Light and scanning electron microscopy as well as histochemical reactions were employed to investigate the leaf structure and secretory activity of an adventive perennial pest that is rapidly dispersing in the Mediterranean region. Interesting structural characteristics, not common among the well-adapted native Mediterranean phanerophytes, were observed. Although ‘unconventional’, these structural characteristics produce an ecologically equivalent, xeromorphic leaf on a plant that proves to be competitive and very efficient under the Mediterranean climatic conditions.

Extensive investigations on the anatomy of the two leaf types in a seasonally dimorphic subshrub revealed interesting variations between summer and winter leaves. Summer leaves of Sarcopoterium spinosum possess a thick epidermis composed of tannin-containing cells and large amounts of mucilage secreted through the inner periclinal walls towards the mesophyll. A thick cuticle is also present on the surface of the leaf. In winter leaves the epidermal cells produce no mucilage while phenolics are accumulated in granular form only. Besides these, some other variations between summer and winter leaves are also discussed in respect of the ability of the plant to withstand the unfavourable Mediterranean conditions.

Seasonal dimorphism is considered from the anatomical viewpoint. Significant differences can be observed between winter and summer leaves obtained from the same individuals. Winter leaves are thick, amphistomatic with a large number of chloroplasts in the cells of the loose mesophyll. The thinner summer leaves are hairier and hypostomatic, with compact mesophyll and cells that accumulate phenolics and crystals. CbJoroplasts are fewer in number and less developed, and at the ultrastructural level they present pronounced differences from those of the winter leaves

Lippia citriodora H.B.K. is an aromatic plant indigenous to South America. It is cultivated and commercialised as an ornamental for its lemon-like scent emitted from its leaves and flowers. The present morphoanatomical and histochemical study revealed that leaves of L. citriodora possess one type of setae (non-glandular) and at least five types of glandular trichomes, with the latter differing anatomically and in the composition of their secondary metabolites. Scanning electron microscopy revealed that the same types of trichomes exist also on the calyces. Histochemical tests indicate that the glandular trichomes contain a combination of terpenoids, flavonoids, carbohydrates, phenolics and alkaloids. However, within the vacuoles of the mesophyll cells, terpenoids, phenolics, flavonoids and tannins are stored. The presence of lamellar cytoplasmic inclusions in the mesophyll cells, possibly precursors of tannins stored in the vacuoles, is reported for the first time.