56 PLANT LIFE ON THE FARM. 
CHAPTER IV. 
SENSITIVENESS. 
. 
Movements dependent on external conditions. — Gravitation, light, 
heat, moisture.—Action of gravity on roots.—Geotropism.—Influ- 
ence of light, heat, moisture, and contact on roots.—Passage of 
roots through the soil.—Action of gravitation on leaves.—Helio- 
tropism.—Sleep of leaves.—Action of heat and moisture on leaves. 
—Defensive arrangements.—Selection of hardy varieties.—lnfluence 
of contact on leaves.—Action of gravity, light, heat, moisture, and 
contact on stems. — After-effects. — Climbing plants.— Combined 
effect of external and internal agencies. 
Closely analogous to the growth-movements are a series 
of alterations of position dependent upon various circum- 
stances, such as gravity, the influence of heat and light 
or their absence, the result of contact or irritation, and 
soon. They are probably essentially of the same nature 
as the growth movements, but, unlike them, they are 
not confined to structures still in a growing state ; more- 
over, in some cases they exhibit a sort of reflex action, 
contact or irritation of one part bringing about a move- 
ment of some other part at a distance. It is often diffi- 
cult to dissociate the effects of these several movements ; 
for a living plant and its parts are subjected at the same 
time to the combined influence of several of these 
agencies, and the force and direction of growth are neces- 
sarily essentially modified by them. 
It may be well in this place to indicate very generally 
in what manner roots, stems, and leaves are sensitive to 
the effects of gravity, light, moisture, and actual contact 
or irritation, and then to specify equally briefly what is 
the general character of the results produced by these 
several causes acting singly, or in combination. 
The Action of Gravity on Roots.—Geotropism.—The 
downward tendency of the main root is one of its most 
