A theory for inflorescence development and flower formation based on morphological and biophysical analysis in Echeveria

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Abstract
Floral development is generally viewed as involving interactions between recently made organs and generative activity on the apical dome; one set of floral organs is thought to induce the next. To investigate such interactions, flowering in Echeveria derenbergii (J. Purpus) was studied at two levels of structure. At the larger, morphological, level the inflorescence apex is shown to have simple cyclic development. Seen from above, it elongates horizontally, then forms a transverse cleft to demarcate a flower primordium in one of two rows. The meristem then elongates at 90° to its previous axis, also horizontally, and demarcates a flower in the other row. Activity on the apical surface correlates well with the nature and activity of adjacent sub-apical organs. For example, the 90° shifts in elongation of the meristem correlate with that tissue's being attached, laterally, to successive large growing bracts whose bases lie at 90°. Also, on the flower primordium, the five sepals arise in a spiral sequence which correlates with one of increasing age, since formation by the cleft, of the edges of the primordium. The second level of study was to test whether the developmental correlations could have a biophysical explanation. By biophysical theory, organs arise where the dome surface is structurally predisposed to bulge. This is a function of the cellulose reinforcement pattern in the surface. Successive patterns of cellulose reinforcement in isolated surface layers from floral organs were determined using polarized light. This was done for the cyclic activity of the inflorescence meristem and the development of the flower. The results indicate that patterns of cellulose reinforcement on the apical dome surface could lead to the production of organs, through local promotion of bulging of the tunica. Subsequent growth of the base of each organ stretches the adjacent dome tissue in a directional fashion. Cytoskeletal responses of these stretched cells lead to new cellulose alignments on the dome which generate the reinforcement pattern for the next round of organs.