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ts of perigone leaves, three in each, and these are usually much alike except the lower (through the twisting of the ovary) of the inner set. This petal, known as the "lip" or "labellum," is usually larger than the others, and different in color, as well as being frequently of peculiar shape. In many of them it is also prolonged backward in a hollow spur (see Fig. 89, _B_). In all of the orchids except the lady's-slippers (_Cypripedium_) (Fig. 90, _B_), only one perfect stamen is developed, and this is united with the three styles to form a special structure known, as the "column" or "gynostemium" (Fig. 89, _B_, _C_). The pollen spores are usually aggregated into two or four waxy masses ("pollinia," sing. pollinium), which usually can only be removed by the agency of insects upon which all but a very few orchids are absolutely dependent for the pollination of the flowers. [Illustration: FIG. 90.--Forms of _Orchideae_. _A_, putty-root (_Aplectrum_), x 1. _B_, yellow lady's-slipper (_Cypripedium_), x 1/2. _C_, the column of the same, x 1. _an._ one of the two perfect stamens. _st._ sterile, petal-like stamen. _gy._. stigma. _D_, _Arethusa_, x 1/2. _E_, section of the column, x 1: _an._ stamen. _gy._ stigma. _F_, the same, seen from in front. _G_, _Habenaria_, x 1. _H_, _Calopogon_, x 1. In the last the ovary is not twisted, so that the lip (_L_) lies on the upper side of the flower.] In the lady-slippers there are two fertile stamens, and a third sterile one has the form of a large triangular shield terminating the column (Fig. 90, _C_, _st._). The ovules of the orchids are extremely small, and are only partly developed at the time the flower opens, the pollen tube growing very slowly and the ovules maturing as it grows down through the tissues of the column. The ripe seeds are excessively numerous, but so fine as to look like dust. The orchids are mostly small or moderate-sized plants, few of them being more than a metre or so in height. All of our native species, with the exception of a few from the extreme south, grow from fibrous roots or tubers, but many tropical orchids, as is well known, are "epiphytes"; that is, they grow upon the trunks and branches of trees. One genus, _Vanilla_, is a twining epiphyte; the fruit of this plant furnishes the vanilla of commerce. Aside from this plant, the economical value of the orchids is small, although a few of them are used medicinally, but are not specially valuable.
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