20 



THE POPULAR SCIENCE MONTHLY. 



dimorphism as its metliod of securing cross-fertilization. The 

 beautiful white blossoms of this species open early in summer. 

 The stamens of some individuals are exserted, with the stigmas 

 below the mouth of the corolla, while in others these conditions 

 are reversed. Another common example of a low-growing plant 

 with dimorphous sexual organs is that of the familiar bluets.* 

 The dainty blossoms of this species are small individually, but 

 grow so abundantly on New England hillsides as often to color 

 them like a light fall of snow. A sectional view of the two forms 

 of flowers is shown in Fig. 3 : a represents the long-styled form 

 with the stamens in the lower portion of the corolla tube and the 

 stigma exserted, while in h the stamens are near the mouth of the 

 corolla and the stigma is below. These blossoms are mainly pol- 

 lenized by small bees and butterflies, one of the commonest New 

 England visitors being the meadow fritillary. \ When an insect 



Fig. 3. Long-styled A^fI) Short-styled Forms of Houstonia 



sucks the nectar from the base of the corolla of the short-styled 

 blossom ih), it will get at a certain place on its tongue some of 

 the pollen from the anthers. If next it visits a long-styled blos- 

 som (a), it will be likely to brush some of this pollen on to the 

 exserted stigma, while a point near the tip of the tongue will 

 receive a fresh supply of pollen grains. If now it again visits a 

 short-styled blossom, this last-received pollen will be at the right 

 elevation to be deposited on the included stigma. Consequently, 

 cross-fertilization will almost certainly occur. 



In the case of the mayflower it is evident that the structural 

 conditions described above will necessitate for the production of 

 seed the transportation of the pollen from the staminate to the 

 pistillate blossoms. The only agents to be called into play for 

 this errand are the insects and the wind. The structure of the 

 plant shows that under any ordinary conditions the wind would 



* Houstonia ccerulea. 



j- Brenthis hellona. 



