The Lycopodiophyta is the oldest group known that is still around. Its ealiest fossil record extends back to the Devonian (408-326 MYBP) or perhaps even as far as the Silurian (438-408 MYBP). It reached its greatest abundance, however, in the Carboniferous (326-290 MYBP). Indeed, the name 'Carboniferous' refers to the large coal deposits that date from the lycopod forests of those days. All extant lycopods are rather low growing herbaceous plants but many extinct species were arborescent. Some were up to 40 m tall and 3 m thick at the base.
Stems
Members of the Lycopodiophyta are either dichotomously branched or combine monopodial and dichotomous branching. The stele is a protostele or siphonostele. Protosteles are older than siphonosteles. Rhyniophytes had protosteles. In simple protosteles, the xylem forms the core and the phloem surrounds it. In more complex protosetles, the xylem and phloem are intermingled as strands of plates. This is what is found in the extant Lycopods.
Protostele: 
Siphonostele: 
Siphonsteles are less expensive to construct than protosteles because xylem and phloem are energetically more expensive cells to make than parenchyma cells. This give plants with a siphonostele an advantage over those with protosteles. Apparently it was not enough to be a lycopod with a siphonostele there are none around today. Some extinct species di not have just one siphonstel, but multiple siphonosteles in their stems. Still was not good enough to ensure their survival. What I do not know is whether multiple siphonosteles were characteristic of the taller lycopds.
The stems are densely covered with spirally arranged microphyllous leaves. In extant taxa and most extinct taxa, the leaves are less than 10 mm long but some extinct arborescent lycopods had leaves up to 30 cm long. As with all microphyllous leaves, the stele does not have a leaf gap where the vascular strand from the leaf joins it. The leaves of lycopods are considered enations. This means that they are outgrowths from the stem surface. How would having enations give lycopods an advantage over those that got their first, the Rhyniophytes?
Reproductive structures
The sporangia are borne either in the axils of leaves or on the upper surface of leaves. A leaf with a sporangium in its axil or on its upper surface is called a sporophyll. Sporophylls sometimes look different from other leaves, but sometimes they look just the same. In some species the sporophylls are scattered among the regular leaves; in others they are aggregated into one or more strobili (sing. strobilus).
If one considers both extinct and extant Lycopods, most species are homosporous. This means that all their spores were of the same size. Some are functionally male, some female, but they look alike. Of the extant families, however, one is homosporous and the other two are heterosporous. Heterosporous plants produce two kinds of spores, large spores that we call female and small spores that we call male. The male spores often have flagella to hel pthem reach the female spores.
In all lycopods, the sporophyte phase is dominant. The spores do not survive unless they are able to grow right away, forming roots, a gametophyte, and mature sex organs. The sperm, which are formed in antheridia, swim through a film of water to the eggs, which are formed in archegonia. One a sperm and egg fuses, the next sporophyte generation has started.
The gametophytes do not have vascular tissue. That is why they can only survive in moist areas. The sporophytes can survive dry conditions because it has vascular tissue. It is not particularly vascular tissue, but it is better than no vascular tissue. It worked when most of the earth had a relatively humid climate AND there were no plants around with anything better in the way of vascular tissue.
Key to Lycopod families and species of Utah