178 



ESSENTIALS OF BOTANY 



211. Flowers with Stamens and Pistils Each of Two 

 Lengths — The flowers of bluets, partridge-berry, the prim- 

 roses, and a few other common plants secure cross-poUina^ 

 tion by having essential organs of two forms (Fig. 134). 

 Such flowers are said to be dimorphous (of two forms). In 

 the short-styled flowers, II, the anthers are borne at the 



top of the corolla tube and 



the stigma stands about 

 halfway up the tube. In 

 the long-styled flowers, I, 

 the stigma is at the top 

 of the tube and the anthers 

 are borne about halfway up. 

 An insect pressing its head 

 into the throat of the corolla 

 of II would become dusted 

 with pollen, which would be 

 brushed off on the stigma 

 of a flower like I. On leav- 

 ing a long-styled flower the 

 bee's tongue would be 

 dusted over with pollen, 

 some of which would neces- 

 sarily be rubbed off on the stigma of the next short-styled 

 flower that was visited. Cross-pollination is insured since 

 all the flowers on a plant are of one kind, either long- 

 styled or short-styled, and since the pollen is of two sorts, 

 — each kind sterile on the stigma of any flower of similar 

 form to that from which it came. 



TrimorpJious flowers, with long, medium, and short 

 styles, are found in a species of loosestrife.^ 



1 See Newell's Reader in Botany, Part 11, pp. 60-63. 



Fig. 



II 



134. Dimorphous Flowers 

 of the Primrose. 



I, a long-styled flower ; II, a, short- 

 styled one. 



