In some plants, especially monocots, parts of the nucellus enlarge or proliferate after fertilization, and have a role as a regulating or storage tissue for the developing embryo. Seed storage tissues derived from the nucellus are termed perisperm. In some monocots (e.g. some members of the order Poales) endosperm is entirely absent from the mature seed, and perisperm represents the primary storage tissue. Seeds of Yucca possess perisperm that contains membrane-bound protein and oil bodies within the cells, together with reserve carbohydrates in the thick cell walls. Some members of the monocot order Zingiberales (gingers, bananas and their relatives) possess perisperm, but this is often entirely compressed in the mature seed, with only the cell walls remaining (e.g. Musa). In other members of Zingiberales (e.g. Canna), mitotic activity during ovule development causes the chalaza region of the nucellus to become massive; this region is then sometimes termed a pachychalaza. In Acorus the perisperm is dermal in origin, formed from nucellar epidermal cells that elongate and become filled with transparent proteinaceous cell contents.