DEVELOPMENT OF PLANTS 391 



been continued, for the perianth is arranged upon a compound 

 ovary, but its parts are often sharply differentiated into calyx 

 and corolla. The sepals and petals comprising these two whorls 

 differ greatly in form, but especially to be noted is the oddly con- 

 structed petal known* as the labellum (Figs. 292, 293, /). This 

 organ is the most striking feature of the orchid and it assumes 

 an almost endless variety of forms and colorations, being a good 

 illustration of Wallace's law that the most highly modified part 

 shows the greatest variation in coloration. In Cypripedium (Fig. 

 292, A) the labellum assumes the form of a moccasin, in other 

 genera it resembles a vase, boat, tongue, body of insect, etc. It 

 is entire or variously lobed, slit, fringed and often prolonged into 

 a tube for the concealment of nectar. The stamens and stigmas 

 are reduced in number and greatly modified, the former organs 

 usually being reduced to one and so fused with the style that the 

 anther is sessile upon it (Fig. 292, B). One or two of the stig- 

 mas are generally modified into a mucilage-secreting organ, called 

 the rostellum (Fig. 293, A). The microspores are usually united 

 into a waxy mass, pollinium (pi. pollinia) and attached to a 

 sticky part of the rostellum (Fig. 293, B, C). The remarkable 

 feature about these gaudy flowers is the relation that the label- 

 lum, anther, rostellum and stigma sustain to each other. The 

 position of these organs is such that an insect visiting the flower 

 touches with some part of his body the sticky part of the rostel- 

 lum and the pollinia are thus made fast to him and carried to the 

 stigma of another flower. These devices are so elaborate in 

 many orchids that the microspores can only reach the stigma 

 through the agency of an insect. In the lower types of orchids, 

 as the moccasin flower (Fig. 292, B), a somewhat different 

 arrangement is found. The bee enters the opening in the upper 

 part of the labellum and feeds upon the glands distributed along 

 the bottom. In leaving the flower he forces his way through 

 the small opening on either side of the style and so he first comes 

 in contact with the stigma and later with the anthers, which are 

 two in number and located back of the stigma. In this flower 

 the anthers are surrounded by a sticky mass and the microspores 

 are thus fastened to the insect's body to be carried to another 



