Abstract

WEBERLING, Focko. The stipules as reliable systematical characters. Bol. Soc. Argent. Bot. [online]. 2006, vol.41, n.1-2, pp.127-150. ISSN 1851-2372.

Stipules are reckoned among the most constant vegetative characters in Dicotyledons. They are formed as marginal outgrowths of the leaf base, at the border of the two integral parts of the leaf, the leaf base ("Unterblatt") and the "Oberblatt", the distal part of the leaf, which forms the lamina and the petiole. Their ontogeny is characterized by an early, often extremely early, appearance and a following precursive development. Therefore the stipules can temporarily exceed the other parts of the leaf, which enables them to function as protecting organs for the terminal bud of a shoot and even for the same leaf. The protecting function is not the only function of stipules. They can also work as assimilation organs or can be developed as spines or glands. In pinnate leaves with basipetal segmentation the stipules often have finished their growth whereas the segmentation of the lamina still is progressing. This confirms the opinion of Eichler (1861) who regarded "Unterblatt" (leaf base) and "Oberblatt" (the distal part of the leaf) as relatively independent components of the leaf. The early appearance of the stipules in the ontogeny of the leaf certainly is responsible for their high constancy and consequently their high systematic value. Form and position of the stipules can vary according to the "principle of variable proportions". In such cases, however, the study of their ontogeny can prove the morphological identity and elucidate the relations between different forms. Often stipules are confused with non homologous structures like basal auricules or pseudostipules, basal leaflets which are more or less transformed. Usually these structures are not as constant elements as the sitpules and they can be easily identified by comparative methods including the study of their ontogeny. On the other hand, stipules often appear to be absent because they are caducous organs or rudimentary. The distribution of stipules within the system of Dicotyledons and some comparative indications point to the possibility that stipules may have evolved several times and independently.

Keywords : Early appearance; Precursive growth; Auricules; Vaginal lobes; Pseudostipules; Rudimentation.

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