Rank of the Subfamilies Loranthoideae and Viscoideae

The family Loranthaceae has been divided into the subfamilies Loranthoideae and Viscoideae (Engler, 1888-89 ; Danser, 1929, 1933a, b). These two subfamilies show great dissimilarity in their morphological as well as their embryological characters.

In the Loranthoideae flowers may be unisexual or bisexual, but a calyculus is invariably present. The ovary may contain a lobed or unlobed mamelon, or the mamelon may be extremely reduced or absent. A collenchymatous pad or tube is invariably present in the ovary. The embryo sac is of the Polygonum type. Many embryo sacs may develop simultaneously in the same ovary and their tips grow up into the style and in some cases reach the stigma. The lower end of the sac sometimes produces a caecum. The endosperm is a composite structure formed by the fusion of all the endosperms developing in the same ovary. The first division of the zygote is always vertical and is followed by transverse divisions resulting in a long biseriate proembryo. The fruit shows 4 distinct layers and the viscid layer is situated outside the vascular bundles of the corolla.

In the Viscoideae, flowers are unisexual and the calyculus is absent except in Viscum orientate and V. articulation. The central ovarian papilla has 2 or more archesporial cells, each of which divides to form two dyad cells. Of these the upper dyad cell is the larger and functions, while the lower soon degenerates. Thus the development conforms to Allium type. A peculiar feature is that after the 4-nucleate stage there is a slow but steady curvature of the embryo sac, which causes its lower end to bend out of the papilla and proceed upwards into the carpellary tissue. The egg apparatus differentiates from the quartet in the originally lower pole, which is later situated at a higher level than the upper. Generally only one embryo sac develops which is said to show inverted polarity. The endosperm is derived from the primary endosperm nucleus of a single embryo sac. The first division of the zygote is transverse except in Korthalsella. The embryo lacks a suspensor. The viscous part of the fruit is situated within the vascular bundles of the perigone.

Thus, the Loranthoideae and the Viscoideae show important differences in floral structure, mode of development of the embryo sac, endosperm and embryo, and in the location of the viscid zone of the fruit. These differences adequately warrant the raising of the two subfamilies to the rank of fatnilies. As proposed by Miers (see Danser, 1929), the name Loranthaceae may be retained for the Loranthoideae, and the Viscoideae may be designated Viscaceae.