10 THE VEGETABLE WORLD. 
Hitherto we have occupied ourselves in considering the roots con- 
stituting the descending and normal system of vegetation. There are, 
owever, some roots 
which are developed 
\ ; along thestem itself. 
Organs, supplemen- 
tary in some sort, 
they come as helps 
to theroots properly 
so called, and re- 
place them when 
by any cause they 
have been destroyed. 
Inthe Wheat-plant, 
the Dog’s-grass 
(Fig. 11), and in 
general in all plants 
of the grass family, 
the lower part of 
the stem gives rise 
to supplementary 
roots, to which these 
common field plants 
owe a portion of 
their vigour an 
their resistance to 
the causes which 
would destroy them 
r 
; 
(Primula) both the 
{=== principal and the 
————=. secondary _ roots 
which spring from 
it perish after some 
years of growth. But the adventitious roots (Fig. 12) springing 
___ from the lower part of the stalk prevent the plant from dying. 
In the tropical forests of America and Asia, the Vanilla, whose 
Fig. 11.—Adventitious roots of the Couct Agro 
L— 3 i mz 
