4 THE HUMAN SIDE OF PLANTS 
spring from their parent stem underground and 
later appear as separate plants; and eventually the 
little connecting link or thread may be entirely de- 
stroyed. It is in this way—“by root,” as we say— 
that raspberry bushes increase or spread out. 
Many grasses spread in the same way, and in all 
directions; some have joints from which upright 
stems arise, and which send down into the earth 
roots of their own, interlacing and binding the soil 
and thus spreading very rapidly. Such grasses 
grow several feet in a few months. The quack- 
grass is a typical example of a fast walker and for 
this reason is greatly feared by farmers. 
Tulips are noted for their ability to walk. If 
planted in dark, shady places, where it is cold and 
damp, they will deliberately walk away from the 
dark place into a more desirable part of the garden. 
They usually move into the sun. The bulb that 
was planted does not itself actually move, but its 
substance is transferred little by little, and only the 
outer wrapping of brown tissue is left. The bulb 
sends out a delicate shoot that runs horizontally 
below the ground until it has reached a distance of 
several inches. Then, near its point, a swelling 
begins to take the shape of a tulip bulb, which 
grows larger and larger as the food-material of 
the old bulb is brought into it. This is done by 
