Early Land Plants
Although the oldest life forms which were found in marine sediments are as old as 3.5 billion years, continents remained uninhabited for many hundred millions years. The colonisation of the land by plants and animals did not occur until some 415 million years ago, at the end of the Silurian. Within the geologically relatively short time-span of only some 50 million years, following the first colonisation of the land by small land plants, the flora diversified rapidly and eventually gave rise to the oldest plants with a tree-like growth habit.
The earliest land plants were only few centimetres high, and consisted of rounded, naked, occasionally bifurcating axes with a central vascular strand. In order to prevent desiccation these axes were covered by a very resistant impenetrable outer layer, which is named the cuticle, and they had stomata for gas exchange. Sporangia, in which the spores necessary for the sexual reproduction were produced, were born terminally.
As early as the Early Devonian
up to two decimeters high, more complex plants developed which partly also
had naked axes, but some had small spine-like protrusions, the so-called
emergences. Real roots were still lacking; water and nutrients were taken
up by so-called rhizoids. These are multicellular hair-like appendages
which developed where the axes laid down on the humid substrate. On the
basis of their overall morphology, the anatomy of the vascular bundle and
the shape and position of the sporangia, several
groups of plants can be distinguished in the Early Devonian, including
the completely extinct Rhyniophytes, the Zosterphylls and the first representatives
of the lycophytes or clubmosses, a group which is still existent today.
Towards a tree-like growth habit
In the Middle Devonian the next evolutionary steps became apparent. These were the development of true roots, which is of course also of importance for the formation of soils, and the development of even more complex vascular systems which from now on also provided stability. Middle Devonian plants could reach a height of a few meters. At the same time further prerequisites for a tree-like growth habit were attained like the formation of secondary wood. In the lycophytes which have long and narrow, or small needle-shaped leaves the evolution of the leaf was already completed, whereas in other groups of plants a differentiation into larger axes and shorter strongly branched lateral axis systems had just begun. These latter lateral axis systems can be interpreted as the forerunners of fern-like fronds.
Thus in the Middle Devonian all basic conditions
for a tree-like growth habit and the formation of larger-scaled forests
The earliest forests
As early as the Late Devonian the earliest tree-like plants appeared and formed forest-like stands. These plants include the earliest tree-like lycophytes which have been documented from several areas such as Spitsbergen and Ireland. Another well-known plant with a tree-like growth habit is Archaeopteris of which numerous, up to 10 m long silicified stems have been found in North America; their trunks basally may reach a diameter of up to 1.5 m. This plant had frond-like axis systems with fan-shaped leaflets and was heterosporous (having small micro- and much larger macrospores). The wood is anatomically remarkably similar to that of the primitive conifers. The formation of micro- and macrospores is the first evolutionary step towards the development of seeds. Because of this combination of characters Archaeopteris is generally regarded as the ancestor of the first gymnosperms.
The first seed plants appeared towards the end of the Devonian. They belong to the pteridosperms or seed ferns, a rather heterogeneous, completely extinct group which was, however, very successful in the late Palaeozoic. These plants had fern-like fronds and did not reproduce with spores but with real seeds and pollen grains. With the appearance of the gymnosperms which are, because of their life cycle, not so strongly bound to humid environments as spore plants, the hitherto uninhabited hinterland could be colonised successfully. Nevertheless, many early forms still preferred humid habitats.
Within a, in a geological perspective, relatively short time span of 50 million years a large diversity of plant groups had developed (Gensel & Andrews 1984). Most of the presently still existing groups (lycophytes, sphenopsids, ferns and gymnosperms) had appeareded by the end of the Devonian. The earliest trees and forests are Late Devonian in age.
During the Devonian floral provinces started to differentiate. This can be related to differences in geographical position and climate. This trend continued until the Permian (Cleal 1991). This contribution focuses on the so-called Euramerican floral province which consisted of large parts of Europe and North America which then were parts of the same continent.
|The Carboniferous coal swamp forests|
|The floral change at the end of the Westphalian|
|Is there a floral break in the Permian?|
|© Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster||