Morphogenèse comparée de protocormes du Cypripedium acaule (Orchidaceae) cultivés in vitro avec ou sans sucre

Seeds of the terrestrial orchid Cypripedium acaule from temperate regions contain an undifferentiated embryo consisting of about one hundred cells. These cells contain lipid and protein storage reserves. The development of the seeds was studied in vitro on a germination medium with or without dextrose (glucose). On the medium containing dextrose, proembryos develop and form a protocorm. From the anterior pole of the protocorm appears a "promeristem", a structure that will form a scale and the apex of the seedling. The formation of the root never precedes that of the apex. In the protocorm, the amount and size of starch grains show an increasing antero-posterior gradient. Only the apex, the procambium, and the secretory cells do not accumulate starch grains. On the medium without dextrose, the proembryos develop slowly into a protocorm, and a semi-meristematic region can be recognized at the anterior pole. However, no other tissue differentiation occurs and the protocorm soon degenerates without having undergone organogenesis. The protocorm accumulates a few starch grains, more voluminous and more numerous at the posterior pole, that agglomerate around the nucleus; but during the development, these rapidly disappear from the cells. Generally, all cells are strongly vacuolated. These observations indicate that orchid seeds cannot reach the seedling stage without a sugar input, which is ensured by a fungal symbiont in nature. Key words: Orchidaceae, morphogenesis, protocorm, starch, sugar effect, in vitro germination.