118 PLANTS OF NEW ZEALAND 



be withdrawn just as they he on the summit of the cohimn, 

 they would hardly be in a position to strike the stigmatic 

 sm-face of another flower in the hollow below the rostellum, 

 but in being withdrawn, the cap of the anther pulls them 

 slightly downward and depresses them considerably, so that 

 they are easily placed on the stigma of a second flower." 



Pollination in the genus Pterostijlis has been well described 

 by Mr. T. F. Cheeseman.''' The lip is sensitive, and, when 

 touched, closes up against the column. An insect landing upon 

 it is thus entrapped, and the only means by which it can escape 

 is by the narrow channel left between the column and the lip. 

 In passing along this channel, it must strike first the viscid 

 stigma below the rostellum, and afterwards the anthers above 

 the rostellum. The back of the insect will now be 

 sticky, so that it will carry off with it the polhnia, or portions 

 of them, to the next flower which it visits, and place them 

 upon its stigma. As each plant bears only a solitary flower, it 

 is clear that cross-pollination must be effected not only 

 between different flowers, but between different plants. The 

 flowers of this genus are pollinated by certain species of flies. 

 This method of entrapping insects seems to be unique in 

 the order. The large green hooded flowers of this genus are, in 

 spite of their size, not very conspicuous, and it is not always 

 easy to determine what attraction they possess for the insects. 



The Ahaorptioii of Moisture. 



Orchids are not only remarkable for the structure of the 

 flower, and for their methods of pollination, but in many other 

 ways. Thus, they have often highly specialized methods of 

 absorbing moisture. There is in New Zealand, a minute 

 species of BolbopJiyUitm, B. Pygmceum. It is not uncommon, 

 but is frequently overlooked, as it is so inconspicuous. 

 The genus has received its generic name, because, at the base 

 of each leaflet, is a small bulb, in which are concealed the 



'Trans. Vol. V., p. 35J. 



