THE ANGIOSPERMAE 



1343 



other orchid genera, such as Dendrobiiim, Cynoches, and Mor modes. In 

 Catasehim (Fig. 1252), the cohimn is large and rises above a basin-shaped 

 labellum. At the top of the column is the anther and below it the rostellum. 



Fig. 1252. Caiasetum tridentatum . A, Male flower in vertical 

 section. B, Flower after stimulation showing pollinia with elastic 

 band and viscid disc being discharged. See text. {After Kerner.) 



Below this again the column is deeply hollowed and its edges are drawn out 

 into two tail-like appendages which hang down into the labellum. Attached 

 to the underside of the rostellum is the viscid disc, connected to the two 

 pollinia by an elastic band which passes over the front of the rostellum. 

 If one of the tails of the column is touched, however lightly, a stimulus 

 is transmitted to the rostellum which leads to the liberation of the viscid 

 disc. The elastic band thus freed, it straightens violently and shoots the 

 disc forward, tears the pollinia out of the anther and projects the whole 

 structure outwards with considerable force. The disc goes in front and if 

 an insect is visiting a flower, the pollinia will naturally be firmly attached 

 to it when the disc strikes its body. 



{c) Bristle mechanism. A third type, to which reference may be made 

 here, is found in the genus Pingiiiaila. In this genus there is a remarkable 

 difference in the pollination mechanisms shown by the various species. 

 P. vulgaris is pollinated by bees; P. villosa is probably pollinated by butter- 

 flies; P. liisitanica is self-pollinated, while in P. alpina there has been 

 developed a trap mechanism quite unlike those described above. 



Pingiiiciila alpina (Fig. 1253) is a mountain species with white flowers. 

 The tubular corolla is bilabiate, the upper part being made up of two petals 

 and the lower half of three. It is extended backwards below the calyx in the 

 form of a spur. The two stamens and the ovary are inserted at the base of 

 the corolla and lie below the upper lip. The stamen filaments are curved 

 around the ovary and the small anthers are compressed and lie against the 



