20 THE STORY OF PLANT LIFE 
anthers are large. The stamens also ripen together. 
The pollen is dry, and to protect it from rain the 
anthers open only in dry weather. Bracts in the 
catkin-bearing trees protect it above. The stamens 
in the Nettle group are folded up till ready to 
burst, and do so explosively. The pollen in the 
catkins is blown by the wind by the looseness of the 
catkins. Where the plant does not ripen its pollen 
before the leaves, as in Grasses, the gregarious habit 
is an equivalent aid to the effective pollination of 
the plant. 
The pollination of flowers by insects is called 
entomophily. It is the most general method. All 
classes of insects take part in pollinating flowers. 
The higher classes visit the higher plants. 
Insects first learnt to find flowers were food- 
supplying by their scent, and the honey and pollen 
were discovered subsequently. These gifts to the 
bees, etc., are the prices paid for pollination. 
The object of this is to secure cross-pollination. 
For it is found that plants that self-pollinate them- 
selves are not so strong and healthy as those that 
are cross-pollinated. And, in fact, the pollen of 
some plants is sterile when applied to the stigma of 
the same plant. 
Flowers conceal the honey in their nectaries so 
placed that pollen is collected in the act of seeking 
it, whilst some insects seek pollen specially. Special 
devices, such as hairs and a hooded corolla, etc., are 
provided to protect the pollen from rain, flies, etc. 
