THE MONOCOTYLEDONES 



2105 



Fig. 2043. — Listera ovato. Tway-blade. A, Flower. B, Pollinia. C, Top of ovary with 

 part of the labellum, showing stigma (hatched) above which is the long rostellum; above 

 this the anther lies almost horizontally. (After Knuth.) 



The next genus we may consider briefly is Spiranthes of which some 

 fifty species are recognized. Some are found in Britain, of which S. 

 spiralis (Fig. 2044) (Lady's Tresses) is the best known. It is a small plant 

 with a tuberous root from which arise a group of radical leaves and a spirally 

 twisted inflorescence of pale, sweet-scented flowers. S. romafisoffiana may 

 be mentioned because of its curious geographical distribution. It is a native 

 of both North America and of Kamchatka, but is also found wild in 

 Ireland, and in the islands of Colonsay and Coll in the Hebrides. It occurs 

 nowhere else in Europe. 



Darwin studied the pollination mechanism in this genus. In S. spiralis 

 (Fig. 2045) the small, whitish, horizontal flowers possess the scent of 

 hyacinths, which is attractive to insects, and they alight on the reflexed 

 part of the labellum. In the lower part of this structure are two globular 

 nectaries, the nectar from which accumulates in a small receptacle situated 

 below them. The rostellum forms a long, thin, narrow process which is 

 united to the stigmas by two divergent shoulders. The central part of the 

 posterior side of the rostellum forms a receptacle for mucilage. Slight 

 contact causes a longitudinal slit to open in the anterior wall of this recep- 

 tacle, which gradually extends to the back of the rostellum and exposes the 

 cavity of the anther. Each loculus contains two very crumbly pollinia which 

 are separated above but are united in the middle by elastic threads. The 

 upper part of the anther, pressed against the back of the rostellum, dehisces 

 before the flower opens and the pollinia thus come into contact with the 



