164 TYPES OF BRITISH PLANTS 
and on heaths you may often find the Field Gentian, with 
its dull violet flowers and deep tube, four-cleft dt the top. 
Two other Gentians are 
of such striking beauty 
that they deserve a men- 
tion. The Buck-bean, or 
Bog-bean, growing in soft 
marshes, is recognised in 
a moment by the three- 
fold leaf, from the sheath 
of which springs in early 
summer a long stalk, 
bearing at its summit a 
cluster of rose and white 
flowers, each delicately 
fringed upon the edge. 
A second, and rarer, 
beauty of the group is 
the Water Villarsia, which 
lives in a few slow-running streams, its round leaves 
floating like the Frog-bit’s on the surface, and sending 
up at midsummer single flowers of a delicate yellow, the 
petals edged again by a fringe. 
Primroses and cowslips need no general description, 
but they have a device to make sure of cross-fertilisation, 
which deserves a few words. If you examine several 
primroses, you will notice that the top of the tube is 
sometimes occupied by a knob, which is obviously the 
stigma, and sometimes by small pockets, which are 
equally obviously stamens. At first you may think that 
this is merely another case of keeping stamens and 
pistils on different plants; but if you open the flower, 
you will find that both stamens and pistils are always 
CENTAURY. 
