Because of the enormous quantities of coal formed during the period centering on about 300 million years ago, it has been termed the Carboniferous. This is not to say that coal deposits of considerable industrial importance do not occur in other geological periods. Quite the contrary, other periods were also carboniferous (with a small ‘c’) but once the term had been used, different names were chosen for other sub-divisions of geological history. The result of this nomenclature is that it is possible to refer to Carboniferous plants (with a capital ‘C’) with the knowledge that it comprises the entire flora known during the Period (geologists use a capital ‘P’ for the name of the sub-division), whether they were important coal-forming plants or not.

Undoubtedly the most remarkable Carboniferous plants were the clubmosses; the most well-known being Lepidodendron [Fig. 12]. It seems likely that members of this genus grew to maturity relatively rapidly, produced a crop of strobili and then died. These lycopods were heterosporous. Both microsporangia and megasporangia were contained in each strobilus borne by the mature sporophyte tree. The scattered megaspores would eventually give rise to embryos when the ova in their archegonia had been fertilized by spermatozoids from the microspores. Each new sporophyte would then root itself in the soil and start to grow upwards. In a given area only a very few would find it possible to grow to mature trees in the gaps left by dead ones. Once they had done so, the life-cycle would be repeated again. Another genus, Lepidocarpon, had already proceeded to the point where it only produced one large megaspore (and three minute aborted ones) of seed calibre in each of its megasporangia. However, it would appear that it was not a true seed, but it was well on the way, evolutionarily speaking, to becoming one.

Within the Carboniferous horsetails, as stated previously, there were two sub-groups:

  1. Those resembling the modern Equisetum.
  2. The extinct one, the sphenophylls.

The former group was heterosporous, but the latter were homosporous. In their case all of their sporophytes had to make a start from a relatively small amount of food as compared with those of Calamites.

Undoubtedly the group that gave, at this time, the best start to the next generation were seed plants related to Medullosa [Fig. 15]. Some of them produced seeds which were over four inches long and two inches in diameter; bigger than the stones in avocado pears, for example. Of course, not all contemporaneous seeds were as large. Many Carboniferous seed plants (gymnosperms, as they are called, for reasons to be discussed later) had seeds that were much more modest in size but all nevertheless passed on to the next generation more food material than did most of the heterosporous plants described above.