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-pollina- 
tion by having essential organs of two forms (Fig. ae 
Such flowers are said to be dimorphous (of two forms). 
the short-styled Hower, II, the anthers are borne at es 
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- 
Fic. 184. Dimorphous Flowers ing a long-styled flower the 
of the Primrose. bee’s tongue would be 
I, a long-styled flower; II,a short dusted over with pollen, 
styled one. 2 
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. 
Trimorphous flowers, with long, medium, and short 
styles, are found in a species of loosestrife.t 
1 See Newell’s Reader in Botany, Part II, pp. 60-63. 
