightly crushed exotegmen, b) an endotegmen with cuticular thickenings that are concentric with respect to the micropyle, c) hemispherical deposists of cutin on the anticlinal walls of the endotegmen, and c) a thin layer of endosperm that covers the radicle. During its cellular stage of development, the endosperm has conspicuous transfer walls at the chalazal end next to the nucella. The embryo is small and has a conical cotyledon.

A floristic inventory of the cloud forest of some localities of Landa de Matamoros municipality, Querétaro, was undertaken; this area is located in the Sierra Madre Oriental and is part of the Sierra Gorda Biosphere Reserve. In this zone the forest is  fragmented, but it has a relatively good conservation status in some localities such as Camarones, Neblinas, La Yesca, and La Joya del Hielo. A floristic list of vascular plants composed by 130 families, 465 genera and 774 species was obtained. A  distribution map of this vegetation type is included.

In the San Felipe desert of Baja California 324 species of vascular plants were recorded which belonged to 206 genera in 68 families. Of these, 9 are endemic species. The families with the greatest diversity were Asteraceae (42 species) and Fabaceae (30 species), which respectively accounted for 13.0% and 9.3% of the total flora. With 129 species, annual herbs were richest growth form including 129 species, followed by suffrutices (57), perennial herbs (55), shrubs (40), succulents (22), vines (8), trees and rosettes (5 each), and parasites (4).

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The aim of this paper is to review and assess scientific literature relevant to arbuscular mycorrhizal fungi (AMF) population dynamics: reproduction and dispersal mechanisms, and establishment and distribution in natural ecosystems. Arbuscular mycorrhizal fungi (AMF) are obligated root symbionts with an extraordinary capacity for growing, dispersing and surviving, but their life history is not well understood yet. Although there is information concerning AMF dispersal, distribution and establishment, some data are still ambiguous and contradictory. Arbuscular mycorrhizal fungi (AMF) life cycle responds to surrounding environment but even when they only reproduce asexually, their populations encompass a high genetic and functional diversity within ecosystems. Environmental disturbances create new habitats for AMF dispersal, which can take place by root to root contact, animal transport and erosion processes. These agents are the common dispersal mechanisms of AMF in nature. Distribution of AMF is influenced mainly by the environment and soil and plant communities, but the specific AMF-host plant association is thought to be secondary because the AMF must invade and colonize any host root for establishment. Thus, inoculum density, host and fungal genetic compatibility, edaphic factors and plant-microbial activity determine the formation of mycorrhizal infections.

The Calakmul region, at the center of the Yucatan peninsula, contains the largest forested area of the Mexican tropics. Our objective is to provide the classification, description and spatial distribution of the plant communities of Calakmul region. In spite of the relatively lack of topographic features there is large spatial heterogeneity in the vegetation. Five plant associations are underlined because of their regional, national and world relevance: guayacán forest (Guaiacum sanctum), jobillo forest (Astronium graveolens), low deciduous forest, tall forest and mixed seasonally flooded forest. We discuss the main factors influencing the vegetation, including rainfall gradients, soil development, natural disturbances and anthropogenic factors. This study underscores the importance of using vegetation classification with enough detail to assess the representation and effectiveness of natural protected areas.

The climate in which the Pliocene plant community of Santa María Amajac, Hidalgo, developed, was inferred using Climate Leaf Analysis Multivariate Program (CLAMP), which correlates leaf morphology with climate. The environmental parameters calculated were: mean annual temperature: 16.7 ??1.2?C; warmest month mean temperature: 26.4 ??1.6?C; coldest month mean temperature: 7?C; growing season length: 9.2 months; mean growing season precipitation: 72 mm; three wettest months: 280 mm; three driest months: 62 mm: mean annual precipitation: 650 ??120 mm; relative humidity: 58%; specific humidity: 7.8 g/kg, and enthalpy: 309.5. These values suggest that the climate was a subhumid, temperate one, Ca(w)(w0)(e’).

A survey of the diversity, distribution, and conservation status of the family Lauraceae in southern Mexico was conducted. The diversity account resulted in a list of 120 species representing 10 genera: Aiouea, Beilschmiedia, Cassytha, Cinnamomum, Licaria, Litsea, Mocinnodaphne, Nectandra, Ocotea and Persea. The states of Chiapas, Oaxaca, and Veracruz have the largest numbers of species, particularly for the cloud forest and the tropical rain forest. Endemic species account for 47.5% of the surveyed taxa, which concentrate in Chiapas, Oaxaca, and Veracruz. Only 58 of the species considered in the study are present in the system of nature reserves in southern Mexico. The general implications of these findings for the persistence of this component of the Mexican flora are discussed.

Architectural models help to describe the development and build-up of tree crowns. This knowledge is indispensable for understanding the behavior of trees in the forest and is useful in studying the evolutionary relationships between taxa. This contribution presents a list of 108 tree species from the tree flora of the Yucatan Peninsula and their corresponding architectural models. These species represent 25% of the total number of species and of 68% of the families in the tree flora of the Peninsula. A total of 15 different architectural models were recognized. Data were acquired from field observations and literature study.