126 
PLANT STUDIES 
out of the pollen-sacs or to be removed from them. The 
devices used by flowers containing both stamens and carpels 
to prevent self-pollination are very numerous, but most 
of them may be included under the three following heads : 
(1) Position. In these cases the 
pollen and stigma are ready at the same 
time, but their position in reference to 
each other, or in reference to some con- 
formation of the flower, makes it un- 
likely that the pollen will fall upon the 
stigma. The stigma may be placed 
above or beyond the pollen sacs, or the 
two may be separated by some mechan- 
ical obstruction, resulting in much of 
the irregularity of flowers. 
In the flowers of the rose acacia and 
its relatives, the several stamens and 
the single carpel are in a cluster, en- 
closed in the keel of the flower. The 
stigma is at the summit of the style, 
and projects somewhat beyond the 
pollen-sacs shedding pollen. Also there 
is often a rosette of hairs, or bristles, 
just beneath the stigma, which acts as 
a barrier to the pollen (see Fig. 131). 
In the iris, or common flag, each 
stamen is in a sort of pocket between 
the petal and the petal-like style, while 
the stigmatic surface is on the top of a 
flap, or shelf, which the style sends out 
as a roof to the pocket. With such an 
arrangement, it would seem impossible 
for the pollen to reach the stigma un- 
aided (see Fig. 132). 
In the orchids, remarkable for their 
strange and beautiful flowers, there are 
FIG. 132. A portion of 
the flower of an iris, 
or flag. The single 
stamen shown is 
standing between the 
petal to the right and 
the petal-like style to 
the left. Near the 
top of this style the 
stigmatic shelf is 
seen extending to the 
right, which must 
receive the pollen 
upon its upper sur- 
face. The nectar 
pit is at the junc- 
tion of the petal and 
stamen. While ob- 
taining the nectar the 
insect brushes the 
pollen-bearing part 
of the stamen, and 
pollen is lodged upon 
its body. In visiting 
the next flower and 
entering the stamen 
chamber the stig- 
matic shelf is apt to 
be brushed. After 
GRAY. 
