44 THE HUMAN SIDE OF PLANTS 
ing hairs, poison darts, and various other weapons 
of defence; while acrid juices, poisons, and tough, 
fibrous growths assure safety from both insect and 
larger animal life. Some plants shoot their heads 
above the surface of the water, to avoid the attack 
of water insects; and then, in order to prevent their 
enemies from climbing up the stems to the heads, 
they exude a thick, viscid varnish, which both op- 
poses the passage of insects and protects the plant 
against inclement weather conditions. 
Against the second general danger to plant life 
—voracious animals and voracious or hostile plants 
—there are four common means of defence: thorny 
or dagger-like weapons; acrid or poisonous qual- 
ities; offensive odours; and simulation and flight. 
The first defence is very common. Cattle have 
learned in the past to avoid spinous or prickly 
plants, the knowledge of the danger of interfer- 
ence with such plants, which ancestor cattle have 
learned by painful experience, having become an 
inherited antipathy in their descendants. 
Against plants and grasses which secrete silica 
this antipathy is apparent also; and with good rea- 
son. All animals intuitively have learned to avoid 
the rough, “cutting” grasses and the bristly plants. 
As the humans in ages past mixed iron with their 
copper to make the weapons more durable, so have 
