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Herbaceous primary stems have the same basic anatomical pattern of a structural hollow tower. Mechanical strength is concentrated in the periphery and supplied by lignified xylem, lignified sclerenchyma or unlignified collenchyma. This Flannel Flower (Actinotus helianthii) stem shows this hollow rod structure. The details vary from species to species depending on the deposition of both vascular tissue and supporting (i.e. sclerenchyma and collenchyma) tissue.