CHAP. iv.J REPRODUCTION OF PLANTS. 137 



as in tlie primrose, are called Dimorphous ; those with 

 three forms, as loosestrife (Lythrum salicaria), are called 

 Trimorphous. The facilities afforded by heteromorphism 

 in connection with cross-fertilization, is clearly shown 

 by the species of Primula, Pig. 39, which includes the 

 primrose and cowslip. An insect in obtaining nectar 

 from the base of the coralla-tube of a long-styled prim- 

 rose flower would dust its "tongue" with pollen at a point 

 which would exactly come in contact with the stigma 

 when it visited a short-styled flower. Conversely, 

 when visiting a short-styled flower the pollen would 

 adhere to that portion of the " tongue " that would come 

 in contact with the stigma of a long-styled flower. 



A further guard against self-fertilization evolved by 

 many plants depends on the fact that the pollen of a 

 given flower when placed on the stigma of the same 

 flower is slower in forming pollen-tubes, and thus effect- 

 ing fertilization, than the pollen from another flower of 

 the same kind, and in some plants this idea is carried so 

 far that the pollen will not fertilize the ovules of the same 

 flower producing it, but when placed on the stigma 

 actually induces a toxic effect that results in the death 

 of the unfertilized ovules. 



The sign that a plant has reached the stage of almost 

 or altogether entire dependence on insect aid for effect- 

 ing fertilization is indicated by the irregularity or unsym- 

 metrical structure of the corolla, accompanied by the 

 growing together to a greater or less extent of its 

 component petals. Several important groups of plants 

 belonging to this category have the corolla consisting of 

 petals of unequal size, and so arranged that a median 

 line divides it into two equal parts, but the petals are 



