Oil and mucilage cells in Annona (Annonaceae) and their systematic significance

The morphology and distribution patterns of oil and/or mucilage cells, i.e. idioblasts, in the leaf of 37 Annona species are described. Idioblasts are always present in the spongy parenchyma in all species and in most cases also in the palisade parenchyma. Usually both oil cells and mucilage cells occur; in some species either oil or mucilage idioblasts are present. Their number ranges from few to abundant Both idioblast types possess a suberized wall layer. Mucilage cells are significantly larger than oil cells. The distribution patterns of oil and/or mucilage cells often do not coincide with the current classification into sections. Leaf features such as lamina thickness, the presence of sclereids in the mesophyll, crystals in the adaxial epidermis, hairs, an adaxial hypodermis, a thickened cuticle and the presence of a papillate abaxial epidermis and sunken stomata are compared with the distribution of oil and mucilage cells. Following an analysis of the systematic value of oil and mucilage cells it is concluded that combinations and frequencies of oil and mucilage cells are often useful in species identification, and sometimes indicate relationships between species. In general, the significance of oil and mucilage cells as phylogenetic markers is extremely limited as shown in tentative cladistic analyses of leaf anatomical characters.