52 



THE PERIGONE 



alternating, adhesion necessarily involves the ultimate effect of cohesion. 

 In the case of cohesion extended very high, peculiar effects, often 

 puzzling to the beginner, are produced, as in the case of Oenothera 

 (Fig. 29). 



Here the calyx, after adhering to the entire surface of the ovary (e), 

 is continued upward in the form of a long, slender tube resembling a 

 flower-stem. Inside of this tube the petals and stamens are adnate, 

 and do not become free until they reach its summit. 



Cohesion. — Cohesion, like adhesion, may be partial or complete. In 

 its slightest forms, with a mere band of union at the base, it may 

 escape observation, as in the case of the corolla of Lysimachia (Fig. 84). 

 In such cases a decision is best reached by carefully pulling away the 



Fig. 84. Adnate corolla and androecium of Lysimachia, the parts of each coherent at the base only- 

 87. Completely separating calyptra of Eucalyptus. 88. Partly attached calyptra of Mitranthes. 

 89. Corolla of Oenothera escaping through a fissure in side of calyx. 90. Corolla of Ayenia, its petals 

 coherent at summit only. 91. Completely coherent petals of Ipomoea, leaving the margin merely 

 sinuate. 



corolla. If there is a union, however slight, the corolla may thus be 

 removed as one body. Agglutination will occasionally cause an appear- 

 ance of cohesion, but upon applying the test here specified, the parts 

 will be found to separate readily, without the tearing of any tissue. 



The Calyptra. — A peculiar and extreme form of cohesion is that in 

 which the sepals refuse to separate even at the apex wdien the flower 

 expands, and the calyx is torn loose from its basal attachment, falling 

 entire as a Calyptra, as in the corolla of Eucalyptus (Fig. 87), or remain- 

 ing attached at one side as in Mitranthes (Fig. 88). A modification of 

 it permits the remainder of the flower to escape through a rent in the 

 side, as sometimes in the case of Oenothera (Fig. 89). Very rarel}^ 

 cohesion exists at the apex only, a remarkable instance being the corolla 

 of Ayenia (Fig. 90). 



