In most flowers belonging to other orders the stigma secretes a little viscid matter.

Now, in certain orchids similar viscid matter is secreted, but in much larger quantities by one alone of the three stigmas; and this stigma, perhaps in consequence of the copious secretion, is rendered sterile.

When an insect visits a flower of this kind, it rubs off some of the viscid matter, and thus at the same time drags away some of the pollen-grains.

From this simple condition, which differs but little from that of a multitude of common flowers, there are endless gradations--to species in which the pollen-mass terminates in a very short, free caudicle--to others in which the caudicle becomes firmly attached to the viscid matter, with the sterile stigma itself much modified.

In this latter case we have a pollinium in its most highly developed and perfect condition.