A Synopsis of Pterostemon (Iteaceae), a Group Endemic to Mexico

The genus Pterostemon Schauer (Iteaceae), is a group of three species restricted in distribution to Mexico. It includes: Pterostemon mexicanus Schauer, P. rotundifolius Ramírez, and P. bravoanus J. Jiménez-Ram. & Martínez Gordillo. On the basis of field observations and herbarium specimens, we review the morphology (vegetative and reproductive), and distribution to give an updated taxonomic treatment for the genus. Additionally the utility of morphological characters is evaluated for taxomic use, and detailed taxonomic descriptions and ecological associations are given for each species.

Ptersotemon is distinguished primarily by the combination of its woody habit, pubescent stems with lenticels, leaves with stipules, production of resin, the presence of a pubescent hypanthium, 10 stamens (5 fertile and 5 staminodia) with flattened filaments and toothed apices, and the capsular fruit containing one or rarely two seeds with endosperm.The most important characters to separate different species are the shape of the leaves, the size of the petioles, the number of flowers per inflorescences, the petals and their position during anthesis.
Natural populations of the three species of Pterostemon are usually small and highly localized, and even though usually a large proportion of flowers can be seen, in at least one of the species the production of seeds is very scarce and the establishment of seedlings is over all very low.

Material and Methods
In order to review and consolidate the taxonomic information available for the genus, herbarium material was reviewed from the herbaria ENCB, FCME, IEB, MEXU, MO, TEX, and XAL.In addition, field work was conducted to allow for collection of fresh material as well as recording of direct observations of the species in the wild.The field work was conducted in the states of Puebla, Hidalgo, Oaxaca, and Guerrero.Plant material was collected for further analysis under the Scanning Electron Microscope, phytochemical as well as embryological studies.For all the species in the genus, a taxonomic description, as well as phenological and environmental data are included.

Conclusion
The genus Pterostemon Schauer is included into the Iteaceae, a paleo-endemic family restricted to Mexico.It comprises three species of woody shrubs or small trees in arid and semi-arid regions.Pterostemon bravoanus is distinguished by presenting leaves with elliptic to orbicular lamina, inflorescences with more than 50 flowers, petals erect at anthesis, pollen spheroidal, and their distribution is restricted to the state of Guerrero; P. mexicanus has leaves with obovate to widely-obovate lamina, inflorescences with fewer than 20 flowers, petals extended at anthesis, pollen prolate, and distribution in the states of Guanajuato, Queretaro, Hidalgo, San Luis Potosi, and Puebla; P. rotundifolius has leaves with suborbicular to oblong-orbicular lamina, inflorescences with 2 -50 flowers, petals reflexed at anthesis, pollen spheroidal, and is restricted in distribution to the states of Oaxaca and Puebla.In all three species the fruit is an indehiscent capsule with a solitary seed.

Figure 3 .
Figure 3. Cellules at the base of the trichomes of Pterostemon mexicanus, adaxial surface (from Cruz 5999 at FCME).tropous, bitegmic; tegmens of epidermic origin, tristratified.Fruit an indehiscent capsule, floral whorls persistent (Figure 11).Seeds 1, rarely 2, ovoid, maroon.Distribution and habitat.Pterostemon along with the family Iteaceae is a paleo-endemic restricted to Mexico.It is found in arid and semi-arid regions of central and southern Mexico.It comprises three species with narrow and scattered populations in the states of Querétaro, Hidalgo, Puebla, Oaxaca, Guerrero and San Luis Potosí.It can be found growing in well drained calcareous soils, in dry scrublands, tropical deciduous forest, Quercus forest, and Pinus and Juniperus forest between 500 and 2350 meters above sea level.

Figure 6 .
Figure 6.Exudates from glands in the adaxial surface of the leave of Pterostemon bravoanus (from Cruz & Ponce 5899 at FCME).

Figure 7 .
Figure 7. Spherical gland in the adaxial surface of the leaf of Pterostemon rotundifolius (from Vega 2 at FCME).