166 BOTANY. 



In the lady-slippers there are two fertile stamens, and a 

 third sterile one has the form of a large triangular shield ter- 

 minating 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 exces- 

 sively 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. 



Of the five thousand species known, the great majority are 

 inhabitants of the tropics, but nevertheless there are within 

 the United States a number of very beautiful forms. The 

 largest and showiest are the lady's-slippers, of which we 

 have six species at the north. The most beautiful is the 

 showy lady's-slipper (Cypripedium spectabile), whose large, pink 

 and white flowers rival in beauty many of the choicest tropical 

 orchids. Many of the Habenarias, including the yellow and 

 purple fringed orchids, are strikingly beautiful as are the 

 Aretliusece, (Arethusa, Pogonia, Calopogon). The last of these 

 (Fig. 90, H) differs from all our other native orchids in having 

 the ovary untwisted so that the labellum lies on the upper 

 side of the flower. 



A number of the orchids are saprophytic, growing in soil 

 rich in decaying vegetable matter, and these forms are often 

 nearly or quite destitute of chlorophyll, being brownish or 

 yellowish in color, and with rudimentary leaves. The coral 



