The pollen grains
of Dracontium are exclusively monosulcate. The monosulcate aperture type has
been generally regarded as the most primitive for angiosperms as well as monocots
(Muller, 1970; Walker & Doyle, 1975; Zavada, 1983) and Araceae in particular
(Grayum, 1992). Monosulcate pollen may be described as heteropolar, boat-shaped
and bilaterally symmetrical, and therefore, it may be assumed these conditions,
which are all intimately correlated with the monosulcate aperture type (Walker
& Doyle, 1975), represent the most primitive stages of polarity, pollen shape
and pollen grain symmetry in Araceae (Grayum, 1992, p. 123). Grayum (1992)
also suggested that boat-shaped, elliptic grains are more primitive than boat-shaped,
elongated ones; both types are found in Dracontium.
Dracontium pollen grains, as in
all other aroid genera except Xanthosoma and Chlorospatha (Grayum, 1992),
are shed in monads (Fig. 74- 82, Table 3). Monads are considered basic for
angiosperms as a whole.
The size of Dracontium pollen grains
ranges from 17.9 x 31.8 (D. amazonense) to 31.3 x 48 um (D. changuango). Grayum
(1992, p. 128), using measurements of only the longest axis, suggested that
the primitive pollen size of Araceae lies in the range of 22-34 um and there
is a pervasive trend in the direction of larger pollen. This implies that
Dracontium has both primitive and derived pollen types. However, pollen size
may vary according to the quality of mineral nutrition, local climate, and
other environmental factors (Muller, 1979). Different pollen sizes were found
in different populations of the same species (D. spruceanum).
Nonapertural exine sculpturing of
Dracontium pollen may be reticulate (D. changuango, Fig. 83), subreticulate
(D. amazonense, Fig. 84), foveolate (D. spruceanum, Fig. 85), punctate (D.
angustispathum, Fig. 86). Reticulate exine is widely regarded as primitive
for monocots (Walker & Doyle, 1975), since the earliest known fossil pollen,
Liliacidites, has this type of exine sculpturing (Doyle, 1973; Wolfe et al.
1975). However, Thanikaimoni (1970) and Harley (1990) suggested that the psilate
exine type (similar to punctate exine type) is primitive in palms. In Araceae,
Grayum (1992, p. 130) also characterized punctate exine as nearly as primitive
as psilate type; he observed that foveolate and reticulate exine types were
equally common and could not decide which of the two "subtypes" was more primitive.
In any case, the exine type of Dracontium is among the most primitive Araceae.
In Dracontium, a species may exhibit
more than one exine type or subtype. This indicates that the evolution of
exine characters has occured in the genus. A cladistic analysis involving
all other characters will be required to establish whether thee is a transformational
sequence from punctate to foveolate then reticulate exine in Dracontium. Then,
the evolutionary scheme of exine sculpturing in Araceae can be reconsidered.
Grayum (1992, 130-132, fig. 158) described a reduction in size of the brochi
toward the 'ends' in Dracontium prancei, but the significance of this is unknown.
Exine stratification has rarely
been studied in Araceae (Grayum, 1992) and was unknown in Dracontium until
the present study. Pollen exine stratification from three populations of D.
spruceanum have been investigated using TEM. The ektexine of Dracontium pollen
consists of a tectum and an infratectal granular layer (Fig. 87). This granular
type of infrastructure, much like the granular wall structure found in the
Nymphaeaceae, Magnoliaceae, and Annonaceae (Zavada, 1983), is found commonly
in gymnosperms (Xi & Wang, 1989). Endexine is very rare in monocots, and when
present is different from the endexine observed in dicots (Zavada, 1983).
It has been sought in only a few genera of Araceae (Grayum, 1992). The endexine
of Dracontium pollen is lamellate, with 4-6 layers. As far as I am aware,
lamellated endexine has never been reported for angiosperms. It exists in
most gymnosperms except some species of Podocarpus (Xi & Wang, 1989). I suggest,
therefore, that Dracontium is among the most primitive of aroids.
