Anatomical descriptions can be effective for solving systematic issues, but these studies are relatively scarce for cycads.
Therefore, we present here a leaflet and cuticle anatomical study on the genus Dioon, to provide a set of epidermal traits that clarify species delimitation and relationships between species and their habitats.
We used standard micro-technique for leaflet sectioning, and cuticular peel preparation for light microscopy. Also, we used the chromium trioxide method for scanning electron microscope observations on cuticles. Measurements were taken on 10 randomly chosen replicates of each cell or tissue type, for each of the leaflets sampled per taxon.
Micromorphological variation among species was calculated for each trait. Finally, we reconstructed the ancestral states of the observed epidermal fibre-like cell and pore shapes, by tracing the characters on the species phylogenetic tree of Dioon. We were able to describe the leaflet anatomy, cuticles, and epidermal features for 14 Dioon species.
The quantitative analysis was useful to reveal five geographically structured species groups. Character tracing on the phylogenetic tree of Dioon has amplified our current understanding on species relationships with respect to habitats.
The presence/absence data suggest that the evolutionary acquisition-deletion of structural shapes is phylogenetically independent, thus climate seems to play a very important role in the variation of cuticular and stomatal traits.
Many epidermal traits, especially adaxial cuticle thickness and epistomatal pore width and depth, might be adaptations resulting from a long-term influence of climate, since they appear to have correlation with climatic conditions in relation to their biogeography.
We conclude that the variation of all traits are mostly sustained and intrinsic to the species, and are of promising taxonomic value. The combination of the epidermal traits with other characters has potential for taxonomic resolution at species level.
A morphological study of the leaflets of a hybrid cycad obtained from a cross between Ceratozamia mexicana and a new species, Zamia monticola, shows that some of the structures distinctly resemble either one parent or the other, and that other structures show characteristics of both parents.
The stomata illustrate the latter point clearly, as they combine distinctly the peculiarity found in the contour of the guard cells in both parents.
A scanning electron microscopy (SEM) study of Cycas cuticle characteristics was undertaken in order to expand our knowledge of microscopic characters not observable under light microscopy and to clarify unresolved affinitites among some species within the genus.
Whole leaf and isolated cuticle specimens from the middle region of leaflets of greenhouse-grown plants of Cycas revoluta, Cycas rumphii, Cycas circinalis, Cycas media, and Cycas normanbyana were examined using SEM for interior and exterior features.
Characteristics in common include hypostomy, hair bases on abaxial and adaxial surfaces, adaxial cells randomly arranged, adaxial exterior cuticle smooth, and stomata sunken to various degrees but stomatal pit always formed by two layers of epidermal cells.
Stomatal complex is of the polyperigenous type.
Stomata randomly dispersed and oriented, and except C. revoluta, are not contiguous.
Stomata deeply sunken in C. revoluta, intermediate in C. rumphii and C. normanbyana, and less sunken in C. circinalis and C. media.
Aperture between guard cells extends the entire stomatal length in C. rumphii and C. normanbyana, ∼80% in C. circinalis and C. media, and ∼50% in C. revoluta.
Cuticular features of C. revoluta show the greatest difference from the other species in complex relief of exterior cuticle and interior cuticular structure of subsidiary cells; C. media and C. circinalis show close similarity to each other and their stomatal complex dimensions fall within the same unique cluster using principal component analysis under normalized variables.
C. normanbyana and C. rumphii show the most similarity to each other in cuticular micromorphology. Stomatal complex dimensions of these two species fall into a second cluster that also includes C. revoluta. These data contrast with current taxonomy placing C. normanbyanasynonymous to C. media.
* Detail of the author’s work : Xylotomy and leaf epidermis of recent Cycadaceae, in preparation now –
We hope to offer some help to botanists dealing with recent Cycadaceae but also to paleontologists, when attempting to determine the individual genera by the structure of leaf epidermis on the basis of the following key of determination and two photo-plates. The magnification of the leaves of characteristic representatives of the individual genera (both from surface an d backside) is uniformly 300X .