ANATOMICAL STRUCTURE OF THE PERENNIAL STEM IN DIFFERENT ROSA L. SPECIES

The observations comprise a comparative study of the anatomical structure of the perennial stem in the species Rosa canina L., R. damascena Mill., R. rubiginosa L., and R. rugosa Thunb., cultivated in the Botanical Garden of Iasi. Cambial activity gives rise to a relatively thin outer ring of secondary phloem and to several rings of secondary xylem, which indicate the age of the examined branch. Within the phloem ring, conducting elements (sieve tubes and companion cells), phloem parenchyma cells (some containing idioblasts), and irregularly dispersed libriform fibres with thickened, lignified walls can be distinguished, these fibres being more abundant in R. rugosa. In R. rubiginosa, the phloem fibres are arranged in two layers separated by phloem parenchyma. At the periphery of the phloem ring, in contact with the inner layers of the primary cortex, thick bands of sclerenchymatous fibres with very thick, heavily lignified walls (with a punctiform lumen) are visible, especially in R. canina and R. damascena. Analysis of the secondary xylem shows its ring-porous aspect in all observed species. Each growth ring displays on its inner side vessels of larger diameter (representing the earlywood), separated by libriform fibres with strongly thickened, intensely lignified walls. On the outer side, each ring contains fewer vessels of smaller diameter, with libriform fibres predominating – a feature common to all examined species. In contact with the pith, remnants of the primary xylem can still be observed, completely lignified and protruding as small bundles into the pith. All annual rings of secondary xylem are traversed by numerous multiseriate medullary rays, usually of uniform width or variable in R. canina. In all cases, the rays consist of radially elongated cells with relatively thin but lignified walls. In all species, the oldest ring of secondary xylem is thicker than the subsequent ones, in which large-diameter vessels predominate. The pith exhibits a network-like appearance, in which very large, thin-walled parenchyma cells are separated by much smaller cells with moderately thickened but strongly lignified walls.