2003 - Vegetative anatomy

Note: The translation is an unofficial document provided for information purposes only. The official source is the original publication: https://www.lankesteriana.org/indiceGENERAL.htm

The subfamily Cypripedioideae includes the genera Cypripedium, Selenipedium, Phragmipedium, Paphiopedilum and Mexipedium (Albert 1994). The latter was recently identified as a monospecific, xerophytic genus, with a rupicolous habit and is endemic to Oaxaca, Mexico (Albert and Chase 1992). Numerous papers with diverse approaches have been published on the descriptive and systematic vegetative anatomy of the first four genera (Albert 1994, Arditti 1992, Atwood and Williams 1978, 1979; Lawton et al. 1992, Pridgeon 1981, 1986, 1987; Pridgeon et al. 1983, Rosso 1966, Williams 1979). However, the anatomy of Mexipedium xerophyticum had not been elaborated.

The present study aims to present the anatomical description of the mature vegetative organs of Mexipedium xerophyticum; to give an ecological interpretation of some anatomical characters and to compare them with the anatomical data previously presented for the other genera of the same subfamily. Leaf, stolon and root samples of M. xerophyticum (G. Salazar 3740) were fixed in a solution of formalin-alcohol-acetic acid-water, to be processed according to conventional histological techniques of kerosene embedding. 43 qualitative and quantitative characters of the dermal, fundamental and vascular tissues of each organ were analyzed. Observations and photomicrographs were made under an Axioskop-Carl Zeiss photomicroscope, measurements were taken with a micrometer eyepiece adapted to the microscope. The images obtained were digitized and subsequently edited with Paint Shop Pro 7.5 software. The details of the root velamen and exodermis were observed under a scanning electron microscope.

The anatomical description of this species includes qualitative characteristics; the quantitative values recorded represent the arithmetic mean of 20 measurements for each characteristic. These results were compared with data from other authors for the rest of the genera of Cypripedioideae.

The distinctive anatomical characteristics of Mexipedium xerophyticum are: At the leaf level, cells of the abaxial epidermis not differentiated into costals and intercostals, an inconspicuous central vein and absence of endodermal cells in the sclerenchyma sheath of the vascular bundles; at the root level, the presence of spongy tylosomes. Most of the anatomical characteristics of M. xerophyticum are interpreted as xeromorphic, which allows it to withstand conditions of high physiological or environmental water stress. These xeromorphic characters are: erect leaf position, reduced leaf size, leathery texture, presence of small scales and waxy plates as well as a thick cuticle, abundant but small stomata only on the abaxial surface and semi-sunken, with external cuticular ridges as a cuticular collar forming an insulating chamber above the stomata. It has been established that these hyperstomatic cavities reduce transpiration by increasing the pathway along which water vapor must diffuse to leave the leaves and trap stagnant moist air outside the stomatal pore. The development of these cuticular ridges seems to predominate in dry habitats (Sinclair 1987). Large adaxial epidermal cells with thickened walls, specialized for water storage, and a thick cuticle are also adaptations to xeric conditions (Arditti 1992).

The xeromorphic features of the roots are: abundant trichomes; velamen with a high number of strata, numerous perforations between its walls, making the transport of water to the internal tissues more efficient and allowing the free passage of air, water or nutrients through the tissue. Spongy tyloses, exodermis and endodermis with thick-walled, strongly lignified cells and a parenchymatous medulla are other characteristics that allow the species to adapt to dry environments.

Mexipedium xerophyticum grows at ground level, where the incidence of light is usually low and diffuse. The presence of micropapillae as epidermal ornamentations allows a better light capture by different oblique angles; the leaves are erect, so the greatest light capture is carried out on the abaxial surface and it is here where a better development of micropapillae was observed. The parenchyma cells of the abaxial end of the mesophyll have the greatest number of chloroplasts. These two traits appear to be an adaptive advantage for greater and more efficient photosynthetic activity. This position of the leaves is due to the development of structures that allow additional mechanical support, while at the same time resisting strong winds to which the plant is exposed. This is made possible by the development of thick anticlinal walls in the epidermal cells, a relatively high number of vascular bundles per millimeter, and each bundle with a sclerenchyma sheath at both ends. In addition to mechanical strength, the greater number of bundles allows for greater and more efficient water transport in the plant, which is necessary to respond to short periods of water availability. The presence of abundant calcium oxalate crystals in the leaf mesophyll and stolon pith is due to the fact that Mexipedium xerophyticum is a plant that grows in rocky soils with high carbonate and limestone content.

Albert and Pettersson (1994) mention that the molecular and morphological similarities between Paphiopedilum sensu stricto, Phragmipedium and Mexipedium are more prominent than their differences. From the anatomical analysis, it can be stated that these same similarity relationships are found among the three genera. The characters shared by these three genera are: thick leaves, conduplicate vernation, inconspicuous lateral veins, undifferentiated adaxial epidermal cells such as costals and intercostals, epidermal cells with straight anticlinal walls, epidermal cells with thick outer tangential walls, large cells in adaxial epidermis, hypostomatic leaves, presence of micropapillae in the abaxial epidermis of some of its species, lower proportion of mesophyll in the leaf, fleshy root, Calanthe-type velamen, more than four strata in the velamen, presence of filamentous fungi in the velamen; no evident content in the cortex cells and 8 to 17 arches in the root vascular tissue. In addition Mexipedium shares with Phragmipedium the presence of a thick cuticle, absence of trichomes and the presence of tylosomes although they are also present in Paphiopedilum fairrieanum. Mexipedium shares with Selenipedium the absence of tessellated leaves, the presence of a single stratum in the root endodermis and cells of the endodermis with U-type thickening. Mexipedium shares with Cypripedium the absence of tessellated leaves and the presence of thin-walled pericycle cells (except in C. irapeanum).