Abstract
The ratio of primordium size to the meristem size (<em>P/M </em>ratio) is regarded by some geometrical models of phyllotaxis as the parameter, which determines the quality of spiral and whorled patterns of lateral organ arrangement. This assumption was tested on floral meristems in four genets representing four <em>Magnolia </em>taxa: <em>M. </em>× <em>salicifolia</em>, <em>M. stellata</em>, <em>M. denudata </em>and <em>M. acuminata</em>. In successive zones of <em>Magnolia </em>flower, lateral organs are initiated in specific phyllotactic patterns and at specific values of the meristem and primordia sizes. The elements of perianth, usually positioned in three trimerous whorls, are initiated as large primordia on relatively small meristem. The switch in the identity of primordia, from tepals to stamens is accompanied by an abrupt increase in the size of the meristem and decrease in the primordia size. Small values of <em>P/M </em>ratio and frequent occurrence of qualitative transformations of phyllotaxis contribute to the exceptionally rich spectrum of spiral patterns in androecium zone. New spiral patterns emerge when bigger primordia of carpels are initiated on the meristem, which at the same time starts diminishing in size either abruptly (<em>M. </em>× <em>salicifolia</em>, <em>M. stellata</em>, <em>M. acuminata</em>) or slowly (<em>M. denudata</em>). Spiral patterns identified in gynoecia have lower numbers of parastichies than the patterns of androecia and occur in frequencies specific for the genet. Although noted ranges of the meristem and primordia sizes, justify the occurrence of phyllotactic patterns observed in successive zones of <em>Magnolia </em>flower, they do not explain genet-specific frequencies of the patterns observed in gynoecium zone. The lack of straightforward relationship between frequency of the patterns and <em>P/M </em>ratio in gynoecium suggests that more complex geometrical factors or factors of non-geometrical nature are engaged in determination of <em>Magnolia </em>floral phyllotaxis.
Highlights
Architecture of the aerial parts of plant is determined in the shoot apical meristem generating primordia of leaves, flowers and floral organs in regular phyllotactic patterns
We focused on four different taxa – M. × salicifolia, M. stellata, M. denudata and M. acuminata, taking into account all three zones of the flower – perianth, androecium and gynoecium (Fig. 1, Fig. 2), to have a better view of developmentally changing floral phyllotaxis
During ontogeny of Magnolia flower the changes in the initiated organ identity are accompanied by the changes in the meristem and primordia sizes and alterations in organ arrangement
Summary
Architecture of the aerial parts of plant is determined in the shoot apical meristem generating primordia of leaves, flowers and floral organs in regular phyllotactic patterns. PHYLLOTACTIC PATTERNS Analysis of phyllotaxis in Magnolia gynoecia had a very detailed character – in addition to the meristems a large number of the embryonic, generative shoots was collected from each tree during autumn and winter.
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