ECOLOGY OF ASCLEPIAD LIANES 
101 
Stem 
The stems of Marsdenia and Rhyncharrhena 
have a continuous vascular cylinder traversed by 
narrow rays (Fig. 10) as in many other asclepiads 
(Metcalfe & Chalk 1957). Both species have 
simple uniseriate hairs (Fig. 10A) and many 
druses attached to the wall of cortical cells (Fig. 
10B). The cells of the hypodermis look very' 
similar to those of the epidermis in both spe¬ 
cies. 
Phloem occurs as small strands internal to the 
primary xylem (Fig. 10) in both species; such 
intraxylary phloem is universal in the family 
(Metcalfe & Chalk 1957). Some vessel elements 
have larger diameter in Rhyncharrhena than in 
Marsdenia (Fig. 10). Both species have thick- 
walled fibres adjacent to the external phloem. In 
both, broken cells indicate the natural dis¬ 
integration of the central pith to produce a 
lysigenous cavity (Fig. 10). 
While laticifers are thought to occur in the 
shoots of all asclepiads (Metcalfe & Chalk 1957), 
they can be difficult to distinguish (Metcalfe 
1966) and we were unable to identify any with 
certainty. Cut stems exude milky latex in Mars¬ 
denia as in most asclepiads, but not in 
Rhyncharrhena which we assume has watery 
latex as seems to occur in Asclepias tuberosa 
(Wilbur 1976), Ceropegia cumingiana (Bruyns 
& Forster 1989) and some other asclepiads 
(Williams 1984). 
Foliar glands 
A number of plant species including asclepiads 
have foliar glands or squamellae adaxially at or 
near the leaf base (Ramayya & Bahadur 1968). 
As their nature in asclepiads is almost unknown, 
they were examined in Marsdenia and Rhyn¬ 
charrhena. 
Both Marsdenia and Rhyncharrhena have a 
pair ol glands per leaf adaxially at the leaf base 
(frgs 11,12). The glands are up to 0.3 mm long 
and exude copious amounts of a sticky, trans¬ 
parent substance through a large pore in the 
c^ofthe^and head (Fig. 11). Both stalk and 
ead are nearly cylindrical but are more convex 
abaxially; the head sits obliquely on the stalk. 
ascular tissue is absent. Necrotic cells on gland 
tips of older leaves may show that glands gradu- 
auycease to function once the leaves mature (see 
also Lapinpuro 1976). 
In the closely-related Apocynaceae, the gland 
secretion is thought to be a high polymer resin 
an its suggested role is to provide a protective 
coating on adjacent young lateral buds 
(Ramayya Sc Bahadur 1968); this may apply also 
in Marsdenia and Rhyncharrhena. Such coatings 
might protect against climatic extremes in arid 
areas (Dell & McComb 1978) or against herbi¬ 
vores (Juniper & Jeffree 1983). 
The anatomy and morphology of the glands 
are very similar to those of the other taxa exam¬ 
ined in detail, both Apocynaceae (Ramayya & 
Bahadur 1968) and the asclepiad Marsdenia 
liisae (Lapinpuro 1976, Williams 1989). 
GERMINATION 
Methods 
The coma was removed from seeds of known age 
and provenance (Appendix 2) and any broken, 
shrivelled or empty seeds discarded. Seeds were 
surface sterilized in 3% sodium hypochlorite 
and sown onto 9 cm Whatman 182 filter papers 
in sterile glass petri dishes, with five replicates of 
ten seeds per dish. The dishes were placed in 
growth cabinets with lights supplying 180-240 p 
E m -2 s -1 for a 12 hr photoperiod at 10/10°C, 
16/14°C, 25/15°C and (. Marsdenia only) 
30/20°C. For Marsdenia only, seed of four ages 
(Appendix 2) stored at room temperature and 
humidity was tested for germinability at 
25/15°C. Comparable Rhyncharrhena seeds 
were not available. 
Results and discussion 
No seeds germinated at 10/10°C. For Marsdenia, 
final germination percentage was very similar at 
25/15°C and 30/20°C but the germination rate 
was faster at 25/15°C. By comparison, 16/14°C 
reduced both the amount and the rate of germi¬ 
nation in both species. Germination was highest 
and fastest in Rhyncharrhena at 25/15°C (Fig. 
13) . 
Germinability of Marsdenia seed declined 
steeply with time and was zero at 14 yr (Fig. 
14) . 
The germination maximum of 25/15°C for 
Marsdenia is broadly similar to that for other 
asclepiads like Morrenia odorata (20-25°C; 
Singh & Achhireddy 1984) and Ampelamus albi- 
dus (30°C; Soteres & Murray 1981). The present 
data for 10/10°C and 16/14°C suggest that ger¬ 
mination may be reduced or prevented by win¬ 
ter temperatures in the field. All viable seeds of 
both species tend to germinate quickly given ap¬ 
propriate conditions; there is no evidence of 
dormancy as in some cool temperate species like 
Asclepias syriaca (Bhowmik & Bandeen 1976). 
