REPRODUCTION AND DISPERSAL 847 



lateral display of zygomorphic flowers (fig. 1174), which thus are well suited for in- 

 sects that alight on lips or hover before the flowers. 



Odor. — The attractiveness of flowers to insects is in large part due to 

 their fragrance. Most fragrant flowers are also showy (as in the Hlacs, 

 roses, crabs, and water-lilies), but some very fragrant flowers are incon- 

 spicuous (as in the grape and mignonette), just as some very showy 

 flowers are without appreciable odor (as in the poppy) . In many plants 

 (as in Smilax herhacea and Trillium erectum) the odor, though offensive 

 to human nostrils, attracts certain insects. Some flowers that are rela- 

 tively odorless by day are very fragrant at night (as in species of Silene) . 

 Flower fragrance commonly is due to the escape of volatile oils into the 

 atmosphere. A remarkable case of floral dimorphism is seen in Renan- 

 thera, a tropical orchid; most of the flowers are white and inodorous, but 

 at the base of the inflorescence are two fragrant yellow flowers which 

 bloom first and remain fresh and fragrant until all the other flowers 

 have gone. 



The sensitiveness of pollinating insects to color and to odor. — It is 

 believed commonly that odors and bright colors in flowers are of great 

 importance as indicators (or " signals ") to insects of the presence of 

 nectar or pollen, and some observers even go so far as to suppose that 

 these features have arisen through natural selection, the insects preferring 

 the more fragrant and showy flowers, while others go unpoUinated, so 

 that the plants bearing them have no progeny. There is no evidence 

 whatever for the selection theory of the prevalence of showiness and odor, 

 and even the theory that insects are attracted by color and by fragrance 

 rests too little on experiment and too much on the untenable assumption 

 that the theory must be true, because nobody knows any other r61e for 

 these floral features. It is a tenable hypothesis that such features are 

 without value to the flowers possessing them, and the " signal " theory 

 deserves support only as it is proven experimentally. 



It is not certain that insect attraction is the only possible role of colored corollas; 

 it has been suggested that they may play an important part in the chemistry of fruit 

 maturation. Pigmented plastids may be important in food making, and pigmented 

 cell sap may indicate the formation of useless by-products. It is to be noted that 

 some wind-pollinated flowers are very showy, as in the larch and the red maple. 

 Corollas also are of some importance as protective organs for the pollen and stigmas, 

 especially in flowers whose corollas close at night and in stormy weather. 



The possession of a keen sense of odor by pollinating insects is un- 

 doubted, inconspicuous fragrant flowers being visited much more than 



