Hybridization has played an important role in Saxifraga evolution causing reticulation and a high number of described hybrids, but little is known about how hybrid speciation had occurred in the genus. We focus on a group of closely related Saxifraga species of the subsection Triplinervium from Pyrenees, the phylogenetic relationships of which remain unsolved. Trying to unmask cryptic (or ancient) hybridization processes, we analyze one nuclear (ITS) and three plastid regions (rpl32‐trnL, trnS‐trnG‐trnG, and 3′trnV‐ndhC), as well as nuclear DNA content. Pollen and seed morphology and viability studies were carried out to evaluate the status of spontaneous hybrids. DNA ploidy levels were also inferred for the two Madeiran taxa (of the same Saxifraga subsection), where recent hybridization processes are not expected. Molecular markers revealed multiple reticulation events, which, as suggested by DNA content and chromosome numbers, have occurred in homoploidy (without genome doubling after hybridization). In addition, autopolyploidy has occurred in some species or populations, especially in the Madeiran archipelago colonization. Chromosome number variation appears to be related to centric fission events, which also could lead to the formation of the B chromosomes inferred in some taxa. Spontaneous hybrids do not produce viable seeds and this could be due to differences in parental chromosome numbers. Hybrid speciation has probably been successful by chromosomal arrangements, which also generated new, more or less intermediate, chromosomal numbers in this group of taxa.

Under controlled conditions (14 h light : 10 h dark; 27 ± 1 and 19 ± 1 °C), common ashes, Fraxinus excelsior L., display a rhythmic growth. The average duration of the first growth flush is 106 days, followed by a phase of nonlengthening from 9 to 11 months, depending on individuals. This rhythm was materialized in the shoot organization. The end of each growth stage was characterized by progressively shorter internodes and by the presence of scales around the bud. Histological studies reveal that no leaf is preformed at the embryo stage and that the meristem activity carried on after the stem growth stops. The common ash growth rhythm is therefore not the direct consequence of the arrest of meristem activity. The nonlengthening phase is an apparent resting phase during which the initiation of new leaves carries on. Key wordsff growth, development, common ash, meristem, plastochron, rhythm. [Author's translation]

The composed pennate adult leaf of Guarea guidonia (Meliaceae) has a multiannual rythmic growth shown by the successive sequential setting of leaflet pairs. This is demonstrated at the level of the foliar central vein by the differentiation of periodic rings in the wood. Microsurgically isolated foliar primordia produced large leaves, the tops of which supported several foliar sequences. Early removal of foliar primordia suppressed the endogenous rythmic growth pattern and the functioning of the foliar apex became continuous. Isolated cuttings of the composed leaf were readily rooted, the rythmic growth being maintained. This was also possible with cuttings of isolated nodes. Data show that rooted cuttings of this plant could be used as a model for the study of endogenous rythmic growth of woody plants. Key words: Guarea guidonia, composed leaf, endogenous rythmic growth, root cutting. [Journal translation]

No abstract

Nous proposons une technique biochimique nouvelle basée sur les modifications de la régulation de l'énergétique cellulaire, en rapport avec l'état de dormance. L'analyse consiste à mesurer l'aptitude du matériel végétal à augmenter sa teneur en nucléosides tri-phosphates non adényliques (NTP) à la suite de l'apport d'un précurseur exogène de l'ATP, l'adénosine. Les résultats de cette étude sont que les conceptions de V EC ts, pour la première fois, sont dévcloppécs biochimiquement. Les optimums de température de croissance des bourgeons apicaux de frêne à la fin de la période de dormance sont mieux précisés ce qui n'a jamais été fait jusqu'à présent.