230 Plant Life and Evolution 



a few of the more striking cases. For a fuller 

 account of these special contrivances to insure cross- 

 pollination the reader must be referred to the works 

 of Darwin, Kerner, Miiller, and other students of 

 these interesting problems. 



Prevention of Self-fertilization. One of the com- 

 mon methods by which cross-pollination is secured, 

 is the maturing at different times of the stamens and 

 carpels. A common example of this is seen in the 

 scarlet geranium, where all the pollen is shed before 

 the pistil is ready to receive it, so that the flower 

 must be pollinated from a younger one, and this 

 must be done through the aid of insects. In the 

 nasturtium much the same, conditions exist, but the 

 pistil, when it is ready to receive the pollen, takes 

 a position exactly the same as that occupied by 

 the stamens at the time that the pollen is shed, so 

 that the bee or humming-bird, coming from the 

 younger flower, and bearing with it the pollen, 

 touches the same part of the body to the pistil in 

 the older flower, and thus deposits upon it the pollen 

 which it has brought from the younger one (Fig. 

 21, C, D). 



Heterostylism. In a number of plants, including 

 species of primrose and some of our native plants, 

 e.g., the trailing arbutus and partridge-berry, what 

 is known as heterostylism occurs, i.e., there are pis- 

 tils of two lengths in different flowers, and the 

 stamens are of reciprocal lengths. Thus long- 

 styled flowers have short stamens, and vice versa. 



