Endothelium - Ovule

In plants bearing unitegmic ovules, the nucellus degenerates during early stages of ovule development and the embryo sac comes in contact with the innermost layer of the integument. The cells of this layer become specialised to supply nutrients to the embryo sac. These cells elongate radially, their cytoplasm becomes dense and store starch and lipids. This specialised layer is called endothelium. Endothelium is also reported in some bitegmic ovules. Tapetal cells are similar to endothelium cells of anther in storing carbohydrates, proteins, RNA, ascorbic acid and other metabolites; it is secretory in function is hence referred to as the integumentary tapetum.