All three genera have simple, entire leaves with mucronate apices. All show winged or ridged stems with the ridges merging with leaf petiole bases. Leaf phylotaxy was used by Hill (1915) to justify merging Colpoon with Osyris because both alternate and opposite leaves were seen in the former. Variation in phylotaxy was noted by Stauffer (1961) where C. compressum was scored as decussate, rarely with the two leaves somewhat shifted. Bean (1990) scored C. compressum as ‘leaves usually opposite to subopposite, less often alternate’ and the newly described species Colpoon speciosum as ‘leaves as often alternate as subopposite, less often opposite’. Although Rhoiacarpos leaves were scored as opposite by Hill (1915), it too can show the subopposite condition (Figure 4b). Thus, phylotaxy appears to be a rather plastic morphological feature in Osyrideae. Shifting (displacement) of leaf position through development can be a form of metatopy (Weberling 1989) possibly influenced by genetic or environmental factors.

Rhoiacarpos is easily distinguished from the other two genera by leaf features. Its leaves are nearly sessile, bifacial (shiny above and dull below), with cordate bases and somewhat revolute margins. The leaves of Osyris and Colpoon differ in these characters, but are quite similar to each other overall. Hilliard (1994) states for both Osyris and Colpoon ‘both leaf surfaces closely and minutely white-dotted’, but this feature has not been observed by this author in the field, herbarium or in photographs.

The inflorescences of Osyris are mainly axillary whereas those of Colpoon and Rhoiacarpos are terminal. It is interesting that Hill (1915) describes the inflorescence of Rhoiacarpos as a panicle composed of 3-flowered axillary cymules. In Figure 4 of Stauffer (1961), both Rhoiacarpos and Colpoon are diagrammed with 3-flowered units. In other Santalales, this 3-flowered cymule unit is equivalent to a dichasium. But the situation in Osyrideae appears more complex, mainly owing to compression and reduction (loss of flowers). Hilliard (1994) indicates flowers in Osyris lanceolata ‘may occur in 2–3-flowered dichasia’. Technically, a dichasium is 3-flowered whereas a monochasium is 2-flowered. Upon first examination, the inflorescence units may appear to be dichasial such as in Osyris (Figure 2c), Colpoon (Figure 3b), and Rhoiacarpos (Figure 4c), but in these cases the pair of flowers (or buds) subtending the older, terminal flower are unequal in age. Moreover, the older of the two subtending buds is often present as a lateral innovation. For these reasons, it seems best to describe these units as monochasia or compound monochasia. Simple (2-flowered) monochasia are frequently seen in the female inflorescences of Osyris (Figure 2d).

The terminal peduncles in the panicles of Colpoon are often swollen (Figure 3a and d). In his diagram, Stauffer (1961) coloured black those inflorescence axes that become compressed. Thus, the cluster of flowers seen arising from these swollen apices in Colpoon is interpreted as being a series of reduced monochasia (or dichasia according to Stauffer 1961). The phenomenon of syndesmy (incorporation of partial inflorescence units and their axes into the main axis) is well documented in cymoid types resulting in a coenosome (Weberling 1989). Looking at the lower inflorescence branch in Figure 3a, the six flowers and buds are of different ages: two past anthesis, one at anthesis, one older bud and two younger buds. This situation could be interpreted as two dichasia, but because the ages of the flowers are wrong, this situation might best be interpreted as three monochasia.

The components of the inflorescence axes (peduncles) are often subtended by small bracts. Stauffer (1961) indicated that these abscise in Osyris, abscise or are persistent in Colpoon and are persistent in Rhoiacarpos. The study by Bean (1990) confirms that the persistence of these bracts in C. compressum is polymorphic (compare Figure 3a and d) and further shows that in C. speciosum the bracts are persistent and enlarge upon fruiting.

The flowers in Colpoon and Rhoiacarpos are bisexual, whereas in Osyris lanceolata flowers appear to be unisexual. Hilliard (1994) stated, ‘the flowers of Osyris lanceolata are hermaphrodite and male, the plants being androdioecious’, but no evidence was provided that supported the presence of bisexual versus female flowers. Female flowers possess stamens (Figure 2d), but if the anthers can be shown to not produce pollen, then these are staminodes and the flower is functionally female.

Flower merosity is notoriously variable in Santalaceae, often showing three or four different forms within the same inflorescence. But for differentiating the three genera considered here, this character has some value when the most common number is considered. Thus, Osyris generally has 3-merous flowers, occasionally having 4-merous flowers. Colpoon typically has 4-merous flowers, but 5- and 6-merous flowers can also be seen. Most Rhoiacarpos flowers are 5-merous, but 4- and 6-merous flowers are also known. The number of lobes of the glandular disk in these genera follows the number of petals.

In all three genera, many fewer fruits develop to maturity compared with the number of flowers originally present on the inflorescence axis. In Colpoon, evidence of flower abortion exists (Figure 3d) such that a single fruit is present per axis with the scars of abortive flowers present at the swollen peduncle apex (Figure 3f). The fruits of Rhoiacarpos differ from the other two genera in having persistent petals at the apex (Figure 4f and g). A cluster of fruits is shown in Figure 4f; however, each fruit appears to be present on a separate inflorescence branch with evidence of numerous abortive flowers and their associated bracts below (Figure 4g).

In Osyris (Figure 2f) and C. compressum (Figure 3f), the petals abscise and are not persistent on the mature fruit. Bean (1990) indicates that in C. speciosum the petals are persistent; thus, this feature is polymorphic within the genus. Although it is not clear whether the character of fruit shape will remain valid following further observations, it appears that the fruit in Colpoon is more obovoid than in the other two genera that have spheroid to ellipsoid fruits.