The next group of plants are sometimes referred to as "seedless vascular plants." The plants in this group have true xylem and phloem which makes them vascular by anyone's definition. They are "seedless" because they reproduce by means of spores. Beyond this definition, these organisms have the same feature set as the bryophytes we have just finished studying.
Seedless vascular plants are in Kingdom Plantae, have chlorophylls a and b with carotenoid and xanthophyll accessory pigments. They store starch and have cellulose-pectin cell walls. The plants are all oogamous and have a sporic life history. The gametophyte is quite small compared to the sporophyte, but the sporophyte is dependent upon the gametophyte at least in its early development. However the sporophyte is more-or-less indeterminate in growth and quickly becomes independent. The gametophyte is ephemeral.
Need to add here a clear presentation of stele forms... protosteles: haplostele, actinostele, plectostele.... siphonosteles: solenostele, dictyostele, eustele, atactostele.
The organisms that fall into the seedless vascular plants are quite diverse and belong to at least four phyla. The following table provides some organization to the major groups:
|Whisk Ferns||Club Mosses||Selaginella||Horsetails||Ferns|
|Sporangium||axil of enation;|
|axil of microphyll;|
|axil of microphyll;|
|underside of megaphyll;|
|Gametangia||All have archegonia (neck, venter) and antheridia (sterile jacket)|
|Gametes||All have sessile egg and flagellate sperm|
As we go through the examples, we will try to consistently walk through the life history. But as we look at the differences among these seedless vascular plants, we will remember that they all share the same basic life-cycle features.
The spores of Psilotum are shed into the wind. They land in soil but they are very small and thus do not have enough reserves to survive for long. Germinating in soil they must quickly establish a relationship with soil fungi. These live symbiotically for the duration of the life of the gametophyte in the subterranean environment...and perhaps beyond!
The Psilotum gametophyte produces both antheridia and archegonia. Both have sterile jackets which open upon maturity in moist soil. The sperm swim in soil water to the open archegonium. Syngamy occurs and the zygote grows at the expense of the gametophyte and fungal partner at least initially.
The Psilotum sporophyte develops an underground rhizome system. A rhizome is an underground stem. This rhizome is covered by epidermal cells that grow out into the soil, which are called rhizoids. There are no roots on the sporophyte. This will mean that the large sporophyte will need to live in very moist soils. The rhizome system will send up aerial shoots. These stems grow up into the light and carry out photosynthesis.
The vascular system in both rhizome and aerial stems is a protostele. It is a solid vascular cylinder consisting of a fluted cylinder of xylem (actinostele) completely surrounded by phloem. The xylem maturation is exarch. If you recall the dicot root vascular cylinder (solid, ridged xylem cylinder with radial xylem/phloem arrangement), Psilotum has a similar format. So if Psilotum is a "primitive" vascular plant, then a dicot root (haplostele form of protostele) is more "primitive" than a dicot stele (eustele form of siphonostele). Outside the stele is a cortex for storage in the rhizome and for photosynthesis in the aerial shoot. The epidermis of the aerial shoot is cutinized and produces stomata for gas exchange. The epidermis of the rhizomes produces hair-like cells functionally equivalent to root hairs.
The aerial shoot does not have leaves, but does produce enations. These epidermal flaps occur at intervals along the length of the shoot. They have some cortical mesophyll, but lack any kind of vascularization. There may be a vascular trace branching off from the stele going into the cortex, but does not leave the plane of the epidermis. It heads toward the enation but does not enter it. Whether this is a plesiomorphic characteristic or an apomorphic reduction could be debated as we shall see below. Interestingly, the only other psilophyte, Tmesipteris has stem appendages that intergrade from enations to microphylls (having a single vascular bundle entering and running the length of the leaf).
In the axil of the enations, a sporangium may form. Because this has three chambers, it is often called a synangium. The three chambers open by separate slits, which is why the thinking is that these are three sporangia that are fused into one synangium. Meiosis takes place in sporocytes in these sporangia, the slits open, and the spores are shed to the wind.
There are extinct organisms such as Rhynia and Cooksonia, that are some of the oldest vascular plants on earth going back to the Devonian! These have rhizoids on rhizomes and aerial shoots with enations. Their vascular system is a centrarch protostele of the haplostele type. They are at least superficilly similar to extant Psilotum. Recent analysis of macromolecular sequences however puts Psilotum very close to the ferns (arguably the most advanced forms of seedless vascular plants). So while the resemblance may be quite intriguing, current thinking is that this is misleading. Further study supports the status of Psilotum as a true fern that has apomorphically reversed some of its character states so that it superficially resembles the extinct species. Perhaps Psilotum and Tmesipteris belongs with the Ophioglossum ferns in a group called Monilophytes.
It could easily be argued that Psilotum should not even be associated with the Devonian fossil organisms...or presented here in the sequence of the seedless vascular plants. Certainly apomorphic transitions that most surely are reversals make this organism highly evolved rather than "primitive." However, I have chosen to leave this organism and its relative here in the sequence, more for its features (however reversed they may be) shared with those ancients than for its correct placement as a fern that has undergone further evolution and might therefore be arguably last in the presentation sequence!
It is also intriguing to consider Tmesipteris microphylls. Could we be seeing apomorphic reversals that reflect in full reversal of the enation→microphyll→ megaphyll evolutionary pathway? If so, then Tmesipteris could be the intermediary reversal between the Ophioglossum-group of ferns, and Psilotum. If this idea is found to be sound, then this primitive-looking Psilotum is in fact the more highly evolved in this group.
The spores of Lycopodium are shed into the wind. They land in soil but they are very small and thus do not have enough reserves to survive for long with active metabolism. Fortunately spores are dormant and can remain so for long periods of time. However, once metabolism is activaed and these tiny spores are germinating in soil, they must quickly establish a relationship with soil fungi. These live symbiotically for the duration of the life of the gametophyte in the subterranean environment...and perhaps beyond! Exceptions to this are apparently extant...with photosynthetic gametophytes germinating from spores at the soil surface which, in spite of being photoautotrophic, associate with fungi.
The Lycopodium gametophyte produces both antheridia and archegonia. Both have sterile jackets which open upon maturity in moist soil. The sperm swim in soil water to the open archegonium. Syngamy occurs and the zygote grows at the expense of the gametophyte and fungal partner at least initially.
The Lycopodium sporophyte develops an underground rhizome system. A rhizome is an underground stem. This rhizome produces true roots along its length. The sporophyte is also competitive in fairly moist soils, though some species are specialized for desiccating environments. The rhizome system will send up aerial shoots. These stems grow up into the light and carry out photosynthesis.
The vascular system in both rhizome and aerial stems is called a plectostele (sometimes a root-like actinostele). It is a solid vascular cylinder with exarch xylem maturation. The ridges of xylem in the actinostele and the edges of the ribbons of xylem in the plectostele are the location of protoxylem. The phloem completely surrounds the actinostelic forms and penetrates the ribbon-like xylem areas of the plectostele. Outside the stele is a cortex for storage in the rhizome and for limited photosynthesis in the aerial shoot. The epidermis of the aerial shoot is cutinized and produces stomata for gas exchange.
The aerial shoot does have leaves, but they are very small. These are called microphylls because they have just a single vascular bundle running along their length. The microphylls have photosynthetic cortical mesophyll, and so are very bright green in color. The microphylls are generally spiral in arrangement, though some of the leaves may be small and appressed against the stem epidermis in some species. Because of the leaf arrangement and appearance, Lycopodium species have common names such as "club moss" or "princess pine" or "ground pine" or "running pine." All of these describe some superficial resemblances of these sporophytes of other plants (more or less primitive) to Lycopodium.
In the axil of the microphylls, a sporangium may form. In such a case the microphyll is also called a sporophyll. This sporangium may be wider than long, but it consists of just one chamber. The sporocytes inside undergo meiosis and produce spores. The sporangium then splits open to shed the spores to the wind.
In some of the "more primitive" species of phylum Lycophyta (such as Huperzia lucidula), the sporophylls for a particular season are produced in one short segment of the stem. As you look along the stem you can count these areas to determine the age of the sporophyte.
In "more advanced" lycophytes (such as Lycopodium obscurum) the sporophylls are produced in a terminal cluster called a strobilus. These sporophylls often lack chlorophyll and show through the yellow colors of the sporangia in their axils. The strobilus is sometimes called a "cone" in casual settings.
Further-advanced lycophytes (such as L. clavatum) produce a cluster of strobili at the shoot apex...increasing the number of spores that can be produced by the sporophyte.
Finally the most-advanced lycophytes (such as L. complanatum produce a very long stem section below the strobili, elevating them higher into the wind. This will provide a better distribution of spores downwind.
In the fossil record are forests of Lepidodendron. These large tree-like lycophytes had secondary growth (wood) and very large strobili between fist-size and pineapple-size. These strobili shed massive quantities of spores that collected in deep layers. The spores contained some flammable biochemicals. In fact in the 1800s, Lycopodium "powder" was used as a "flash powder" that was ignited above ancient cameras to provide a "flash" exposure! I will give you a demonstration of this in class. Since the ancient spores were not degraded by animals or other organisms, the flammable molecules were overrun with sediments and became coal deposits. Now we dig down to those deposits and use the coal to fuel our power plants. Maybe next time you turn on the light switch you can thank some extinct lycophytes!
We will skip over some very special lycophytes (Selaginella and Isoetes) for now because these have an apomorphic life history that is quite distinct from all these other seedless vascular plants. We will come back to these plants however!
The spores of Equisetum are shed into the wind. They land in soil but they are very small and thus do not have enough reserves to survive for long. Germinating on the soil surface, they quickly become photosynthetic. They resemble the gametophyte of thallose liverworts. So is this ontogeny recapitulating phylogeny?
The Equisetum gametophyte produces both antheridia and archegonia. Both have sterile jackets which open upon maturity when covered with a film of water. The sperm swim in this water film to the open archegonium. Syngamy occurs and the zygote grows at the expense of the gametophyte at least initially.
The Equisetum sporophyte develops an underground rhizome system. The rhizome produces true roots along its length. The sporophyte is also competitive in fairly moist soils, though some species are specialized for desiccating environments. The rhizome system will send up aerial shoots. These stems grow up into the light and carry out photosynthesis.
The vascular system in both rhizome and aerial stems is a eustele. It is a ring of vascular bundles. These bundles consist of bundle of phloem with xylem on the radial and inner tangential sides. The xylem maturation is endarch. So this is an advanced system in terms of stele and xylem maturation, but rather primitive in the xylem/phloem arrangement. The vascular system is surrounded by an endodermis. Outside that is a cortex for storage in the rhizome and for extensive photosynthesis in the aerial shoot. The epidermis of the aerial shoot is cutinized and produces stomata for gas exchange. But the most unique feature of this epidermis are cells that are invested with much silicon dioxide. This deposit is very glass-like. Running through a stand of Equisetum hyemale sounds like shattering glass in a vague way. Clumps of E. scirpoides look vaguely like a steel-wool pad, and were used in colonial days to scrub pots because of the scouring power of these glass deposits. This feature gave Equisetum one of its common names: scouring rush.
The aerial shoot does have leaves, but they are very small and generally are not photosynthetic. These are microphylls and have just a single vascular bundle running along their length. The microphylls are whorled arrangement at the nodes of the stem. In Equisetum hyemale, the stem has very strong apical dominance and so it is a stem with the tiny leaves appressed against the stem epidermis. In Equisetum arvense apical dominance is very weak. So a branch appears coming out from the base of each of the microphylls at the node. So a whorl of branches grows out at each node. This give a much-branched plant body that looks somewhat like an up-side-down horse tail. And this appearance resulted in the other common name, horse tail, and its translation into Latin, Equi-setum.
At the apical bud of the stem, or at the apex of a fertile stem, a strobilus forms. This strobilus looks superficially like a pineapple with hexagonal scales covering the surface. Each of these scales is the outer surface of a sporangiophore. This scale is attached to the axis of the strobilus by the stalk of the sporangiophore. All around this stalk, and attached to the underside of the scale are several sporangia with sterile jackets. The scale shrinks in diameter and the stalk grows and curves upward as the strobilus matures. When the sporangia dry out, they crack open and release the spores to the wind. The spores of Equisetum each have four wall strips, called elaters, that reflex outwards to push the spores away from each other. This assures that the spores do not clump and make it into the wind for dispersal.
In the fossil record are forests of Calamites. These short tree-like horsetails had secondary growth (wood) and large strobili, about fist-size. The rhizomes were very large in diameter.