194 



EVOLUTION OF PLANTS 



the New World, is the Bromeliacese. These are char- 

 acterized by modifications of the vegetative parts rather 

 than by the flowers, which are rather simple in structure. 

 Most of the order are epiphytes, and they form one of 

 the most striking features of the tropical American flora. 

 The best known of these are the so-called " Spanish 



FIG. 47 (Monocotyledons, Scitaminene, Orchidaceae) . A, flower of Canna : 

 the flower is strongly zygomorphic, with inferior ovary, o, and the 

 stamens reduced to a single one; B, the single stamen" an, and the 

 upper part of the pistil, st, of A; C, flower of an orchid (Arethusa), 

 showing the marked zygomorphy, inferior ovary, o, and the "lip," I; 

 D, a section through the "column," or coherent stamen and pistil of 

 Arethusa, showing the single anther, an, and the stigma, st: the rela- 

 tive positions of the anther and the stigma are such that insect-pollina- 

 tion is absolutely necessary ; E, flower of the wild yellow lady's-slipper 

 (Cypripedium pubescens), one of the orchids; I, the sac-shaped lip; 

 F, the column of the lady's-slipper, showing one of the two fertile sta- 

 mens, an, the stigma, st, and the third, sterile stamen, x. 



moss," of the southeastern United States (Tillandsia 

 usneoides), and the cultivated pineapple. Several spe- 

 cies occur in Florida, but it is further south that they 

 reach their greatest development. In the West Indies 

 they are abundant and varied, and form a very conspic- 

 uous feature of the vegetation, covering the branches of 



