Download Laboratory 9 - Paleobotany of Lower Vascular Plant Groups I

22
Laboratory 9 - Paleobotany of Lower Vascular Plant Groups
I. Lycophyta
A. Herbaceous representatives
1. Lycopodites
The genus Lycopodites includes eligulate lycopods with helically-arranged microphylls (either
isophyllous or anisophyllous) and disperse or compact strobili similar in organography to
Lycopodium. Observe the specimen from the Carboniferous of Kentucky and determine the mode of
preservation. Is anatomical deta1il present? How can this genus be distinguished from terminal axes
of the arborescent lycopods? Specimens, such as this one represent much of the known
morphological diversity of the living genus Lycopodium, are found from the early Carboniferous to
the present.
2. Ligulate forms
The extinct genus Selaginellites represents bisporangiate strobili similar to the living genus
Selaginella and have been found connected to the stem genus Paurodendron. Although
representatives of these genera are not available in this laboratory, it is important to note similarities
in the presence of ligules, meristelar organization of the stem and the presence of a hollow chamber
in the center of the stem. Representatives of the form genus Isoetites, also unrepresented here,
consist of sporophylls which strongly resemble those of the living genus Isoetes. The oldest
specimens apparently date back to the Triassic. What implications would this have for the postulated
ancestry of Isoetes from Pleuromeia and Nathorstiana?
B. Arborescent representatives
The arborescent lycopods were tall, heterosporous, ligulate plants which apparently dominated the
coal-forming swamps of the early Carboniferous. In gross organography, these plants possessed a
number of unique features. The most conspicuous of these is the leaf cushions which bore the long
microphylls found attached to axes of Lepidodrendron, Sigillaria, and Lepidophloios, among other
genera. The organization of the leaf cushions on the stem after extensive secondary growth suggests
that leaf cushions were capable of continued growth throughout the life of the plant. Observe the
organization of the leaf cushions of Lepidodendron both externally by observing compressed
specimens and internally by studying coal ball peels. What is the organization of the parichnos?
What is their function? Observe the location of the ligule. Is the function of the ligule any less
enigmatic in fossil lycopods? Examine the compressed terminal axes of Lepidodendron and note the
external similarity to Lycopodites.
The internal anatomy of the arborescent lycopods was marked by extensive secondary growth,
producing large amounts of secondary growth visible to the exterior of the protostelic center of the
stem. Although phloem has been observed in such stems, it is minute in comparison with the xylem.
This has been taken as an indication that the vascular cambium of these plants was typically
unifacial--producing xylem, but no phloem. The wood of lycopods is distinctive because of the
presence of fimbrils between the scalariform thickenings, which are also known as Williamson
23
striations. Rays were poorly developed as evident in the coal ball peels before you. Examine the
specimens available. What degree of development did the cork cambium express?
The underground portion of the stem is also entirely dichotomous, and originates from a root stock
known as Stigmaria, a commonly found root compression genus. The roots as in Isoetes bore roots
in a definite rhizotaxis, with the rootlets in extinct species commonly similar in structure to the living
genus. In these extinct species, it is even more evident that it is possible that the roots of these
arborescent forms arose in a manner similar to stems, and perhaps possessed an entirely different
evolutionary history than did the roots of derivatives of the Rhyniophytes. Observe the rootlets of
Stigmaria on display and the pictures of several spectacular stumps in your textbook and elsewhere.
The reproduction of the tree lycopods is particularly fascinating since it appears to reflect many
trends encountered in achieving the seed habit. The vast majority of the lycopods were monosporic,
possessing either microspores or megaspores. An exception to this is Barinophyton which has
identifiable spores of both morphotypes present in the sporangia. Although in all cases, the strobilus
was organized into sporophylls bearing sporangia, the extent of dimorphism between sterile
microphylls and the sporangium-bearing sporophylls was variable. Also variable was the degree of
reduction of the number of megaspores produced in the megasporangium. The most extreme cases of
reduction in spore numbers in the female occurred in Lepidocarpon, where only a single, large
functional megaspore remained after megasporogenesis. Whether the megaspore was capable of
fertilization while on the tree is not well-known, but it is evident that the resulting megagametophyte
was similar in many ways to previously discussed heterosporous lycopods. The presence of a line of
dehiscence near the apex of the megaspore and the obvious lack of true integuments differentiate
these structures from seeds; however, the trends are evident and striking. A trend for protection of
the megasporangium in these lycopods relied primarily on the elaboration of the laminae of the
sporophyll and reached various degrees of completion and complexity.