42 THE HUMAN SIDE OF PLANTS 
coming out only when they know the pests will 
not be around are in somewhat the same class as 
the morally weak plants which buy off or pay 
“blackmail” to their enemies. The bird-cherry is 
one of this type; many of the vetches also bribe 
their crawling ravagers by feeding them from a 
special supply of nectar that is stored in epidermic 
cells on the under side of the leaves. Insects, 
mounting the stems in quest of alluring sweets, 
make the most of the first supply that comes to 
hand, revelling on this until their desire is satis- 
fied, and then dropping off in a state of drunken 
satiation, entirely fooled by the ingenious plant and 
thwarted in their quest for the rarest wines which 
have lain just beyond. It is the oldest of human 
tricks! 
Weapons are the most evident means of defence 
man has; and to plants, thorns, spears, and dag- 
gers form a very useful and common protection. 
But the thorns and daggers on plant stems have 
not, as many people think, like Topsy, “jes’ 
growed.” Those plants which have adopted exter- 
nal weapons as a means of self-defence have done 
so with thought and consistency. One who notes 
the position and arrangement of thorn armour will 
see that all spikes and daggers point down to meet 
the scaling forces. The guerilla worm or insect 
