| Abstract: | Summary Alyssum leucadeum Guss. is an endemic Apulian chamephyte, growing on rocks; material collected at Santa Maria di Leuca has been studied anatomically and the following observations have been made. In the basal thick woody stem the sclerenchimatic tissue is well developed, and is divided up, by parenchima, in many bands which are normally disposed in regular concentric rings while in some cases a single band can be separated by the parenchimatic tissue into «islands» forming tangential rows. A few tubes are to be found in the sclerenchimatic bands but in very small number compared with the tubes observed in the parenchima. At times a sclerenchimatic band can be connected to the next one towards the outside through radial sclerenchimatic strands, making the stem more resistant mechanically. The parenchimatic bands interposed between the sclerenchimatic ones are composed of parenchima cells rich in starch. Starch consumption proceeds from the inside towards the outside and the outer bands are richer in starch. Inside the parenchima bands are islands of conducting tissue: the tubes being neither numerous nor large, as is to be expected in a xeromorphic plant. The islands of conducting tissue are sometimes surrounded by a sclerenchimatic sheath. The alternate succession of concentric rings of soft and hard tissue, together with the arrangement of the conducting elements in isolated groups inside the soft tissue, the groups being sometimes surrounded by a mechanical sheath, show that the stem is provided with great plasticity, is able to shape itself according to the space it finds in the rocks, and can undergo deformations, compressions, etc. An even greater plasticity can be obtained if necessary by resolving the sclerenchimatic bands in several smaller islands. Such a structure, apart from the width of tubes lumina, recalls somehow the secondary structure of certain lianas where the succession of soft and hard tissues gives a great plasticity to the stem. The structure of the root is similar to that of the stem, the parenchimatic bands being thicker in the radial direction and having cells completely packed with starch. Alyssum leucadeum is therefore a rock plant with a well defined anatomical constitution and particularly adapted to its habitat. It is a xeromorphic plant, with small tubes, rich in sclerenchimatic and reserve stisues, defended against transpiration by a complex covering of hairs, utilising the winter-spring season for its growth, and the summer as its resting period. Such an annual rithm shows the oriental affinities of this species while the presence of endogenous buds and of a thick layer of cork are other characteristics of plants particularly adapted to a rocky habitat. |
