INTERDEPENDENCE OF ORGANISMS 



33 



by insects. Such an one is the common blue violet (Viola 

 cucullata) . Other species of violets are commonly visited 

 by bees ; and this one is apparently finely adapted for such 

 visitation. Yet the bees rarely visit it, and the showy 

 flowers, being incapable of self-pollination, produce no 

 seed. 



The accompanying figures show the structure of the 

 flower. It is strongly bilateral, with a saccate lower petal en- 

 veloping two spurred stamens : it is blue, with pretty "guide 

 marks" about the entrance: it secretes a little nectar, and 

 exhales a slight perfume : its entrance is 

 blockaded against improper visitors, 

 but it is narrowed and curved conven- 

 iently to admit the proboscis of a bee 

 standing head downward upon its front. 

 Furthermore, it is well adapted to 

 profit by the bee's visits. A proboscis 

 plying between the spurs of the two 

 FIG 25. Tip of pistil lower stamens would dislodge the dry 



of the violet as seen J 



from the front, pollen from the anthers, and catch it as 



showing pollen 



i p ow h o e f d the t0 st? eh a o1 " it; falls ' and cari T ^ out ' and when prob- 

 ing the sac of the next flower visited, 

 would deposit it on the stigma: for the stigmatic surface 

 is contained in the hollow of the pistil tip, turned toward 

 the entrance (fig. 25); the lower edge of it would scrape 

 up pollen from an entering proboscis, but would only evade 

 pollen that was being withdrawn. What better device 

 could be imagined for securing cross pollination? 



The trouble with the mechanism is that it no longer 

 works. The bee stays away. Did it visit the flowers, it 

 would transfer their pollen perfectly and they would be 

 very fertile. This anyone may demonstrate by transfer- 

 ring the pollen with a tooth pick and watching the result in 

 seeds produced. The failure seems to lie farther back in 



