A mechanism involving transport, storage and retrieval of a symmetry-breaking message controls the relative growth rate of the cotyledonary buds of plantlets of Bidens pilosa L. The asymmetry was induced by administering a few needle pricks to one cotyledon of each plant. The storage of the symmetry-breaking message was independent of the number of pricks ("all or nothing" process) and irreversible. However, various treatments could render the plants either able to retrieve the stored symmetry-breaking message (in which case, the bud opposite to the pricked cotyledon began to elongate statistically sooner than the one associated with the stimulated cotyledon) or not (both buds then had an equal chance to be the first to start to grow). The retrieval process was also associated with a temporal oscillation. At the level of the whole plants, bud growth was observed only after the removal of apical dominance, and its degree of asymmetry was expressed by use of a parameter g ranging from zero (symmetrical case) to ± 1 (full asymmetry in favor of one of the cotyledonary buds). The highest g-values observed in the present contribution were of the order of 0.5. At the cellular level, the pricking of one cotyledon caused a number of cells, which were within the meristem of the bud associated with the pricked cotyledon and were in cell-cycle phases S or G2, to undergo cellular division and then be blocked in phase G1, whereas the cells of the opposite bud were practically unchanged.
Stomata of leaves from in vitro grown rose plantlets remain opened in the dark. The ultrastructure of their guard cells was studied after a 7 h light and a 7 h dark period, and compared to that of functional stomata from plants which have been acclimatized to greenhouse conditions. Qualitative and quantitative observations concerning the shape of the guard cells, mitochondria, plastids and starch grains, demonstrated the similarity in guard cell ultrastructure. The peculiarity of guard cell ultrastructure of in vitro cultured plants was the inability to close in the dark; vacuolar area was 40% of the whole guard cell area during both light and dark period whereas, in guard cells from greenhouse plants, the vacuolar area was 40% of the whole guard cell area during the light and only 25% during the dark period. These results indicate that stomata from in vitro plants are duly developed and possess an ultrastructure suitable for a typical functioning. The inability to close in the dark results from atypical water relation.
63170 Au hi ere arrive le 12 .fevrier 200 I, accepte le I 0 septembre 200 I Resume.-Au sein des bourgeons vegetatifs dormants de Pecher l'organogenese se poursuit lentement pendant Ia periode hivernale. Des experimentations en conditions environnementales controlees montrent que les meristemes de jeunes arbres places en novembre a une temperature constante de 18 •c presentent une activite lente et rythmique, doni Ia periode est de l'ordre de plusieurs semaines. Un traitement prealable de trois mois a 5 •c stimule sensiblement l'organogenese dans les bourgeons d'arbres places ensuite a Ia temperature de 18 •c landis qu'un pre-traitement de un a trois mois a 23 •c l'inhibe completemen!. Le probleme de Ia regulation de l'activite du meristeme au sein d'un bourgeon dormant est discute a Ia lumiere de ces resultats.Mots-c/es : organogenese -meristeme -dorrnance -temperature -Pecher.Abstract.-Within the vegetative dormant buds of the Peach tree, the organogenesis continues on a slow rate all along winter. In experiments with controled environmental conditions, the meristems of young trees placed, in November, at 18 ·c. shows a slow and rythmic activity. A 5 •c treatment before 18 •c conditions, hastens the organogenesis while a 23 •c pretreatment inhibits it completely. The question of the regulation of the meristematic activity within a dormant bud is discussed.
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