BOTN 201 - Department of Biology - TAMU
Images of the Navasota Flora - 14 May 1995
The genus Physostegia (Lamiaceae/Labiate) includes 12 species that are limited to North America. Seven of these are found in Texas and this one, P. pulchella is a state endemic, i.e., its range of distribution does not extend beyond Texas. On side view the flower is typical of the Lamiaceae, with the bilabiate, sympetalous corolla and unit calyx. Basic zygomorphy is evident from the pollen vector's perspective with the androecium positioned above the corolla tube. After pollination and fertilization, the gynoecium matures within the persistent calyx. The four-parted fruiting structure of the mint family is evident if the calyx is dissected away.
Another member of the Lamiaceae that is now coming into flower throughout the Navasota Valley, Monarda punctata, takes a different tack for pollinator attraction. While the flowers show a general structure that is typical of the family, the corollas - not brightly colored - are nearly hidden within a series of colored bracts. In this case, the bracts and verticillate inflorescence (a feature common to many mints) produce a display that attracts pollen vectors.
From the human perspective, the most important family of flowering plants in the Poaceae (Gramineae) or grass family. Given the common vegetation type of the Navasota Flora, Post Oak Savannah and Blackland Prairie the family is also of primary importance from an ecological point of view. Grass identification is based on the spikelet, clusters of flowers or a single flower subtended by a series of bracts. Ryegrass (Lolium perenne) produces a distinctive array of spikelets that are two-ranked (placed on opposite sides) and sessile (directly attached with no stalk). A close look at the Lolium spikelet reveals a series of florets (flower with subtending bracts - lemma and palea that are subtended by two 'sterile bracts', the glumes. Lolium has only two species in Texas, both introduced from Eurasia. This species is often planted to stabilize disturbed soil. It will then 'escape' to inhabit other areas and thereby displace native species. Given the abundance of L. perenne in the Navasota Flora, its ecological impact on the native flora is significant.
Another conspicuous element of the Poaceae now coming into flower is the genus Bromus (Bromegrass or 'Chess'). This is a larger taxon, ca. 150 species, with 15 in the Texas flora, some of them native. A close look at the Bromus spikelet reveals a structure identical to that of Lolium, i.e., it is composed of a series of florets (flower with subtending bracts - lemma and Palea that are subtended by two 'sterile bracts', the glumes. In contrast to those of Lolium, Bromus spikelets are not sessile. Thus, the Bromus inflorescence is a lax, diffuse, often drooping contrast to the erect, tight, planar aspect produced by Ryegrass.
Return to the Navasota Flora Homepage
Return to the BOTN 201 Homepage
Last updated by HDW on 16 May 1995