The ovule primordia of Poncirus and Citrus are trizonate and the outer integument is of subdermal origin. The seed-coats of Poncirus, Citrus, Fortunella, Citropsis and Murraya are very similar in structure but differ in minor points. The mechanical layer is a derivative of the outer epidermis of the outer integument. In the five taxa studied the seed coat is formed out of the outer integument and usually also contains remains of the inner integument and the nucellus. The innermost layers of the outer integument often contain crystals. A thin layer of endosperm one to a few cell layers thick is present.