AN INDIVIDUAL PLANT IN ALL OF ITS EELATIONS 139 
much more influenced in direction by other external 
causes, especially the presence of moisture. As a rule, 
the soil is not perfectly uniform, and contact with different 
substances induces curvatures, and as a result of these and 
other causes, the root system may become very intricate, 
which is extremely favor- 
able for absorbing and 
gripping. 
96. Direction of the stem. 
As soon as the stem tip 
is extricated from the seed, 
it exhibits sensitiveness to 
the light influence (heliot- 
ropism), being guided in 
a general way towards the 
light (see Fig. 1430). 
Direction towards the 
light, the source of the in- 
fluence, is spoken of as 
positive heliotropism, as 
distinguished from direc- 
tion away from the light, 
called negative heliotro- 
pism. If the main axis 
continues to develop, it 
continues to show this posi- 
tive heliotropism strongly, 
but the branches may show 
every variation from positive to transverse heliotropism ; 
that is, a direction transverse to the direction of the rays 
of light. In some plants certain stems, as stolons, run- 
ners, etc., show strong transverse heliotropism, while other 
stems, as rootstocks, etc., show a strong transverse geot- 
ropism. 
97. Direction of foliage leaves. The general direction of 
foliage leaves on an erect stem is transversely heliotropic ; 
FIG. 143. Germination of the seed of 
arbor- vitae (Thuja). B shows the 
emergence of the axis (?) which is to 
develop the root, and its turning to- 
wards the soil. C shows a later stage, 
in which the root (r) has been some- 
what developed, and the stem of the 
embryo (7t) is developing a curve pre- 
paratory to pulling out the seed leaves 
(cotyledons). E shows the young plant- 
let entirely free from the seed, with its 
root (r) extending into the soil, its stem 
(h) erect, and its first leaves (c} hori- 
zontally spread. After STRASBURGEK. 
