252 



PLANT STRUCTDKES 



and stigmatic surface into an indistinguishable mass in 

 the center of the flowers. The pollen-grains in each sac 

 are sticky and cohere in a club-shaped mass (pollinium), 

 which is pulled out and carried to another flower by the 



Fig. 235. A flower of an orchid (Habena- 

 riay. at 1 the complete flower is shown, 

 with three sepals behind and three pet- 

 als in front, the lowest one of which has 

 developed a long strap-shaped portion 

 (lip) and a still longer spur portion, the 

 opening to which is seen at the base of 

 the strap, and behind the spur the long 

 inferior ovary (epigynous character) ; 

 the two pollen sacs of the single stamen 

 are seen in the center of the flower, di- 

 verging downward, and between them 

 stretches the stigma surface ; the rela- 

 tion between pollen sacs and stigma sur- 

 face is shown in 2; within each pollen 

 sac is a mass of sticky pollen (pollini- 

 um), ending below in a sticky disk, 



which may be seen in 1 and 2 ; in 3 a pollen mass (a) is shown sticking to each 



eye of a moth. — After Gray. 



visiting insect. The whole structure indicates a very 

 highly specialized type, elaborately organized for insect 

 pollination. 



Another interesting epigynous and zygomorphic trop- 

 ical group, but not so elaborate as the orchids, is repre- 

 sented by the cannas and bananas (Fig. 120), common in 

 cultivation as foliage plants, and the aromatic gingers. 



From the simple pondweeds to the complex orchids the 

 evolution of the Monocotyledons has proceeded, and be- 

 tween them many prominent and successful families have 

 been worked out. 



