Article,
Evolution, polymorphology and multifunctionality of the phloem system
Perspectives in Plant Ecology, Evolution and Systematics, (December 1999)
DOI: 10.1078/1433-8319-00069
Abstract
Current research discloses that the phloem system is not only responsible for the allocation of photoassimilates, but has several other functions. Despite the knowledge acquired recently, the phloem remains the most puzzling plant tissue due to its inaccessability to experimental approach. Since well-preserved fossile remnants of phloem tissue are rare, evolution of sieve elements and the whole phloem was inferred from the phloem structure in present plant taxa. Special attention is paid to the evolution of the sieve elements being the conducting modules of the phloem. Development of sieve elements probably was a polyphyletic event. It may have occurred independently in various groups of algae and in the land plants. The emergence of highly specialized accessory cells sustaining the sieve element is restricted to the Spermatophyta. An attempt is made to explain the presumptive evolutionary development of the phloem system in terms of physiological fitness. In particular, the diversity of the leaf phloem in dicotyledons is discussed. It is an example of progressive phloem evolution in a plant organ that is permanently challenged by daily variations and more persistent environmental changes.
