The genera Polygonum and Bilderdykia (Polygonaceae) in southern Africa: morphology and taxonomic value of the ocrea and fruit

The external morphology of the fruit and the ocrea of 16 taxa of Polygonum and Bilderdykia in southern Africa was studied. Fruits are either lenticular or trigonous. Six types of fruit surfaces were distinguished. Five types of ocreae were found, characterized by: a green undulating limb; a silvery hyaline sheath; a brown tubular sheath without terminal hairs; a terminal fringe of short cilia or setae; and a bristly hairy sheath fringed with long rigid setae.


INTRODUCTION
The aim of this study was to examine the morphology o f the ocrea and fruit of 16 of the southern African species of the genera Polygonum and Bilderdykia, and to determine their taxonomic importance.

MATERIALS AND METHODS
The herbarium voucher specimens mentioned in the captions o f the figures are all kept at the National Herbarium (PRE), Botanical Research Institute, Pretoria.

Organography o f the fruits
In Polygonum and Bilderdykia the ovary is superior, ses sile, 1-locular.with a solitary basal, sessile or stalked ovule.The fruit is a nut enclosed by the persistent perianth (Dyer 1975).During dispersal the seed remains enclosed in the entire fruit wall (Harder & Firbas J965).
Based on the shape of the fruit, two types are recog nized (Table 1): 1. trigonous or 3-angled fruits (Figure 1A) and 2, lenticular fruits (Figure IB).In both types the sur faces are either concave (Figure 1A) or convex (Figure 1C).Trigonous fruits and lenticular fruits with convex surfaces are com m on, whereas lenticular fruits with concave surfaces are found only in Polygonum lapathifolium and P senegalense (Table 1).

Organography o f the ocrea
The ocrea (plural: ocreae; sometimes spelled ochreae), is a tubular sheath formed when the stipules are united into a hood, which covers the stem apex at first; later it is ruptured and remains as a membranous tube surrounding the stem at the nodes (Harder & Firbas 1965).
Ocreae o f the southern African species o f the genera Polygonum and Bilderdykia can be divided into five different types (Table 1): 1, a tubular membranous sheath ending terminally in a spreading or recurved, green, leaf-like undulating limb with a strigose margin (Figure 3A).This type occurs only in P. limbatum.
2, a silvery hyaline sheath, conspicuously veined, reddish at the base or red all over, lacerating easily (Figure 3B).This type is found in five species.
3, a brown membranous tubular sheath, conspicuously veined without or rarely with short terminal hairs (Figure 3C).This type occurs in six species.4, a tubular membranous sheath fringed with short rigid cilia or setae (Figure 3D ).This type occurs in three species.5, a brown tubular sheath, bristly hairy and fringed with numerous long rigid setae and tearing readily on one side (Figure 3E).This type is found only in Polygonum pulchrum.Ocreae can be smooth and hairless (Figure 4A) or pubescent with multicellular trichomes (Figure 4B).

DISCUSSION AND CONCLUSION
From Table 1 a key has been compiled using only fruit shape, fruit surfaces and ocreae found in species of Polygonum and Bilderdykia.In the Flora o f southern Africa (Vol.9,1 in prep.)fruit shape, fruit surface and ocrea type play a major role in the keys.In this paper it is the first time that these features have been illustrated.In the genera Polygonum and Bilderdykia the fruit and the ocrea are the two most important organs for distinguishing between the taxa.