Pseudocycles and pseudocyclic similarity of shoot systems in conifers
Russian State Agrarian University, Department of Botany
127550 Moscow, Timiryazevskaya, 49
e-mail: botanika2@timacad.ru
The results of a study on shoots and their systems in modern Pinopsida are summarized. In total, about 250 species belonging to 42 genera of conifers from collections of Russian Botanical Gardens are studies. The object of investigation was systems of elementary monorhythmic shoots which are generated from a bud during a single period of growth. These shoot systems can branch sylleptically. In such systems, as a result of complication and reduction, lateral branches acquire some features of leaves (limited growth, flattening) and functionally replace them. Leaves themselves enter into the composition as subordinate structures. This is indicative of pseudocyclic evolution of shoot systems in conifers. In different families, specific or analogous sequences of such transformations are observed. There exist at least two types of pseudocycles: reductional and synclad ones. The reductional one is characteristic for family Pinacea, the synclad one — for families Cupressaceae and Podocarpaceae. Structural evolution of shoot systems in conifers by pseudocycle type displays close similarity in families Araucariaceae, Cupressaceae, and Podocarpaceae, which, as per my view, infers their common origin and prolong evolution with compatible modi of shoot system transformation. Occurrence of a specific pseudocycle in Pinaceae indicates their rather early splitting off from the main stem of all conifers evolution. Evolution of shoot systems in conifers by pseudocycle type happens as a consequence of leaves limited feasibility. Because of the inability for edge growth, in humid climate it turns out to be preferable to replace leaves with modified shoots.
