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Journal Number 95
May 2005
ORIGINAL PAPERS
Pterostylis Venosa can be distinguished from
- Pterostylis Humilis
By Bruce Irwin and Dan Hatch
When Vic Vercoe published photographs of Pterostylis venosa and/or P. humilis in Journal 73:
p28, we felt that we had information which might assist in a positive identification, but Bruce
had misfiled his best drawings of P. venosa. Fortunately they have reappeared, so here is our
belated response.
It is difficult to visualise an orchid from a botanical description published more than a hundred years ago, especially when the author didn't bother to mention characters which are today of diagnostic importance (cf Cunningham, who omitted to mention the leaf shape of his Acianthus rivularis. Look at the trouble he caused!).
When Colenso described Pterostylis venosa, he made no mention of its stigma. How was he to
know that 30 years later another Pterostylis species would be found, which while similar to
P. venosa in outward appearance, would possess a most remarkable stigma? What would be
the point of his describing the unremarkable stigma of P. venosa?
As it turned out the other plant was P. humilis, and it had a remarkable stigma because it was
self-fertile. Unfortunately this stigma, hidden within the flower, didn't show in photographs.
In distinguishing P. venosa from P. humilis the shape of the stigma is paramount. The other
items listed in the table are more useful to confirm identification than to establish it.
The following table of characters is based on published descriptions and detailed drawings -
those of P. venosa from Mt Egmont only (it doesn't occur on Ruapehu); those of P. humilis from
both Egmont and the Ruapehu area. Characters considered helpful in identifying photographs
are marked with an asterisk *. |
After fertilisation of the flower, the stem elongates, largely above the floral bract, and in
P. humilis there is a tendency for the leaves to scatter up the stem, but this is a variable
character.
Although the foliage of P. venosa is usually yellowish-green and that of P. humilis bluish-
green, leaf colour is unreliable because the leaves of both species can become bright yellow,
depending on the environment/substrate, the age of the plant and the time of the year.
If after photographing the plant, you are not sure of its identity, prise the flower open and
examine the stigma - you won't hurt it.
Pterostylis venosa
Drawings by Bruce Irwin of a Mt Taranaki plant.
Photographs by Ian St George of plants found near Berwick, Otago.
Pterostylis humilis
Drawings by Bruce Irwin of plants from Ruapehu and Mt Taranaki.
Photographs by Ian St George of a plant found at Whakapapa, Ruapehu.
 
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