The modern flora of cycads includes eleven genera and about 100 species distributed across the warm, subtropical environments of the Americas, Africa, and Australasia. Virtually all extant genera have restricted geographical ranges, which reflect both relict and endemic distributions. Cycads have a variety of ecological ranges; they occur as understory plants in tropical and subtropical rain forests, or thrive in dry, low nutrient regions. Yet despite their broad geographic and ecological ranges, it is likely that in the next geological period, cycads will join Ginkgo in extinction.
Fairchild Botanical Gardens in Florida maintained one of the world's most comprehensive collection of living cycads including many that are endangered in the wild. Fairchild Gardens was severely hit by hurricane Andrew in 1992 and most of the non-native plants, including many cycads, were severely damaged or killed. A botanical SWAT team from Missouri Botanical Gardens was helicoptered in to Fairchild as the storm clouds were clearing. Their goal was to save as many of the plants as possible and to preserve the rest for future study. Today, one of the most important captive cycad collections is Lotis Land in Santa Barbara, California. Begun as a private collection, this garden is now open to the public and researchers. It is home to several plants of one species for which only pollen-bearing individuals are known.
You can study modern cycads, both in lab and around campus (VG 1:10). Notice the short stout, unbranched stems, armored with persistent leaf bases. Cycads bear their pinnately compound leaves in a crown around the apical meristem, like a tree-fern or a palm. Their internal stem structure is characterized by a eustele with endarch protoxylem, where a small amount of manoxylic wood is produced from a bifacial vascular cambium. As the leaf trace arises from the primary xylem sympodium is dichotomizes and the two branches set off in opposite directions. The pair of traces circumscribe (girdle) the stele as they transverse the cortex, meet again before entering the petiole. These girdling leaf traces are a unique and useful identifying characteristic of cycads, and a feature that certainly distinguishes them from the Medullosans.
Just like Medullosans, the cycads represent a "natural group". Cycads first appeared in the Pennsylvanian with a number of problematical fossils. Spermopteris, for example, is characterized by two rows of ovules attached to the abaxial surface of Taeniopteris, a common simple leaf type in strata of Upper Paleozoic age. This plant has been allied with the cycads mainly on the basis of foliar features, including haplocheilic stomata (Figure 8.6), in which the guard mother cell gives rise to only two guard cells. Venation patterns and a non-bifurcating leaf-base also represent derived characters shared between Taeniopteris and cycads. Haplocheilic stomata occur in several other types of seed plants, for example conifers, ginkgoes, Ephedra, glossopterids, and cordaites. So, is this a good synapomorphy? Nevertheless, it is key when distinguishing cycad foliage from that of another group of plants with a cycad-like growth form that you will study in a couple of weeks -- the "Cycadeoids" (Benettitaleans). Foliage believed to have been produced by cycads is commonly pinnately compound (e.g. Nilssonia (VG 1:11)), but also include some simple, entire leaves, e.g. Nilssonia and Taeniopteris (VG 1:12). In living cycads, note the once-pinnate structure of leaves and pinnae venation patters. The leaves are quite tough, due to the thick hypodermis present on both top and bottom of pinnae lamina. What might be the roles of this reinforced structure?
|Figure 8.6: Haplocheilic stomata in cycads.
During the Permian fossil genera with greater resemblance to living cycads begin to appear, with e.g. Crossozamia. Among these are taxa that produce megasporophylls in a helical arrangement similar to the simple ovulate cones of extant Zamia. The megasporophylls have ovules (VG 1:13) attached to their lower surface. This early co-occurrence of simple and relatively complex reproductive structures has lead to some controversy about the polarity of this character among modern cycads. It also implies an earlier origin for some of these characters than was previously thought.
The cycads had their heyday during the Mesozoic, and have been declining in species richness and ecological importance ever since. Many cycad genera from this the Mesozoic have been reconstructed from fairly complete fossils, for example Leptocycas (Figure 8.7). Based on these reconstructions, some workers think that the Mesozoic cycads in general had relatively slender trunks with widely spaced leaves that abscised, and that cycads with the short, thick stems we recognize today did not evolve until the Tertiary.
|Figure 8.7: Reconstruction of Leptocycas gracilis from the Late Triasssic. Living plants may have been about 1.5 m tall.
The microsprophylls of cycads are arranged in strobili and bear clusters of microsporangia on their abaxial surface (VG 1:14). Since ovulate structures are more commonly preserved that the pollen organs, only a few fossil microsporophyll have been described. Lasiostrobus is a relatively modern-looking cone from the Pennsylvanian consisting of helically arranged microsporophylls that has been tentatively interpreted as cycad in origin. However, it also shows features reminiscent of both conifers and ginkgoes.