Journal Number 88
September 2003

ORIGINAL PAPERS

Structure & Development in New Zealand Terrestrial Orchids
By E.D. Hatch

[updated 25 April 2003; previously published as Hatch E.D. Auckland Botanical Society Newsletter p5 November (1971); reprinted Orchadian September p10 (1972) and NZNOG Newsletter 2: p4 June (1982); NZNOG Journal 72: p17 September (1999)].

In New Zealand the terrestrial orchid is a typical monocotyledon. It consists of a creeping, branching rhizome with alternating nodes and internodes. (The nodes are the knobbly bits which carry the leaves, while the internodes, as the name implies, are the smooth stretches of stem between the nodes). Each node bears buds for lead, leaf and branch, and the relative development of these buds depends on their position on the plant.

Below the surface leaf-buds form scale-bracts and branches develop freely. Above the surface green leaves are usual and branches less frequent. In those species which lack chlorophyll, the leaves, not being required for food production, retain their underground scale-bract dimensions. The buds are borne only on the nodes and it follows that any bud-bearing structure must be, or include, a node.

The round tuber in Pterostylis, Corybas, Acianthus etc., is therefore an enlarged terminal node,
adapted for food storage, dormancy and regrowth, while the so-called `root' which precedes it
is a single elongated internode. Not all these branch internodes bear tubers. Some remain slender, have numerous root-hairs and appear to function only as feeders.

In Diplodium those species which have a bracteate-leaved mature form (I have experimented with alobulum, brumale, trullifolium, and the Australian coccineum and obtusum), will throw rosettes of juvenile leaves from the nodes of the flower stem in the event of damage to the plant. Usually the lower nodes but sometimes halfway up the stem, and these branch rosettes will in turn produce their own descending, tuber-forming internodes.

Working with Pterostylis oliveri [1], I discovered that if a flower stem comes into contact with the soil it will throw tuber-bearing branches from the nodes (i.e. from the axils of the leaves).

The elongated tuber in Thelymitra, Orthoceras and Calochilus, and in Spiranthes also, is an initial node combined with a partly enlarged following internode.

In Gastrodia the whole rhizome is enlarged, nodes and internodes together. Scale-bracts and
scars of scale-bracts point the position of the nodes.

The function of the tuber is to tide the plant over the dry season and in those species which form several tubers, to provide a means of vegetable increase. In mountain and far-southern species the dormant period is extended to cover the cold season as well. In species which live under relatively damp conditions throughout the year (Adenochilus, Danhatchia) there is a tendency for the rhizome to be perennial and no tubers are formed.

Molloybas cryptanthus which is normally rhizomatous, will form tubers under adverse conditions, while Townsonia deflexa and Corybas cheesemanii sport both semi-perennial rhizomes and regularly-formed tubers.

Some swamp species are also perennial. Thelymitra pulchella, which sometimes grows in water, will often throw a new leaf along the still-green old one, and the same occurs with Spiranthes. In Spiranthes also, if the plant is too small to flower, it will go on growing and not die back until it has flowered at the end of the second season. Until, that is, it has built up sufficiently large tubers with enough store of food to flower on. Spiranthes can do this because of the swamp environment; "dry" species are often forced to spend several seasons building up the tubers to flowering size. Spiranthes sometimes dies down at the end of the second season without flowering. Bob Bates [2] says "...In the semi-aquatic Microtis orbicularis the new tuberoid may begin to sprout before the old plant has died off".

In species which have different leaf-forms at different stages in their growth, for example the obtusum complex in Diplodium, the growth stage depends entirely on the size of the tuber, that is on the amount of nourishment available. In Diplodium alobulum and trullifolium tubers up to 3mm diameter will produce only rosettes, 4-5mm the intermediate flowering form with both types of leaves, while anything above 7mm will produce the mature flowering form with bracteate leaves only. D. brumale has no intermediate stage. The changeover from juvenile to adult is a tuber diameter of 7mm.

Similarly in the Australian D. coccineum the Rubicon is 12mm. Anything below that diameter will produce a rosette, anything above it a flowering plant. The rosette form which the small plants assume, provides a maximum area of green leaf for photosynthesis and results in the rapid formation of flowering-size tubers.

This variation in form does not affect species with basal rosettes (Pterostylis nutans, curta etc,) since maximum leaf development is always present. If the mycorrhizome of Diplodium brumale chances to develop in a congenial spot it will throw some tubers large enough to flower the following season, 2 years from seed to seed. But this is rare - the tubers are normally smaller and develop rosettes. I have done this experimentally and have also gown Spiranthes from seed and produced flowering plants in 3 years.

The form the plant takes depends on the development of the leaves and the length of the internodes. Large leaves and short internodes produce conspicuous rosettes like those of Pterostylis nutans; large leaves and long internodes forms like P. banksii.

Scale-bracts and long internodes give plants like Gastrodia and Danhatchia, while tubular leaves and long internodes give Corunastylis/Prasophyllum and Microtis.

The tall seeding peduncle of Corybas/ Nematoceras and Chiloglottis is a single internode.
The structure is simple but the possibilities are endless, and so the orchids have proved.

References
l. Hatch E.D. NZNOG Newsletter 26: p6 June (1988)
2. Bates R. Journal of the Adelaide Botanical Garden 7(1): p48 (1984)