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Comparative anatomical studies were conducted on two invasive species (Chromolaena odorata and Tithonia diversifolia) and two non-invasive species (Ageratum conyzoides and Aspilia africana) in the family Asteraceae in Nigeria. The aims are to study the anatomical characters of the invasive species and the noninvasive species with a view to report the anatomical characters in the invasive species responsible for invasiveness and also to correlate these characters with the functions they perform in the invasive species. The foliar and stem micromorphological study of the invasive and non-invasive species were undertaken using Light Microscope (LM). The occurrence of vessels in the pillar of the abundant sclerenchyma tissues are important component of the skeletal system in the invasive species. The prominent tiles of parenchymatous cells for effective conduction of water and nutrients; the occurrence of various vessel types: short and long together with wide and narrow vessels for water conservation and reduced vulnerability of stem to cavitations; the long but coiled trichomes for effective light piping; high stomata sizes with low stomatal index to reduce excessive evaporation that might lead to desiccation and severe disruption of photosynthetic function; all these among others are the characters reported for the invasive species and they are responsible for their aggressiveness and xerophytic nature.
Keywords: Micromorphology, sclerenchyma, characters, aggressive, foliar, stem.