428 



GENERAL BOTANY 



of the common yellow lady's-slipper (Cypripedium pubescens) 

 (Fig. 272) and the pink species (Cypripedium acaule) (Fig. 273). 

 ^ Reproduction. The flowers of the lady's-slipper are simpler 

 than those of the true orchids, but are nevertheless very highly 

 modified in such a manner as to prevent self-pollination. One 

 of the petals is developed into a saclike structure (the labellum, 



or lip) observed in the 

 figures, with a narrow en- 

 trance above for insects. 

 The style is highly modi- 

 fied and projects into this 

 opening to the labellum, 

 bearing the stigma and 

 two stamens on its under- 

 side (Fig. 274). Insects 

 make their way into the 

 cavity of the labellum on 

 either side of the project- 

 ing style. In order to get 

 out they are obliged to rub 

 against the stamens and 



o, ovary ; a, anther of a perfect stamen ; sta, im- are thus dusted with pol- 

 perfect stamen : stig, stigma , , . , 



len. The stigma is situated 



below the stamens, so that the outgoing insect does not pollinate 

 it. The next flower visited, however, is certain to receive on 

 its stigma the pollen from the flower previously visited. 



FIG. 274. Vertical section of a flower of 

 Cypripedium acaule 



ALISMACEAE AND PONTEDERIACEAE 

 SAGITTARIA AND PONTEDERIA 



Habitat and habit. Sayittaria (arrowhead) and Pontederia 

 (pickerel weed) are good examples of Jiydrophytic monocotyle- 

 dons which inhabit shallow water on the margins of lakes or 

 streams. In Sagittaria the flowering stem and leaves arise from 

 stolons which are buried in the mud at the bottom of the lake 

 or stream. In Pontederia the underground stems are rhizomes 

 which give rise to a flower-bearing shoot and long-petioled leaves. 



