180 PHYSIOLOGICAL BOTANY. 



structure, number, or arrangement of the floral organs. 

 In the few flowers which never open, the Cleisto- 

 gamous, such as the late apetalous flowers of the Blue 

 Violet, and also probably those of Gentiana Andrewsii, 

 only self-fertilization is possible. But in the multitude 

 of open flowers with both stamens and pistils exposed, 

 as in the Lily, Rose, Morning G-lory, either self or cross 

 fertilization is possible unless determined by some 

 other special circumstance. The stigma may receive 

 pollen directly from its own stamens, or indirectly 

 from other flowers near or remote, through the agency 

 of winged insects, humming-birds, or of the wind. 

 Again there are flowers in which the organs are so 

 situated that self-fertilization is very difficult, or even 

 impossible. Of this class are the Asclepiads and 

 Orchids, whose pollen, cohering in masses (pollinia), is 

 inclosed in cavities, and only dragged forth by insects 

 to be carried to other flowers. So in Iris, where the 

 extrorse anthers and petaloid stigmas are averted from 

 each other, the former beneath, and shedding its pollen 

 downward. 



445. Dichogamous Plants. In some species the 

 stamens and pistils are not cotemporary in the same 

 plant, but the stamens of one plant mature at the 

 same time with the pistils of another plant, and vice 

 versa. This necessitates cross-fertilization, and the 

 agency of the wind or of insects. We have examples 

 in the Grasses, the common Plantain, in Scrophularia, 

 etc. 



446. Dimorphous Plants are such as the Mints 

 (Mentha), the Yellow Jessamine (Gelsemium), Hous- 

 tonia cserulea, etc. In these the flowers assume two 

 forms, with the stamens and pistils cotemporary in 



