402 VANILLA. PART n. 



their places of attachment, but the whole pollinium is 

 jerked forward over and beyond the tips of the horns, 

 to the distance of two or three feet. The insect, dis- 

 turbed by so sharp a blow, or after having eaten its fill, 

 flies with the pollen adhering to it to fertilize the fe- 

 male plant, which differs from the male in having no 

 antennse. Thus the agency of insects is as requisite to 

 fertilize these semi-dioecious as hermaphrodite orchids. 



The Yanilla, which is cultivated for its aromatic pods 

 in Tahiti, Bourbon, and the East Indies, does not bear 

 fruit without artificial aid, which shows that the Ameri- 

 can insectj which fertilizes it in its own native home, is 

 not indigenous in the places mentioned. It appears that 

 many exotic orchids require a less elevated temperature 

 than has hitherto been supposed. 



The form and position of the necta,ry are exceedingly 

 varied. In certain species, both of the native and 

 tropical orchids, they are always dry ; but Mr. Darwin 

 has discovered that, in these cases, the walls of the 

 nectaries are thick and formed of two coats, and that a 

 liquid is contained between them, to which the insect 

 penetrates by piercing the inner wall. The exotic 

 orchids are, for the most part, larger, and require larger 

 insects to fertilize them than our small ones, whose 

 organs are generally microscopic. A curious instance, 

 both of this and of the extraordinary form of the nectary, 

 is found in the Angrsecum sesquipedale, a Madagascar 

 orchid, with large six-rayed flowers, like stars formed of 

 snow-white wax. It has a green whip-like nectary, 

 sometimes as much as a foot long, and, from the structure 

 of the plant, it appears that the pollinia never could be 

 withdrawn, until a large moth, with a wonderfully long 

 proboscis, attempts to drain the last drop of nectar from 

 the bottom of the nectary. 



Notwithstanding the vast diversity in the form of the 

 orchids, they are homologous in their general structure. 



