PLANT FAMILIES: ORCHIDACE^. 



25S 



plantain, hence the common name of water-plantain. The tlower is regular 

 (all parts of a set are alike), and all the parts are distinct and free. This 

 represents a simpler and more primitive condition than exists in the lily 

 family, where the carpels are united. The floral formula is as follows : 

 Ca3,Co3,A6,G6 — » ; i.e. the parts are in threes or multiples of three. The 

 stamens are in pairs in front of the sepals, and really represent but three sta- 

 mens, since it is believed each one has divided, thus making three pairs. No 

 stamens stand in front of the petals in the water plantain, but in the 

 European genus Butomus one stanjpn in addition stands in front of each 

 sepal. 



497. The arrow leaf (genus sagittaria) occurs in wet ground, or on the 

 margins of streams and ponds. The leaves are very variable, and this seems 

 to depend to some extent on the depth of the water. Several forms of this 

 plant are shown in figs. 493-495. The flowers are moncecious or dioecious. 



498. The orchid family (orchidaceae). — Among the orchids are found the 

 most striking departures from the arrangement of the flower which we found 

 in the simpler monocoty- 

 ledons. An example of 

 this is seen in the lady- 

 slip per (cypripedium, 

 shown in fig. 464). The 

 ovary appears to be below 

 the calyx and corolla. This 

 is brought about by the 

 adhesion of the lower part 

 of the calyx to the wall 

 of the ovary. The ovary 

 then is inferior, while 

 the calyx and corolla are 

 epigynous. The stamens 

 are united with the style 

 by adhesion, two lateral 

 perfect ones and one upper 

 imperfect one. The stamens are thus gynandrous. The sepals and petals 

 are each three in number. One of tlie petals, the "slipper," is large, 

 nearly horizontal, and forms the " lip " or " labellum " of the orchid flower. 

 The labellum is the platform or landing place for the insect in cross polli- 

 nation (see Part III, Pollination). Above the labellum stands one of the 

 sepals more showy than the others, the ' ' banner. ' ' The two lateral 

 "strings" of the slipper are the two other petals. The stamens are still 

 more reduced in some other genera, while in several tropical orchids three 

 normal stamens are present. 



499. There are thus four striking modifications of the orchid flower: ist, 



Fig- 327- 

 Flower of an orchid (epipactis), the inferior ovary 

 twisted as in all orchids so as to bring the upper part of 

 the flower below. 



