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Wood from the stems and roots of the malacophyllous Mediterranean drought semi-deciduous species Phlomis fruticosa was studied. The stem is diffuse-porous with narrow vessels. Though narrow vessels impose high conducting resistance, they are less vulnerable to cavitations, thus providing safety during summer drought and winter freezing. The geographical distribution of the plant (up to 1400 m altitude) may be relevant to the occurrence of narrow stem vessels, which provide high resistance to cavitations from low temperatures during winter. Vessel grouping in the stem adds to the safety against cavitations. Root vessels are mostly solitary and have almost double the diameter of stem vessels. Diffuse-porosity, the presence of narrow vessels in stems and the values of vulnerability and mesomorphy indices, are adaptations to the two stresses imposed by the Mediterranean climate, i.e. summer drought and winter cold.

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In: IAWA Journal
Authors: and

Root and stern wood of the Mediterranean summergreen Capparis spinosa L. was studied. Wood anatomical features favour high hydraulic conductivity, which is necessary for maintaining the high midday stomatal conductance and rates of photosynthesis observed in this plant. Xylem conduits of both stern and root consist of wide and short vessel elements with simple perforation plates. Vessel grouping in the stern secures xylem safety against cavitations. The plant would be highly vulnerable to cavitations due to freezing conditions, although these are rare during the Mediterranean winter. Thus, the anatomical features of the plant, which does not seem to suffer from water stress though growing entirely during the Mediterranean summer drought, are compatible with its adaptive strategy. The significant amount of minerals found in the root vessels, and the abundant starch grains of the wood might be involved in a possible osmotic shifting of water in the xylem.

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In: IAWA Journal
Authors: and

The structure of primary lenticels of the Mediterranean evergreen Olea europaea and the winter deciduous species Cercis siliquastrum was investigated during the year using scanning electron, conventional brightfield and epi-fluorescence microscopy. It was revealed that lenticels of O. europaea do not undergo significant structural changes over this time period. The filling tissue of O. europaea lenticels consists of fully-suberized cells that form small intercellular spaces. The air-exposed filling cells are replaced during spring and early summer by new early-suberized cells. Further notable structural modifications during the year were not observed. By contrast, lentice1s of C. siliquastrum possess a closing layer of suberized cells delimiting an underlying mass of non-suberized filling cells. During the period of high metabolie activity of the plant, i.e. during spring and early summer, the suberized closing layer is ruptured from the pressure exerted by the newly formed underlying cells. During late summer a new closing layer is formed, delimiting again the non-suberized underlying filling cells during winter. The possible role of lenticels in the gas exchange process is discussed. In both species the shade-adapted parenchyma cells of the cortex beneath lenticels shows bright red auto-fluorescence of chlorophyll, a phenomenon that is not yet fully understood.

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In: IAWA Journal

The annual rhythm of cambial activity and the pattern of starch accumulation in pith and ray cells of four seasonal dimorphic subshrubs typical of Greek “phrygana,” Sarcopoterium spinosum, Thymus capitatus, Phlomis fruticosa and Cistus incanus were investigated. Cambial activity in all species examined was found to occur during the winter when temperatures are low. During the rest of the year, cambium remains dormant. At the time of high cambial activity, starch content of pith and ray cells diminishes. High accumulation of starch grains was found to occur after cambial activity ceases in S. spinosum and a moderate starch accumulation occurs in C. incanus whereas there is no starch content in the pith of T. capitatus and P. fruticosa throughout the year. These results indicate that concerning their cambial activity, phryganic species behave like typical desert plants.

In: Israel Journal of Plant Sciences

The intracellular distribution of chloroplasts in the mesophyll was studied in leaves taken from seven plant species from different groups: the evergreen sclerophylls Olea europaea and Nerium oleander; the drought semideciduous and seasonally dimorphic shrub Coridothymus capitatus; the winter deciduous Capparis spinosa, which grows entirely during summer; and the cultivated species Hordeum vulgare, Vigna unguiculata, and Citrus aurantium. In all leaves and mesophyll types, chloroplasts were found to line the parts of the walls exposed to the internal leaf atmosphere. The arrangement of the chloroplasts adjacent to intercellular air spaces might be a universal phenomenon facilitating the inward diffusion of CO2.

In: Israel Journal of Plant Sciences