116 FERTILIZATION. [SECTION 13. 



that under insect-visitation, some pollen is more likely to be deposited upon 

 other than upon own stigmas, so giving a chance for cross as well as for 

 close fertilization. On the other band, numerous flowers, of very various 

 kinds, have their parts so arranged that they must almost necessarily be cross- 

 fertilized or be barren, and are therefore dependent upon the aid of iusects. 

 This aid is secured by different exquisite adaptations and contrivances, 

 which would need a volume for full illustration. Indeed, there is a good 

 number of volumes devoted to this subject. 1 



336. Some of the adaptations which favor or ensure cross fertilization 

 are peculiar to the particular kind of blossom. Orchids, Milkweeds, Kal- 

 mia, Iris, and papilionaceous flowers each have their own special contriv- 

 ances, quite different for each. 



337. Irregular flowers (2.53) and especially irregular corollas are usu 

 ally adaptations to insect-visitation. So are all Nectaries, whether hollow 

 spurs, sacs, or other concavities in which nectar is secreted, and all nectar- 

 iferous glands. 



338. Moreover, there are two arrangements for cross fertilization com- 

 mon to hermaphrodite flowers in various different families of plauts, which 

 have received special names, Dichogamy and Heterogony. 



339. Dichogamy is the commoner case. Flowers are dichogamous when 

 the anthers discharge their pollen either before or after the stigmas of that 

 flower are in a condition to receive it. Such flowers are 



Proterandrous, when the anthers are earlier than the stigmas, as in Gen- 

 tians, Campanula, Epilobium, etc. 



Proterogynous, when the stigmas are mature and moistened for the re- 

 ception of pollen, before the anthers of that blossom are ready to supply 

 it, and are withered before that pollen can be supplied. Plantains or 

 Ribworts (mostly wind-fertilized) are strikingly proterogynous : so is Amor- 

 pha, our Papaws, Scrophularia, and in a less degree the blossom of Pears, 

 Hawthorns, and Horse-chestnut. 



340. In Sabbatia, the large-flowered species of Epilobium, and strikingly 

 in Clerodendron, the dichogamy is supplemented and perfected by move- 

 ments of the stamens and style, one or both, adjusted to make sure of 

 cross fertilization. 



341. Heterogony. This is the case in which hermaphrodite and fer- 

 tile flowers of two sorts are produced on different individuals of the same 

 species ; one sort having higher anthers and lower stigmas, the other hav- 

 ing higher stigmas and lower anthers. Thus reciprocally disposed, a visit- 

 ing insect carries pollen from the high anthers of the one to the high stigma 

 of the other, and from the low anthers of the one to the low stigma of the 

 other. These plants are practically as if dioecious, with the advantage that 



1 Beginning with one by C. C. Sprengel in 1793, and again in our day with 

 Darwin, " On the Various Contrivances by which Orchids are fertilized by Insects," 

 and in succeeding works. 



