Re The Life of the Plant. 207 
leaflets of the bean to be depressed, those of the clover to 
be bent sideways; while sudden exposure to light, or in- 
crease in its intensity, produces the opposite effect. The 
leaves of Oxalis, on the other hand, assume the nocturnal 
position in direct sunlight. Curvatures of this nature, | 
caused by light, are called paratonic. 
While paratonic curvatures are caused only by the pre- 
sence or absence of light, and are independent of the direc- 
tion in which the light falls upon the plant, the contrary is 
the case with Aeliotropic curvatures, which occur only on the 
side from which the strongest illumination comes. If the 
side which faces the source of light curves concavely, the. 
effort after light, or Aelotropism, is said to be positive ; if 
convexly, it is zegatzve. The former phenomenon is ex- 
hibited by most internodes of erect stems which are in ac- 
tive growth, and especially by petioles, which by this means 
place the surface of the leaf in such a position that it will 
receive the greatest possible amount of light. It may be 
seen very strikingly in plants grown in a window, which at 
length become quite crooked, from their young parts which 
face the light becoming ngid in this position, Com- 
paratively few parts of plants are negatively heliotropic ; 
examples occur in the tendrils of the grape-vine, and in 
older branches of the ivy, which by this means become 
firmly adpressed to their support. Heliotropic curvature 
does not contract the part of the plant; but in positive 
hehotropism the shaded side elongates from more active. 
growth more rapidly than the side which faces the light ; [in 
negative heliotropism the reverse]. _ 
One other phenomenon must be mentioned in this con- 
nection, 72z., that known as geotropfism, or the tendency to 
grow away from or towards the centre of the earth. Parts 
of plants which are still growing in length tend to place 
themselves in such a position that the direction of their 
growth is vertical, or coincides with the direction of the 
force of gravitation. If the growing part consists of tissues 
