88 AEETHUSE^. Chap. III. 



upper part of the column, with their edges meeting 

 in front, as may be seen in fig. B. In this drawing 

 the petal on the near side has been cut away, and 

 the labellum is represented in the position which it 

 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 escap- 

 ing by the same passage, it will almost certainly leave 

 at least one of the four pollen-masses on the adhesive 

 stigma, and thus fertilise the flower. 



All that I have here said is taken from the admir- 

 able description given by Mr. Cheeseman * of Pterostylis 

 trullifolia ; but I have copied the figure of P. longifolia 

 from Mr. Fitzgerald's great work on the Australian 

 Orchids, as it shows plainly the relation of all the parts. 



Mr. Cheeseman placed insects within several flowers 

 of P. trullifolia, and saw them afterwards crawl out, 

 generally with pollinia attached to their backs. He 

 also proved the importance of the irritable labellum 

 by removing it from twelve flowers whilst young, and 

 in this case insects which entered the flowers would not 

 have been compelled to crawl out through the passage ; 

 and not one of these flowers produced a capsule. The 

 flowers seem to be frequented exclusively by Diptera ; 

 but what attraction they present is not known, as they 

 do not secrete nectar. Mr. Cheeseman believes that 

 hardly a quarter of the flowers produce capsules ; not- 

 withstanding that on one occasion he examined 110 



* ' Transact New Zealand Institute,' vol. v. 1873, p 352 ; and vol. 

 vii. p. 351. 



