160 STUDY OF COMMON PLANTS. 



The homologies of the flowers of orchids have been 

 discussed at length by Darwin and others. The following 

 may be given as a brief resume of the most essential facts : 



Comparing the flower of an orchid with a simpler one, 

 such as a lily, the several whorls are seen to have under- 

 gone varying degrees of modification. The three sepals 

 are readily identified, although they are usually petal-like 

 in structure, and two, or sometimes all three, may have 

 undergone coalescence. Of the three petals, the two 

 lateral ones are alike, while the third, called the lip, is 

 enlarged and differs widely in form from the other two. 

 The essential organs are consolidated into a single body, 

 the column. In the genus Cypripedium one stamen has 

 become abortive, while the two remaining ones produce 

 pollen ; in the other genera of the family only one stamen, 

 as a rule, is perfect. The ovary shows its origin in three 

 carpels ; but it is one-celled, and the three placentae are 

 parietal. 



Theoretically it is held that originally the stamens were 

 in two whorls of three each, and that in Cypripedium the 

 staminode (abortive stamen) belongs to the outer whorl 

 and the two fertile ones to the inner, while in other 

 genera, in the great majority of cases, this relation is 

 reversed. For a brief but satisfactory statement of this, 

 with good diagrams, see Luerssen, Botanik, p. 469. 



