The inner part of the leaf (free of green rind) or pulp is a clear, soft, moist, slippery tissue. It has been described using several other terms including mucilage tissue, mucilaginous gel, mucilaginous jelly, inner gel, and leaf parenchyma. Here we will use the term 'pulp.' The term 'gel' is often used, but is not an accurate description because it implies a homogeneous entity. In addition, the term 'gel' is also confusing at times because it is unclear whether it is used to refer to the intact pulp or the viscous liquid gel prepared from it.
The pulp consists of large mesophyll cells (Kluge etal., 1979; Trachtenberg, 1984; Fahn, 1990; Evans, 1996). The vascular bundles are tubular structures located in the pulp, but adjacent to the green rind. The number of these bundles varies, depending on the size of the leaves. They are the conducting system of the leaf responsible for transporting nutrients. The non-viscous yellow liquid that flows freely from freshly cut leaves is derived from the pericyclic cells associated with these vascular bundles.
Cell walls and cell membranes can be observed in the pulp (Kluge etal, 1979; Trachtenberg, 1984), although intact cellular organelles such as nuclei, chloroplasts, and mitochondria are not usually detected. The only organelles that have been observed in the mature pulp mesophyll cells are dilated or degenerated plastids. These may be the source of mucilage polysaccharide during the early stage of mesophyll cell development (Trachtenberg, 1984). Thus, unlike mucilage tissues in other succulents, aloe pulp does not contain chloroplasts and is not involved in CAM (Kluge etal, 1979). This is consistent with the fact that the photosynthetic activity only occurs in the rind where chloroplasts are located.
A viscous clear liquid gel or mucilage is contained within the pulp mesophyll cells. One major component of the liquid gel is a neutral acetylated P1 ^ 4 linked mannan. Thus, A. vera pulp is also unique in that the polysaccharide that constitutes the mucilage is a neutral polysaccharide (See Chapter 4). Mucilage polysaccharides found in other plants are mostly acidic polysaccharides (Kennedy and White, 1983).
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