2II0 



A TEXTBOOK OF THEORETICAL BOTANY 



are cultivated in the hot-houses of orchid speciaHsts both in this country 

 and elsewhere. It is clearly outside the scope of this book to discuss this 

 subject. All we can do is to refer to a few outstanding genera, commenting 

 upon the pollination mechanism where the details have been described. 



The genus Dendrohium is among the larger genera, with some 750 

 species distributed throughout the Old World in Asia, Japan, Australia 

 and Polynesia. They are mostly epiphytic and many are in cultivation. 

 Darwin studied the flower of D. chrysanthum (Fig. 2049). The rostellum 



I 



Fig. 2049. — Dendrobium chrysan'hiim. A, Flower. B, Flower in part section, with the side 

 of the labellar pouch cut away to show the column and rostellum, with the anther at the 

 top and the nectary at the base. C, Column, front view, with the two anther cells above 

 partially covering the stigma. The nectary is at the base. (A after Bateman. C after 

 Daridn.) 



has an upper and a lower surface which are composed of a membrane 

 between which a mass of thick milky-white fluid can be forced out. This 

 mucilage, however, is less viscid than in many other orchids and takes about 

 half a minute to set. The large stigmatic surface lies below the rostellum, 

 while the anterior lip of the anther almost entirely covers its upper surface. 

 The stamen filament is of considerable length but it lies behind the middle 

 of the anther. When the flower is expanded the two pollinia unite into a 

 single mass and lie loosely on the clinandrum, i.e., that portion of the column 

 in which the anther is concealed, which is situated behind the rostellum. 



The labellum surrounds this central column, leaving an opening in 

 front, while its middle portion is thickened and extends back as far as the 

 top of the stigma. The lower part of the column is developed into a saucer- 

 shaped nectary. 



As an insect forces its way to the nectar the elastic labellum will be 

 depressed but the projecting lip of the anther will protect the rostellum 

 from being disturbed. As the insect retreats, however, the lip of the anther 

 will be lifted, thus causing the viscid liquid to be exuded and the pollinia 

 implanted on the insect's back. 



