Towards an organismic concept of land plants: The marginal blastozone and the development of the vegetation body of selected frondose gametophytes of liverworts and ferns

An organismic concept of land plants is outlined, which is based on a synthesis of plant morphology and plant anatomy. The entire plant, the living unity, is conceived as the organism being subdivided into cells, which cannot be interpreted as organisms themselves in the sense of elementary organisms. The evolution of land plant tissue systems is discussed in the introductive chapter.To test the proposed concept, some frondose plants were selected from liverworts (Pellia epiphylla, Metzgeria furcata, Pallavicinia lyallii) and comparable fern gametophytes (Dryopteris filix mas, Vittaria lineata, Stenochlaena tenuifolia) and studied with respect to their organization and the principles of development. They all have an archetypic, two-dimensional, open construction, which is described as the repens-type of plant construction. Primary form growth occurs in the marginal blastozone, which controls cell wall integration. One of the most significant processes of form generation is blastozone fractionation. The tissues leaving the blastozone differentiate during extension growth and maturation of the vegetation body. While the plant grows continuously in the blastozone, it decays steadily in the necrozone.The implications of the two-dimensional repens-type are discussed. It appears as a perfect plant construction, fit to start plant evolution on the land surface. Growing upwards into the atmosphere, the repens-type is obscured. But is reappears in all groups of higher land plants. This demonstrates the existence of evolutionary cycles in plants. It is argued that mutation and selection do not suffice to understand cyclical evolutionary patterns. The influence of organismic construction seems to predetermine evolution because of the limited options to change an appropriately functioning construction. Via construction analysis evolutionary options can be detected and thus, evolution becomes predictable to some extent. Instead of being object of mutation and selection, living organisms should be conceived as subjects in evolution (Weingarten 1993).Dedicated to my admired teacher Professor DrWilhelm Troll on the occasion of his 100^th birthday, 3rd November, 1997.