Viscaceae

Kuijt, J., 2013, Prophyll, calyculus, and perianth in Santalales, Blumea 57 (3), pp. 248-252 : 249-251

Viscaceae

While in Loranthaceae prophylls appear to be of minor significance in the vegetative parts of plants, this is not so in several genera of Viscaceae . Nevertheless, here also they have not been given adequate attention. In Ginalloa and Notothixos they have not been mentioned in the relevant literature, even though the inflorescences in the former genus appear to bear them, as in the axillary ‘triads’ mentioned and illustrated in Barlow (1997), where they subtend secondary lateral flowers. In Korthalsella we find an unclear situation in flower­bearing regions (the concept ‘inflorescence’ itself is not always applicable in the genus) but, again, there is no published record of prophylls in vegetative parts of the plant ( Barlow 1997). In fact, Mekel (1935) explicitly denies the presence of any prophylls in the axillary groups of flowers.

Viscum

There appears to be no previous mention of prophylls in the extensive literature on Viscum . This is surprising in what is the most famous, and most written-about of all mistletoes, V. album L. In V. album , there are regular, annual innovations of one internode each, topped by one pair of foliage leaves and a terminal inflorescence. At the very base of each innovation, there is one pair of minute, translucent, fimbriate prophylls. These structures are visible even during the winter, well before new innovations have started to elongate, and turn blackish in later years ( Fig. 1a ). In the axil of each prophyll, an inflorescence often develops in the next growing season. It seems safe to assume that other species of Viscum , even though most have a very different branching pattern, also have prophylls in comparable locations. Thus the position of later inflorescences can demonstrate the presence of prophylls.

Dendrophthora and Phoradendron

It is important to realize that, in contrast to the view of Eichler (1868), prophylls in Phoradendreae are distinct from any so-called basal cataphylls that may also be present. Basal cataphylls are pairs of leaf scales placed at the base of lateral ramifications in many species in both genera ( Kuijt 2003), but they occur in addition to, and distal to, the prophylls. In those species that lack basal cataphylls, prophylls are nevertheless present. In a few species of Dendrophthora , the prophylls of a lateral branch may be fused to form a compound structure ( Kuijt 1959). Especially in those species of Phoradendron where innovations abort terminally (dichotomous species), such prophylls usually subtend inflorescences or vegetative branches. These ramifications themselves are again provided with prophylls, etc., resulting in a highly complex but extremely regular, symmetrical grouping ( Fig. 1b ). The degree of prominence and the shape of prophylls, particularly in Dendrophthora , may provide important specific features taxonomically.

Arceuthobium

As far as I am aware, no mention of prophylls has appeared in the literature dealing with Arceuthobium . As in all Viscaceae , phyllotaxy is paired. Branching patterns in the genus are of two types, flabellate and verticillate ( Kuijt 1970), and this has led Hawksworth & Wiens (1970) to recognize two subgenera, subgenus Vaginata Hawksw. & Wiens and subgenus Arceuthobium , respectively. In the flabellate pattern, younger, secondary lateral branches are added abaxially to the primary one, so that a flat, fan-like arrangement results. No prophylls are present. The branching pattern in male A. americanum Nutt. ex Engelm. is essentially a replica of that of Viscum album , with one important difference: there is no sign of even minute prophylls, even though the relevant secondary branches develop in the expected positions, flanking the primary lateral branches ( Fig. 1c ). A second difference is that female A. americanum has a percurrent stem system, while V. album is uniformly dichotomous by means of a terminal inflorescence. (The verticillate branching pattern illustrated in Hawksworth & Wiens (1996, f. 2-1d) is incorrect). Another species, A. azoricum Hawksw. & Wiens , an endemic to the Azores, is said to be “crucial to an understanding the migrational history of the genus” ( Nickrent et al. 2004). The species is also characterized by verticillate branching, as are the related A. oxycedri (DC.) M.Bieb. from Eurasia and Africa and A. juniperi-procerae Chiov. from Ethiopia and Kenya ( Hawksworth & Wiens 1976). The whorled female flowers of A. azoricum are said to be subtended by ‘minute bracts’ but, unfortunately, no details are available, and I have not been able to inspect female material. A comparison of male A. azoricum with the North American A. americanum leads to some intriguing facts. The main branching pattern of A. azoricum is identical to that of A. americanum ; but here, each secondary lateral is subtended by a minute prophyll that emerges slightly from above the nearby leaf scale ( Fig. 1d ). These prophylls are black or nearly so ( Fig. 1d, e ), and may be basally fused into a single, compound structure ( Fig. 1e ). They are almost entirely hidden by the associated leaf scales, usually extending for no more than 0.5 mm beyond them. The tip of a prophyll may be acute and fimbriate, or it may be blunt to laterally expanded. The existence of prophylls in this species almost certainly means that the A. americanum ancestry also had prophylls in the relevant positions, and that they have been eliminated in the past, even though its ‘axillary’ secondary branches do form. That leaves us with the remarkable notion of ‘phantom prophylls’, organs that have disappeared in the course of evolution while their morphogenetic influence has remained. Endress (2010) lists a number of comparable instances of this phenomenon in angiosperms.

Eremolepidaceae

Flowers of Eremolepidaceae lack prophylls throughout ( Kuijt 1988a).

Prophylls in non-mistletoe families

While I cannot judge the data and illustrations provided by Wanntorp & Ronse De Craene (2009) for ‘ Olacaceae ’ ( Diogoa , Heisteria and Olax ), there are convincing indications that in non-mistletoe families the prophyllar hypothesis must also be rejected. In those flowers where undeniable calyculi develop (sometimes after anthesis), non-articulated pedicels support them, thus making the idea of any morphogenetic contribution by even very small or unnoticed prophylls a very questionable and remote proposition.

Figures

Fig. 1 Prophylls in Viscaceae. – a. Viscum album, male (Victoria, British Columbia). A year-old node, mid winter. Arrows point to the two visible prophylls. The central scar is that of last year’s inflorescence, the scars of last year’s leaves are indicated with ‘l’. – b. Phoradendron nitens, a dichotomous species (redrawn from Kuijt 1969). The aborted apex (centre) is shown in black; foliage leaves in broken lines. Three numbered, successive generations of prophylls, flanking their inflorescence scars, are shown in the upper half of the figure, where most prophylls have been removed. – c. Arceuthobium americanum, male (Moyie, British Columbia). One type of verticillate branching, with two primary lateral branches (la) and four flower buds (one invisible) occupying prophyllar positions. No prophylls are present. – d. Arceuthobium azoricum, male (Wiens 4953, MO). View of an internode, the nearest lateral branch removed (broken line). Prophylls black (arrows). – e. Arceuthobium azoricum, male (Wiens 4953, MO). Top view of a node such as the upper one of Fig. 1d, two lateral stems and the percurrent one removed (shaded). The four prophylls (black) are fused into a single, compound structure.