86 STUDY OF COMMON PLANTS. 



plants notliing is known." ^ This arrangement practically 

 necessitates cross-fertilization. 



2. Dichogamy, or the maturing of stamens before or 

 after the period of receptivity of the stigma. When the 

 stamens shed their pollen before the stigma is receptive, 

 the dichogamy is proterandrous ; if, on the other hand, the 

 stigma is receptive before the pollen is shed, it is proter- 

 ogynous. The former condition is far more common than 

 the latter.2 



3. Prepotency of pollen from other flowers. It has 

 been found by experiment that pollen from another indi- 

 vidual is often decidedly prepotent over that produced by 

 the same flower. This is best shown by placing its own 

 pollen on the stigma of a flower, and after some hours 

 applying pollen of a different colored variety of the same 

 species. The plants raised from seeds of flowers thus 

 fertilized show by the color of their flowers whether 

 crossing has taken place. Darwin found in a number of 

 cases that pollen of another individual was prepotent after 

 twenty-three or twenty-four hours.^ 



4. Heteromorphism. A considerable number of species 

 produce flowers of different forms. In various species of 

 Primula and Houstonia, certain individuals have long sta- 

 mens and short styles, while others have long styles and 

 short stamens. Such flowers are said to be dimorphic, while 

 those of loosestrife, Lythrum Salicaria, L., which have 

 stamens and styles of three different lengths, are trimor- 

 phic. Both conditions involve the same principle and 

 favor cross-fertilization in a remarkable way.* 



1 Different Forms of Flowers on Plants of the Same Species, p. 278. 



2 Cf. Gray, Structural Botany, p. 219, et seq. 

 ' Cross- and Self-fertilization, pp. 395, 396. 



* Cf. Darwin, Different Forms of Flowers on Plants of the Same 

 Species. 



