194 : THE ORCHID REVIEW. (JULY, 1900. 
microscope, are seen to cohere in fours, called tetrads. This is the earliest 
stage of union, and its significance consists in the fact that during the 
formation of the pollen grains one mother cell divides into four daughter 
cells, as they are termed, and the four daughter cells remain in union, instead: 
of forming separate grains, as in ordinary plants. The further union of 
these tetrads into pollinia is secured by modified pollen grains, which form 
connecting threads and the stalk and axis found in sectile pollinia. The 
caudicle of the Ophrydez, which is so well seen in the remarkable South 
African Bonatea speciosa, is also formed of modified pollen grains. 
Thus we see that the rostellum and pollinia, together with the column, 
and in some cases the lip also, have progressed through increasing stages of 
complexity, and we will now endeavour toreturn as nearly as possible to the 
original starting point. This takes us back to the tribe Arethusez, where the 
union between the anther and rostellum has not taken place. The genus 
Pogonia is one of the most primitive of existing forms, and here we find the 
anther attached to the apex of the column by a short but distinct filament, 
situated close to the rostellum, but quite free from it, and versatile. The 
pollinia are powdery and very friable, the compound grains being only held 
together by a few slender threads, so that they often break up with a 
comparatively slight touch. The sepals are somewhat connivent, and the 
lip forms the landing stage on which the insect alights. On entering the 
flower the insect touches the rostellum, liberating some of the glutinous 
matter, which in turn comes in contact with the pollinia, and on the insect 
retreating the pollinia adhere to its body, and are thus carried to the next 
flower visited, when they get left upon the stigma, and pollination takes 
place. 
Pogonia ophioglossoides is a very beautiful North American Orchid, 
which Meehan remarks grows generally in bogs, among sphagnum and 
sedges, and sometimes a bog will be perfectly ablaze with the bright purple 
blossoms. It is widely diffused in suitable localities from Canada to Florida, 
and occurs also in China and Japan. According to Scudder, the pollen 
consists of powdery grains not united by threads, and he also remarks that 
the rostellum is absent, which would make it more primitive in structure 
than other members of the genus—at all events in the extended sense of the 
Genera Plantarum. In this work Nervilia, Cleistes and one or two others 
are included in Pogonia, and species of the former are known in cultivation, 
and possess a rostellum, which secretes viscid matter, but is quite free from 
the anther. Nervilia is widely diffused in the warmer regions of the Old 
World, and Cleistes is exclusively American. : 
Arethusa and Calopogon are allied genera of North American swamp 
Orchids, and the latter is distinguished by having the lip uppermost, and 
bearded on the upper side with Jong, club-shaped hairs. In Arethusa the 
