178 STUDY OF COMMON PLANTS. 



ingly where the most light is admitted. If the 

 insect crawling on the floor of the labellum moves 

 towards the part that is best lighted, which direc- 

 tion will it take ? Are there any path-pointers ? 



4. Examine more closely the pollen masses. Notice 

 particularly their adhesive inner surface. Observe 

 the form and structure of the stigma, and see 

 how the pollen is retained when applied to its 

 surface. 



6. Endeavor to interpret these peculiar arrangements. 

 If practicable, observe the action of visiting 

 insects.! 



ARETHUSA. Arethusa bulbosa, L. 



Study the flower as directed in the case of Cypripedium, 

 with reference to 



I. External features, such as form and position of parts, 

 color, odor, etc. 



II. Morphological characters. 



Examine each whorl critically. Determine the plan of 

 the flower and note modifications. In what important 

 particular does the androecium differ from that of Cypripe- 

 dium? 



Construct a diagram, and compare with that of the 

 flower of Cypripedium.2 



III. Physiological adaptations. 



While plainly dependent on insects for fertilization, the 

 flower of Arethusa presents a very different mechanism 

 from that of Cypripedium. Examine carefully the rela- 



1 Cf. Miiller, Fertilization of Flou'ers, pp. 539-542 ; Gray, Am. Jour. 

 Sci., XXXIV (1862), pp. 420^29 ; Darwin, I.e., p. 230. 

 - Cf . Goodale, I.e. 



