Within each internode and about four plastochrons after its formation, an intercalary growth area appears, leading to a continued lengthening due essentially to anticline mitoses. It temporarily looks like a cambium. The blade of the leaf which is directly above the internode is absolutely necessary for the formation of this growth area. Xylem integrity of the cauline vascular bundle connected with this leaf is also required. Within each leaf, the presence of the blade and the integrity of vascular bundles are also indispensable for the lengthening of the petiole, whose mitotic system resembles the internode's. The hypothesis of a blade influence on the mitotic lengthening system through mechanical pressure due to the vascular development is discussed. Key words: internode, petiole, blade, vascular tissues, Manihot esculenta, intercalary growth.

To characterize the spatial heterogeneity of forest canopies at various scales, leaf area index (LAI) and transmission of photosynthetically active radiation (PAR) were measured. Six Scots pine (Pinussylvestris L.) stands, representing typical growth stages, from regeneration to mature forest, were selected for study, in Alsace, France. In each stand, hemispherical photographs were taken near the ground at regular intervals along transects, digitized, and analyzed. Gap fractions were inverted to provide estimates of LAI. Light transmission through the canopy was calculated by means of a numerical method. In a typical mature stand, the variability of LAI and PAR transmittance was spatially assessed by means of semi-variograms; spatial autocorrelation occurred up to about 10 m. This result provided a guide to optimize the sampling at the forest mosaic level. For the whole forest, LAI ranged from 1.6 to 7.8; PAR transmittance ranged from 0.02 to 0.59 under overcast sky and from 0.01 to 0.48 under clear sky (June). Maximum variability was found in the youngest stand, owing to a row effect, and in the two oldest stands, which include Norway spruce (Piceaabies (L.) Karst.) patches in the understory. Intermediate variability characterized maturing stands. Methodological problems concerning LAI and PAR transmission estimated by indirect approaches are discussed, as well as the spatial structure of these variables.

In order to assess the contribution of each layer to eVective leaf area index (LAI) in strati ed forest canopies, two Scots pine stands were selected in Haguenau forest, Alsace, France. One stand, 80 years old, had a sparse tree layer and a dense herb layer. The other stand, 50 years old, had a closed tree layer and dense herb and shrub layers. Hemispherical photographs, taken at 10-metre intervals along transects beneath each layer, were digitized and analysed. In the two-layered stand, mean LAI was 0.9 for herbs and 1.5 for trees, contributing 37% and 63%, respectively, to total LAI. A positive and signi cant correlation was found between layers, suggesting that the herb layer had not yet adjusted to the recent thinning of the stand. In the three-layered stand, mean LAI was 1.1 for herbs, 0.4 for shrubs and 2.1 for trees, a contribution of 31%, 11% and 58%, respectively, to total LAI. The positive and signi cant correlation for LAI among layers may be explained by the presence of a mixture of light demanding and shade-tolerant species in the undergrowth. Horizontal spatial auto-covariance of estimates was not signi cant, denoting adequate sampling interval for statistically independent measurements. Such in situ assessments of LAI, diVerentiating vegetation strata, are useful from the perspective of remote sensing of forests, since undergrowth plays an important role in the integrated spectral response of whole canopies.

No abstract

In the south of France, a few wild grapevines have been reported along hedges close to the lower Rhône valley. Two other small populations of Vitis sp. were found in two private lands, the Meyranes and the mas Saint-Georges woodlands, in the Camargue delta. In Meyranes, true wild grape vines are very rare, and much less abundant than in the mas Saint-Georges. The mas Saint-Georges population is particularly rich in Vitis individuals (an estimation of 52 individuals).Old maps indicate that the mas Saint-Georges woodland was an ancient wetland, dried up after the Rhône embankment at the beginning of the 20th century. The forest stand is thus ca. 100 years old. Wild grapevines have found refugia in this young woodland.The study aimed to provide insight into this Vitis population through field, ecological and genetic analyses.The genetic composition and diversity were analyzed on 37 samples. Thirty-one (31) individuals collected were true wild grapevines. Their genetic diversity was found to be similar to that of other European populations. Four were hybrids/introgressed between rootstocks and wild grapevines, and two were hybrids/introgressed wild grapevines and cultivars.