108 A TEXTBOOK OF GENERAL BOTANY 
the force of gravity. Such responses of a plant to gravity are 
known as geotropism. When roots grow downward, or in the 
direction of the force of gravity, they are said to show positive 
geotropism, while stems that grow upward against the force 
of gravity exhibit negative geotropism. 
The action of geotropism can be demonstrated very easily 
if we take a small seedling and lay it in a horizontal position. 
The stem will bend 
upward, while the 
root will turn down- 
ward. The stem and 
root will then con- 
tinue to grow in these 
directions. If, how- 
ever, we place a seed- 
ling in a horizontal 
position and revolve 
it around its own 
axis, so that all sides 
will be subjected in 
the same way to the 
force of gravity, then 
the stem and root will 
The parent plant hangs under the edge of a porch ; . t i : 
the new shoots are growing toward the light, and pace NUE LO BOW 
the roots away from the light. (x 1) horizontal direction. 
, The action of the 
negative geotropism of stems is clearly seen in the case of plants 
which have been blown over but continue to grow. In such eases 
the stems grow upward, away from the surface of the earth. 
The action of geotropism is not due to a direct pull of eravity 
on the plant but is a response of the plant itself, as is shown 
by the fact that stems grow upward against the pull of gravity, 
while roots may exert considerable force in growing downward; 
in fact, roots must exert force to penetrate the ground. The 
difference in reaction of stems and roots is not the result of 
any difference in the force of gravity on the parts concerned 

Fig. 106. The tip of a pendent stem of an or- 
chid (Dendrobium anosmum) (see Fig. 2) produc- 
ing a young plant 

