DIRECTION OF VEGETABLE GROWTH. 375 
weight of the liquid in the cells above them. In other 
words, they are subject not only to the equal hydraulic 
pressure originating in the roots, but also to a slight hy- 
drostatic pressure from the overlying cells. This pro- 
duces the greater extension of the lower passive tissues, 
and accounts for the curvature upward. When the stem 
becomes vertical the hydrostatic pressure is equal on both 
sides of the stem, and the latter is accordingly maintained 
in that position. (Hofmeister, Sachs.) 
Effect of Light.—Besides the influence of gravitation 
and of interior tension, that of the solar light must be re- 
garded, as it assists largely in producing the more com- 
plex phenomena of direction in the growth of plants. 
The explanations already given refer to the plant when 
unaffected by light. As is well known, the stems, leaves 
and roots of plants, when growing where they are un- 
equally illuminated, as in a window, in most cases curve or. 
turn towards the light. More rarely is curvature away 
from the light observed, as in case of the stems of ivy, 
(Hedera helix), and the young rootlets of the mistletoe, 
(Viscum album). The common nasturtium, (Zropeolum 
majus), exhibits in its young stems inclination towards, 
in its older stems inclination away from, the light. Its 
leaves turn always towards, its roots growing in water 
often curve towards, often away from the light. 
