CLADE 2: ASTEROXYLALES (= DREPANOPHYCALES)

These plants are extinct with a fossil history in the Devonian period. Stalked homosporous sporangia are scattered on the axes rather than on microphylls (see Figure 3). They do not have true roots in that there is no anatomical distinction between the stele of the stem and of the root. The leaves can barely be classified as microphylls in that vascular tissue invades only the base of the leaves. Asteroxylales is a sister group to the rest of the lycopods. Thus, characters of the sporangia, leaves, and roots likely are primitive. Three genera are known from this phylum: Asteroxylon (Figure 3), Drepanophycus, and Baragwanathia.

CLADE 3 SPOROPHYLLOUS LYCOPODS

All members of this clade have stalked sporangia that occur on the adaxial surface of specialized microphylls called sporophylls. They also tend to cluster their sporophylls into cone-like strobili.

CLADE 4. LYCOPODIALES

These plants are extant but have a fossil history which goes back to the upper Devonian. The microphylls are closely spaced on the stem and sporophylls are clustered in strobili. Lycopodium (Figure 4), the most common genus, grows as a common woodland plant with a creeping prostrate branching system from which sparsely branching upright axes emerge. Many of the axes terminate in strobili, which in Lycopodium are homosporous. The microphylls that cover the stem make it appear to be “hairy” and contribute to its Latin name which means “wolf’s foot”. The gametophyte of Lycopodium is a small carrot-like structure with the gametangia on its upper surface. It is not photosynthetic as a gametophyte but requires an intimate relationship with a soil fungus through which it obtains its food and nutrients (see Figure 5 for the Lycopodium Life History).

CLADE 5. THE LIGULATE LYCOPODS

All members of this clade have ligules, a word derived from Latin that means “tongue”. The tongue-like appendage emerges from the adaxial surface of the sporophyll in association with the sporangium (see Figure 1-B).

CLADE 6. PROTOLEPIDODENDRALES

These plants are extinct and mainly from the Devonian though they did persist into the middle Carboniferous. They were small upright leafy axes that emerged from a prostrate rhizome. The microphylls have expanded leaf bases and forked tips that make them look peg-like. The sporophylls are loosely associated in terminal strobili. See Figure 6 for a habit sketch.

This group truly was a transitional form in that they were homosporous, but had ligules. Possibly, lycopod heterospory evolved in this line. The type of xylem and development of the stele seemed to anticipate the large woody forms that emerged with the Lepidodendronales (see Clade 11).

Gensel and Berry (2001) note that the protolepidonendralians had leaves that were divided into 3 to >5 segments. For example, Protolepidodendron has leaves that are divided into 9 narrow appendages (Gensel and Berry 2001). Fenton and Fenton (1958) suggest that some of these plants may have developed megaphylls by planation of lateral branch systems. If so, they evolved megaphyll-like appendages independently of the fern megaphylls.

CLADE 7. THE HETEROSPOROUS LYCOPODS (see details of a heterosporous life cycle in Figure 8)

LADE 8. SELAGINELLALES

Selaginella (Figure 7) is similar to Lycopodium in its vegetative growth habit. Most of the 700 species of Selaginella grow as part of the understory in tropical and temperate forests. However, the Resurrection Plant (Selaginella lepidophylla) grows in the deserts of west Texas to Arizona and south to El Salvador. It can allow itself to dehydrate, curl into a ball, and enter suspended animation. Following a rain, the tissue rapidly hydrates and resumes its activity of respiration and photosynthesis. The strobili of Selaginella resemble those of Lycopodium except that the sporangia of Selaginella are subtended by ligules (text with tooltip) Ligules are small leaf-like appendages in the axils of certain microphyll-bearing plants. , and the sporangia are heterosporous (text with tooltip) Heterosporous plants have sporangia that produce spores of different sizes: megaspores (large) and microspores (small). Megaspores produce archegoniate gametophytes, and microspores produce antheridial gametophytes. . That is, some of the sporangia, called megasporangia, make megaspores, recognized as four large spores in a single sporangium. When the spore is released, it germinates to produce a megagametophyte, which produces archaegonia. In the case of Selaginella, the megagametophyte does not exceed the bounds of the megaspore wall. Likewise, the microgametophyte is little more than an antheridium bounded by the microspore wall. Such reduction in gametophytes is found elsewhere only among the seed plants [see Figure 8].

CLADE 9. THE WOODY LYCOPSIDS

Members of this group have a stele that exhibits secondary growth. Usually, this requires a cambium (lateral meristem) which produces secondary xylem.

CLADE 10. ISOETALES

Isoetes (Figure 9) is in an order that has a fossil history which dates back to the Cretaceous (although it is probably much older). The living plants are known as quillwort because they have leaves that look like green spikes. A rosette of the spike-like leaves grows from a shortened upright stem called a corm, giving them the appearance of wild garlic. Although the stems are reduced, they do show secondary growth, the necessary prerequisite for the production of wood. Like Selaginella, Isoetes is ligulate and heterosporous.

Isoetes species are wetland plants, often growing as submerged plants, particularly in acid-sensitive waters, which typically are depauperate in carbonate and bicarbonate. Thus, they are particularly well-adapted to growing in waters in which inorganic carbon might be limiting during the day. They take up carbon-dioxide at night and fix it into small organic acids, which then contribute that carbon to the Calvin Cycle during the daytime. Keeley (1981) was among the first to recognize that Isoetes used this photosynthetic strategy, which had until then been known only in xeric plants.

CLADE 11. LEPIDODENDRALES

The ancient club mosses were among the most impressive members of the Devonian and Carboniferous forests. Particularly, the plants of the late Carboniferous (Pennsylvanian Period) coal forests were very impressive (see Figure 10). They grew to 30-40 meters high with strong monopodial (text with tooltip) Monopodial growth is characterized by extreme overtopping which produces a large central stem with smaller lateral branches. growth, large grass-like leaves and strobili, which were heterosporous (text with tooltip) Heterosporous plants have sporangia that produce spores of different sizes: megaspores (large) and microspores (small). Megaspores produce archegoniate gametophytes, and microspores produce antheridial gametophytes. . The over-all growth habit was somewhat like that of a palm tree. The unbranched trunk was ornamented by distinctive diamond-shaped leaf scars. The crown of the plant had a set of simple branches covered with leaves. Many of the crown branches terminated in large strobili. The megasporangia each had a single megaspore surrounded by persistent sporophylls and resembled seeds, the structures seem to have dispersed by falling away as a unit that floated away as a small boat for dispersal. Because the different plant organs were described separately, they were given different form-generic names before fossil finds united them. The trunk with the scale-like leaf scars was called Lepidodendron; the roots were called Stigmaria; the leaves were called Lepidophylloides; and the strobili were called Lepidostrobus. Despite their size, Lepidodendron plants had relatively little wood (secondary xylem growth) in the stems and roots.

Another group that has uncertain status includes Pleuronemia (Figure 11). These plants were upright (ca. 2m), and unlike Lepidodendron, Pleuronemia was unbranched and had long strap-like leaves. The trunk sat on a base with four lobes. They flourished in the Triassic.