ui] THE FERTILISATION OF FLOWERS. 47 
pollen is all exhausted the stigma is protruded in the 
very same place, so that if it should press against a 
bee which has thus obtained pollen from a younger 
flower, cross-fertilisation is certain. The mechanism 
by which this is effected may be easily seen if any 
papilionaceous flower be taken and gentle pressure 
exerted on the wings. The tip of the keel will be 
exposed and a thin curl of pollen forced out, just as 
it would be against the bee’s body. 
Every one is familiar with the flowers of 7ropa@olum 
major, better known as the Nasturtium. The corolla 
is continued backwards as a long hollow spur which 
contains the honey. When the flower first opens all 
-the organs are immature, but soon an anther becomes 
mature and erects itself in front of the entrance to the 
spur, so that no bee could get at the honey without 
pushing against the anther. All the other stamens 
erect themselves in like manner, one at the time, until 
all the pollen is shed, when they hang down out of 
the way, and the pistil, which has been slowly matur- 
ing and lengthening, raises itself to the position 
previously occupied by the stamens. Insects which 
have visited young flowers cannot help fertilising 
older ones when they visit. them. Self-fertilisation 
cannot take place. 
The Primrose, Cowslip, &c., are well known to 
have two forms of flowers. Even the children are 
acquainted with this peculiarity of the genus, and 
call them “ thrum-eyed” and “ pin-eyed,” according as 
the stigma or stamens are most prominent. In the 
one form (fig. 56) the pistil is elongated and the 
stigma on a level with the top of the corolla-tube. In 
