Sentence:  It is either positive, making the animal creep perpendicularly downward, or it causes a horizontal movement, (<WSD>diatropism</WSD>) at right angles to the axis of gravity, the result of which for the organism is the so-called maintenance of equilibrium. 
Meaning of the word marked with <WSD> tags:  The growth or movement of a plant or animal in a direction perpendicular to a stimulus.