The generally accepted and descriptive term for orchid seed is “Dust Seed”, a term that has been derived from a translation of the German term, ‘StaubSamen’, (Barthlott, 1976; Rauh, 1975), which aptly describes the size, lightness and number of seeds within orchid pods, for example: < 4x106 seeds per pod for
Cycnoches
chlorochilo, (Arditti, 2000).
Orchids produce the smallest of seeds. They range in weight from 2Pg to 14Pg and size from 0.4mm to 1.25 mm in length, with a breadth of between 0.08mm to 0.27 mm, (Withner, 1959; Dressler, 1981; Arditti, 1992, 2000). This dust-like character of orchid seed is typical of long distance wind dispersed plants, (Arditti, 2000; Eriksson, 2000; Levin, 2003; McMahon, 1973; Nathan, 2002, 2006).
Orchid seeds encompass a range of shapes: flattened, globular, lenticular, kernel shaped, ovoid, balloon, winged and thread-like, (Dressler, 1981; Baskin, 1998; Arditti, 2000). The structure and morphology of orchid seed is an important characteristic in orchid taxonomy, (Healey, 1980; Arditti, 2000; Tournay, 1960; Burgeff, 1936; Withner, 1959).
Orchid seed development, without a viable endosperm, initiates when a megasporangium is produced with the outer integument later forming the seed testa. The megaspore mother cell forms an embryo sac containing eight haploid cells. Double fertilization, typical of angiosperms, occurs when a pollen tube enters the ovule micropyle. One of the subsequent developments is the triple fusion (n + 2n) that occurs when one of the pollen tube sperm nuclei (n) fuses with the polar nuclei (2n) to produce a 3n primary endosperm nucleus. It is at this stage the monocotyledonous orchid seed differs from other angiosperms in that the primary endosperm nucleus degenerates and endosperm is not formed, (Sharma, 1987; Sood, 1986; Baskin, 1998). This absence of endosperm and an embryo of minimal cells provide the basis for the orchid seeds characteristic lack of weight and sustenance, (Arditti, 2000; Withner, 1959).
Apart from seven genera: Arundina, Bletilla, Dendrochilum, Encyclia, Polystachya,
Sobralia and Thunia, (Arditti, 1992, 2000; Rasmussen, 1995), all orchids lack
endosperm or, in a few cases, have a minimal number of cells forming the embryo.
The size, volume and weight of orchid seed indicate that the embryo volume tends to be very small compared to the testa surrounding it, (Barthlott, 1976; Arditti, 2000; Healey, 1980). This difference creates the so called “balloon effect”. The air space, between the embryo and the testa endows the seed with an aerodynamic facility, which enables the orchid seed to remain airborne for a considerable time.
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It is this aerodynamic facility, which provides one of the major features in the success of orchid long distance dispersal, (Arditti, 2000; Barthlott, 1976; Burgeff, 1936).
The large quantity of seed produced per pod and the seed volume to gross weight of each seed is typical of many other long distance seed dispersal plants, (van der Pijl, 1982; Nathan, 2002; Levin, 2003; Bullock, 2000). Parasitic plants also produce large seed numbers and this has been suggested as a strategy to increase the potential of securing a host, (Teryokhin, 1982). In the case of orchids the necessity to expose the seed to a suitable mycorrhizal fungus is essential since the seed embryo requires additional nutrition that can supplement the storage resource of the lipid droplets the embryo contains, (Arditti, 2000; Burgeff, 1936; Withner, 1959; Quay, 1995; Richardson, 2000; Wilkinson,1997; Zettler, 1997).
No research of orchid seed transport to New Zealand has been investigated, although the evidence of long distance wind transport to and from various land masses within the Southern Hemisphere has often been reported, (Munoz, 2004; Sturman, 1997; Gandawijaja, 1983; Close, 1978).
This section of the thesis investigates the various aspects, important to the initial dispersal of N. iridescens seed, namely, plant peduncle elongation, dehiscence timing advantage, seed size, aerodynamic advantages and germination. Some characteristics of the N. iridescens seed testa, using the external and internal patterning of the testa could possibly be of taxonomic value, (Arditti, 1992, 2000; Rasmussen, 1993, 1995), and is briefly mentioned. An investigation comparing: N.
iridescens, N. papa, N. longipetalum and Singularybas oblongus from within the
genus Nematoceras, was also undertaken
Because of the long germination period found in the majority of temperate terrestrial orchids, (Rasmussen, 1993, 1995; Baskin, 1998), the time available for research and the time constraints imposed by thesis protocols, only some aspects can be presented in this thesis. A general lack of success has dogged previous researchers in their attempts to germinate temperate, terrestrial orchids of the Southern Hemisphere, ( Batty, 2001; Rasmussen, 1995, 1998; Zettler, 1997; Quay, 1995; Collins, 1992; Clements, 1986; Warcup, 1971).
A1.2 Results
Mature pods, of N. iridescens occurred from late November – early February. The ambient weather appeared to “fine tune” dehiscence, if the weather was warm and had low humidity, the mature pods dehisced early. However, if the weather had a high humidity and temperatures were low the mature pods remained closed for a longer period, (Pers.obs). The time taken, after fertilization, for three seed pods
of Nematoceras iridescens peduncles to elongate to an average length of 153
mm to dehiscence was 63 ± 9 days. Initially the peduncle remained short, the base of the pod lying next to the adaxial leaf surface (See Fig.A1.1). The sole pod peduncle gradually lengthened, over a period of 57 days, to 0 – 20 mm followed by an exponential lengthening rate of 40 -100 mm over the final 10 days.(Fig. A1.2 and A1.3).
Figure A 1. 1 Early seedpod formation on a nursery N. papa specimen plant in September 2005.
Figure A1. 2 Fully extended Nematoceras iridescens peduncles,
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Figure A1. 3 Peduncle expansion rates of four species of the CA: N. papa (1), N. iridescens (1, 2 and 3), N. longipetalum (1) and Singularybas oblongus (1)
The first result was obtained by measuring the growth rate of the peduncle length every 9 days and recording this growth (Fig.A1.3.). The seed pod extends, in the 6 weeks prior to maturation, (Fig.A1.1 and A1.2) above the surrounding bryophyte vegetation and litter. Liberation of seed occurs at a climatically optimal time, distance, seasonal temperature and humidity, which provides an advantage of the thermal influx air currents that exist in forest gaps.