[2.2] This section is devoted to the detailed classification of the most primitive plants which first colonized the land.

The earliest appeared about 415 million years ago and the last survivors became extinct at about 370 million years. It must be borne in mind that these dates may be modified by new discoveries in rocks dated outside these limits.

The classification of these plants is based on the way their stems branched, whether or not there were projections from the stems, the way in which sporangia were borne, and the form of the sporangia.

Because the mode of branching is important in the classification, it is necessary at this point to explain the terms used.

• Dichotomous: the growing tip divides into two branches of, usually, equal status. Two unequal branches may, more rarely, result.
• Trichotomous: the growing tip divides into three branches. It is usual for one to be stronger than the others.
• Pseudomonopodal: the growing tip divides to form two branches, one of which is so much stronger than the other that the plant grows with one main stem giving off branches at intervals.

TRIMEROPHYTES

Plants that grew with pseudomonopodal branching. The side branches themselves branched dichotomously or trichotomously. The stems of some species were covered with minute leaf-like projections but in others they were absent. Sporangia were borne on much-branched side branches and they occurred in dense clusters. The sporangia split open lengthwise to release spores.

Examples (no living genera):

Psilodendrion, Dawsonites, Hostinella, Pertica, Psilophyton, Trimerophyton, Psilophytites.

RHYNIOPHYTES

Plants with a dichotomizing form of growth without projections on the stems. Some branches with terminal sporangia.

Examples (no living genera):

Cooksonia, Horneophyton, Rhynia, Taeniocrada, Steganotheca, Dutoitea, Eogaspesiea, Hedeia, Yarravia, Renalia, Nothia, Salopella.

ZOSTEROPHYTES

Plants whose shoot system may be either pseudomonopodial or dichotomizing. Some species have stem projections but others lack them. Sporangia are borne on the sides of stems and they split open on their curved, outer margins.

Examples (no living genera):

Zosterophyllum, Hicklingia, Gosslingia, Sawdonia, Rebuchia, Oricilla, Koniora, Serrulacaulis, Crenaticaulis, Bathurstia.

From about 390 million years, other plants appeared which were clearly ancestral to the major plant groups that have flourished on Earth right up to the present day. The classification of these plants is deferred to the next chapter.

The earliest colonizing land plants had a number of considerable weaknesses in their life-style and it is interesting to see as we go through the various plant groups leading up to present-day forms how these weaknesses were phased out.

Interesting, too, is the way in which some of the changes that took place paralleled the ways in which the earliest land animals gradually transformed their life processes as they became more adapted to the demands of life on land. The earliest land plants produced huge numbers of spores, only a few of which had much chance of eventually producing viable mature plants. Similarly, animals such as amphibia descended from fish-like ancestors would have produced large numbers of relatively small eggs resulting in few mature adults. In both plant and animal kingdoms there was a simultaneous trend to produce larger reproductive bodies. The production by animals of larger eggs protected by shells led to a much greater chance of offspring survival. In the same way plants started to produce two types of spores: a few large ones supplied with plenty of food reserve for the next generation and many small ones whose function was to release spermatozoids (equivalent to sperms in animals) to fertilise the eggs produced in the larger spores.

Eventually, a very advantageous trend in animals led to their retaining the eggs in the bodies of females and never risking them out in the environment at all; a process that resulted in a high degree of infant survival. Plants developed a way of retaining the larger spores in their sporangia to be fertilized there and dispersed when ready. These dispersed structures were of course seeds which, like the attainment of the huge stature of trees, occurred only about 60 million years after the first plants colonized the land. At this time, therefore, we can see the beginnings of all of the features of plant life on land which today results in seed plants growing everywhere on Earth except in the harshest environments such as deserts, the tops of the highest mountains, and the extreme polar latitudes.