WITH MOBILE LIP
The New Zealand native genus Pterostylis is an interesting one in that it is distinctive by having flowers equipped with a mobile lip, similar to that found in Anguloas and Cirrhopetalums. This mobile lip is a pollination adaptation. The pollinating insect lands on the flower, and alights on the lip. Its weight causes the lip to tip, forcing the insect into a position where it pollinates the flower.
Charles Darwin, in his book "The Various Contrivances by which Orchids are Fertilsied by Insects", first published in 1962, included a description of the fertilisaiton process in this genus, based on the species longifolia and 'trullifolia'. The description is .....
"I may here briefly mention some orchids, inhabitants of Australia and New Zealand..........which are remarkable from their labella being extremely sensitive or irritable. Two of the petals and one of the sepals form a hood which encloses the column, as may be seen in the accompanying figure of Pterostylis longifolia.
The distal portion of the labellum
affords.....but when this organ is touched it rapidly springs up, carrying with the touching insect, which
is th us temporarily imprisoned within the otherwise almost completely closed flower. The labellum remains
shut from half an hour to one hour and a half, and on reopening is again sensitive to a torch. Two membranous
shields project on each side of the upper part of the column, with their edges meeting in front, as may be
fig.B In this drawing the petal on the near side had been cut away, and the labellum is represented in the position which is assumes after having been touched. As soon as the labellum has thus risen, an imprisoned insect cannot escape except by crawling through the narrow passage formed by the two projecting shields. In thus escaping it can hardly fail to remove the pollinia, as, before coming into contact with them, its body will have been smeared with the viscid matter of the rostellum. On being imprisoned in another flower, and on again escaping by the same passage, it will al,most certainly leave at leave at least one of the four pollen-masses on the adhesive stigma, and thus fertiliser the flower."
"The Various Contrivances by which Orchids are Fertilsied by Insects"
by Charles Darwin
The New Zealand species Pterostylis banksii is illustrated below.
Lip in 'normal' open position on left, in closed postion on right.
Side of flower has been cut away to show lip.
Pterostylis banksii growing naturally.
Anguloa clowesii is another widely cultivated orchid which has a similar mobile lip.
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