Loranthaceae

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

Loranthaceae

The loranthaceous material used in Wanntorp & Ronse De Craene’s (2009) study consists of one species each of Passovia (‘ Phthirusa ’) and Struthanthus . Each of these genera is characterized by inflorescences bearing various numbers of paired, lateral triads. A triad consists of one central flower subtended by the primary bract (called pherophyll by Wanntorp & Ronse

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De Craene) and a pair of flowers positioned in the axils of its prophylls. Triads may be pedunculate or essentially sessile on the inflorescence axis. It should be added that the term ‘dichasium’ applied by Wanntorp & Ronse De Craene to lateral triads has also been variously circumscribed in botanical literature ( Lawrence 1951, Endress 2010).

The relevant new interpretation that the authors advance, apparently based on SEM images, is that the prophylls of a loranthaceous flower fuse to form its calyculus, as is also said to be the case in Olacaceae . There is abundant evidence in the known structure especially of neotropical small-flowered Loranthaceae to the effect that this interpretation (which here will be called the ‘prophyllar hypothesis’) has flaws and is difficult to reconcile with the relevant literature, as detailed below.

The structure of triads and dyads

When studying the triads of Struthanthus and Passovia as well as those of other Loranthaceae , it is immediately obvious that their prophylls are physically far removed from the ovary of the median flower. It follows that there cannot have been a developmental connection between prophylls and calyculus. This is true not only for triadic small-flowered genera (all of which are neotropical), but also for Psittacanthus , Tripodanthus and others where the lateral flowers of triads (or, in Aetanthus and some Psittacanthus species, dyads) are placed on elongated pedicels. In such cases, there is no evidence that the prophylls of the triad or dyad have a developmental relationship to any of the calyculi.

Monads

Many genera, both in the New and Old World, develop inflorescences bearing single lateral flowers (monads). In many cases ( Cladocolea , Tristerix , Loranthus , the upper inflorescence portions of Peristethium , etc.) flowers show no evidence of associated prophylls. In other genera, however, each flower is accompanied by a distinctive pair of prophylls; such is the case in Maracanthus ( Kuijt 1976a) , Oryctanthus ( Kuijt 1976b) , Oryctina ( Kuijt (1981a), Dendropemon ( Kuijt 2011a) , two species of Tristerix ( Kuijt 1988b) and Panamanthus ( Kuijt 1991) . In all these cases, a regular calyculus is formed without any developmental involvement by prophylls.

Terminal flowers

Inflorescences that are morphologically terminated by a single flower are known in a number of genera ( Kuijt 1981b), for example in Loranthus , Cladocolea ( Kuijt 1975) , Peristethium ( Kuijt 2012) and at least two Struthanthus species. Prophylls by definition are the first two phyllomes of a branch and thus do not accompany terminal flowers. In such inflorescences it would be difficult to assert that the calyculus of a lateral flower would have a different morphological origin from that of a terminal flower.

Phthirusa

Almost uniquely in Phthirusa , inflorescences are absent, the flowers being sessile in leaf axils, often in clusters. Each flower is associated with a distinctive pair of prophylls ( Kuijt 2011b), which again, cannot have been developmentally involved with the formation of the ovary and its calyculus.

Single pedicellate flowers

In Ligaria and Sogerianthe , the individual flowers are stalked, no inflorescence being present, and the possibility that we are concerned with greatly reduced inflorescences remains. In any case, where prophylls are present, the calyculus is far removed from them.