AND CROSS-FERTILIZE THEIR FLOWERS. 
25 
naturally secured by the structure itself in plants with separated flowers, — most 
completely in those which, like Willows, bear stamens and pistils upon different 
trees. And in the majority of plants which have perfect flowers it is commonly 
no less secured by arrangements of various kinds for excluding the pollen from its 
own stigmas, and having it conveyed to those of some other flower of the same 
species. 
50. Comprehending now the full meaning of these curious arrangements, we 
may turn back to some of the flowers already noticed, to observe how exqui- 
sitely they are adapted to the purpose in view, and then advance to new and more 
varied illustrations. 
51. Cross-Fertilization ill Iris (Fig. 12). A little nectar is produced in the bot- 
tom of the tube or narrow cup of the blossom. The only access to it is a narrow 
channel leading down the united bases of the six divisions or leaves of the flower. 
Now the three inner of these are upright, with their tips curved inwards, shutting 
off all access in that quarter. But the three outer and larger divisions recurve 
and afford a convenient landing-place directly before the stamen and the over- 
arching stigma. Here the bee alights. To reach and suck out the nectar with 
his proboscis will bring the head at least as low as the base of the anther. On 
raising his head to depart he sweeps with it the whole length of the anther and 
dusts it with the pollen now shedding. A little higher the shelf of stigma is hit, 
but only the outer face of it, which is smooth and does not take the pollen at all. 
Flying to the next blossom, the first thing which the pollen-powdered head of the 
bee strikes is the stigma, but this time on the upper face of the shelf or real sur- 
face of stigma, which takes some of the pollen brought into contact with it, and 
so is fertilized. Sinking lower, the head next brushes the anther downwards in 
entering for the nectar, then upwards in departing, and receives a fresh charge of 
pollen to be deposited upon the shelf of stigma of the next blossom visited, and 
so on. 
52. In Arethusa (40, 45, Figs. 13—15), We have never seen bees or other in- 
sects about this flower ; but it is plain from its structure that it cannot set seed 
without their help. As already described, the bee, or other insect of considerable 
size, can enter the blossom only in front ; and the large and crested recurving 
petal offers a convenient landing-place. At the bottom of the narrowed cup of 
the flower a little nectar is produced, down to which the insect must reach its 
proboscis. In rising to escape, its head must strike the lower face of the over- 
