Among the best-known plant groups are the algae, the ferns and the fern-like plants, as well as the seed plants with their two subgroups, gymnosperms and angiosperms. The complexity of the plant structures, their functions, and their performances increase in parallel with the evolutionary level. At the same time, diversification and specialization take place at every level of complexity. The more genetically determined structural and functional elements exist, the larger is the number of their potential combinations. This is one of the reasons, why every level of complexity has a higher number of species than its preceding level.
The term level of complexity or level of organization does not imply that evolution occurred by leaps and bounds. Progressions like, for example, the transition of the aquatic to the terrestrial mode of life are caused by a new development and the optimization of numerous features. In the course of optimization, many transitional life forms existed. They became extinct, as DARWIN noticed, because they were replaced by better adapted organisms. When watching recent species, we do therefore only see the better-suited species, products of evolutionary lines that took place more or less independent from each other. The small sample of species living today can be grouped into ‘adaptive peaks’ (G. L. STEBBINS, 1950) that do either differ in their level of complexity or exist on the same or very similar levels of complexities and are unlike only in their combinations of different features.
Increase in the number of terrestrial vascular plants in the course of geological periods of time. The increase occurs in four clearly demarcated phases. The reasons for the end of a phase are most likely drought periods, i.e. climatic changes. The renewed increase in the number of species is caused by the coming into being of evolutionarily advanced plant groups that became dominant in their period of vegetation: a. primitive trachaeophytes, b. pteridophytes, c. gymnosperms, d. angiosperms. The abbreviations of the geological periods are: S: Silurian period, D: Devonian era, C: Carboniferous period, P: Permian period, R: Triassic period, J: Jurassic period, K: Cretaceous period, T: Tertiary period (according to K. L. NIKLAS et al., 1983).
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