190 EVOLUTION OF PLANTS 



fests itself first in the development of a colored peri- 

 anth or floral envelope, hence they are known as the 

 " petaloideous ' Monocotyledons. Most of these have 

 the carpels more or less perfectly grown together into 

 a compound pistil. 



In the great majority of the petaloideous forms the 

 parts of the flower are in whorls of three, the typical 

 arrangement being shown in the accompanying dia- 

 gram (Fig. 46, D). The two sets of leaves constituting 

 the perianth are usually alike in color and texture, but 

 occasionally, e.g. Trillium, the outer leaves are green, 

 and form a calyx like that found in most Dicotyledons. 

 The six stamens are in two alternating whorls, and the 

 three carpels completely coherent. 



Probably the lowest of the petaloideous series with 

 coherent carpels are the lilies, with their regular flowers 

 showing perfect radial symmetry. Here are found 

 many of the most magnificent of all flowers, and the 

 brilliant colors and fragrance of many of them show 

 their adaptation to insect aid in their pollination. 



Starting from the type exhibited by the simpler mem- 

 bers of the lily family, it is easy to see how specializa- 

 tion has progressed in different directions. This is first 

 seen in the coherence of the leaves of the perianth, so 

 that the flower becomes tubular, as in the hyacinth or 

 tuberose. This is sometimes accompanied by a slight 

 inequality in the size of the perianth lobes, especially 

 if the flower is nodding, and in such cases the stamens 

 and pistil are declined so that the flower is more or less 

 markedly two-lipped (Fig. 46, H). Carried further, the 

 cohesion of the perianth extends to the pistil, and the 

 result is a tubular flower with a so-called "inferior' 



