NZNOG Journal 102 - Various Contrivances

Journal Number 102
February 2007

VARIOUS CONTRIVANCES

Thelymitra
The New Zealand terrestrial orchid flora is unique because most self pollinate:
the various contrivances by which the New Zealand orchids are fertilised by themselves are recounted here.

Some of the best wild orchid floral displays are the spikes of Thelymitra aff. longifolia in the
far north: fragrant, pink, all open at once, mimicking the manuka, deceiving the native bees
(see Figs 20,21,OBC).



Fig.20: a small grasshopper on Thelymitra aff. longifolia at Rubbish Dump Hill.	Fig.21: a typically fine display of the T. aff. longifolia of the Far North.	OBC: A Thelymitra having affinities with Thelymitra longifolia Te Paki, October 2006.

But many Thelymitra are self pollinating: scentless, barely opening if at all, and then only one
at a time.

Cheeseman described the process for T. longifolia. As the flower matures, the column lengthens,
the top edge of the stigma rolls back leaving the sensitive surface facing up and back; the anther
cap is drawn upward leaving the pollinia adherent to the rolled-back top of the stigma; fertilization
follows.

Certainly T. longifolia is capable of insect pollination, and I have reported elsewhere the frantic
apparently pseudo-copulatory movements of a native bee engaged with the dark column.
But self pollination by the method described above is at least a secondary capability.

In T. matthewsii the pollinia are so easily dislodged from the anther cap that the slightest
movement sends them falling down onto the stigma.

Thelymitra matthewsii

pollinia intact pollinia fallen onto stigma

RD Fitzgerald wrote [Australian Orchids. Sydney, Charles Potter, 1876-1894] "Mr. Darwin's proposition regarding the contrivances by which Orchids are fertilized' (sic) is, that they `have for their main objects the fertilization of each flower by the pollen of another flower.' As far as I could investigate the subject in Australia, I have not been able altogether to verify this proposition; for though the great majority appear to be frequently impregnated by pollen brought from other flowers, I believe they are also frequently fertilized by their own.

Certain parts of plants and animals being wonderfully designed (or adapted) for certain ends to the palpable benefit of such plant or animal, it is argued that their life or existence being dependant on such design, they must have been so created. But what, except inheritance, can account for the extension of similar parts to others where they are evidently useless?

The flowers of Thelymitra ixioides are of a beautiful blue colour, and are borne in attractive spikes. In the centre of each flower is placed the stigma or female part of the flower; it resembles a shield, covered with viscid matter. At the top of it is inserted, in a notch, a little boss or button called the rostellum, connected with which, and behind the stigma, are the pollen masses, and on either side arms stretch out, supposed to attract and guide to the rostellum. Touch this rostellum, which is covered, as it were, with glue, with the point of a pin, and then withdraw it, and the pollen masses are at once withdrawn from behind the stigma. Return the pollen masses, now firmly attached to the point of the pin, again into the flower, and the greater part of them adhere to the viscid surface of the stigma, and the flower is fertilised.

Such is the process, and the only one, by which it can be impregnated; but in nature the proboscis, or some part of an insect, acts as the pin has done in the experiment. Left to themselves, in a bell-glass, not one flower of Thelymitra ixioides will produce seed."

"Can there be a more perfect example of predetermined design? The bright colour to attract the insect - the arms to guide it - the projecting rostellum for it to touch - the viscid matter on the rostellum to adhere to the visitor - and the expanded shield-like stigma, covered in its turn with gum to lay hold of the pollen, when the insect either returns its head in search of honey or visits another flower, perhaps on the same spike. What trace is there of development? It is a well-adapted whole: a whole adapted to one end."

"Now examine another species of the same genus, Thelymitra carnea. The flowers are of a bright pink. Here are the extended arms - the shield-like stigma - the sticky rostellum and the pollen masses behind the glutinous stigma; but there is a slight modification; the pollen masses are not only behind but over the stigma, and crumble upon it whilst yet in bud, thereby fertilizing the flower, which seldom opens, and never until after fertilization.

What has become of the picture of design? For what is the colour in the flowers, seeing that they so rarely open, and then to no purpose? For what are the arms? What use is the rostellum? Without their aid, Thelymitra carnea is far more fertile than T. ixioides; in fact, every flower produces seed. Why are all those parts, so necessary in T. ixioides, present in T. carnea? Can they be accounted for by any other explanation than relationship through inheritance?"



The immature (left) and mature (right) column of Thelymitra longifolia. As the column lengthens, the pollinia are left adhering to the top of the stigma.		Thelymitra malvina has friable pollen and as the flower opens its parts are liberally covered with fragments.