A portion of ground tissue (PO:0025059) directly internal to an anther wall endothecium (PO:0020002) that develops from an anther wall secondary parietal cell layer (PO:0006007). [source: ISBN:9780003686647, POC:curators]
May be a single or double layer. Usually crushed by the endothecium and therefore almost invariably degenerates early during anther development. In some angiosperms, including Arabidopsis, it develops from an anther wall outer secondary parietal cell layer. In other angiosperms, it develops directly from an anther wall primary parietal cell layer, while in still others, two layers of the middle layer may develop from both an anther wall outer secondary parietal cell layer and an anther wall inner secondary parietal cell layer.