238 .MORPHOLOGY OF ANGIOSPERMS 



indicate a separate origin from Glnmales. A morphological 

 investigation of these families in the tropics is greatly to be 

 desired. 



X. Orchidales.* — The two families of this alliance are 

 Bnrmanniaceae and Orchidaceae, all but about 55 of the 7,000 

 species belonging to the latter family. These two unequal fami- 

 lies are held together by the very numerous and small ovules 

 and by the extreme zygomorphism of the flower, but the Bur- 

 manniaceae have endosperm, often six stamens, and frequently 

 connate perianth-segments, approaching the Amaryllidaceae. 



The chief interest of the alliance centers about the Orchi- 

 daceae, the greatest monocotyledonous family in point of spe- 

 cies and the most highly specialized. The epiphytic habit is 

 extensively developed, and the terrestrial forms are mostly 

 saprophytic or parasitic. These habits have resulted in the 

 development of certain special structures, such as the bulbous 

 leaf-bases and velamen of the epiphytic forms ; and in the sup- 

 pression of some normal structures, as the primary root, and 

 sometimes all roots. The absence of endosperm, the poorly 

 developed embryo, and the extensive use of the suspensor as a 

 remarkably developed haustorial organ are probably but addi- 

 tional results of the unusual habits of the family. The notable 

 floral structures are the modification of one of the petals to 

 form the labellum and spur, the remarkable " gynostemium," 

 the twisted ovary, and the pollinium-mechanism. 



As an illustration of the varying modifications of floral 

 structure, the ordinary orchid may be compared with the Cy- 

 pripedium type. The flowers are pentacyclic, and the cycles 

 are developed in the two types as follows, beginning with the 

 outermost. In both types the first cycle consists of three sepals, 

 and the second of three petals, the posterior (made anterior by 

 the twisting of the ovary) forming the labellum and spur. In 

 the third cycle two lateral stamens are suppressed in both types, 

 but in ordinary orchids the anterior one is functional, while 

 in Cypripedium it is replaced by a staminodiuin. In the fourth 

 cycle the posterior stamen is suppressed in both types, but in 

 ordinary orchids the two laterals are replaced by staminodia, 

 while in Cypripedium they are functional stamens. In the 



* MrcRosPERMAE of Engler. 



