20 THE FLOWERING PLANT. 
a more or less oblique one. If a seedling is placed with its 
primary root in a horizontal direction, this will very soon curve 
round and grow downwards, while the stem curves upwards. By 
keeping such horizontal seedlings slowly rotating on their long 
axes, the effect of gravity is neutralized. The result is that root 
and stem do not in this case grow downwards and upwards, but 
have a tendency to keep the horizontal direction. Heliotropism 
is the term employed to designate the influence of light on direc- 
tion of growth. Roots are generally negatively heliotropic, 7.e., 
turn away from the light. Hydrotropism refers to the influence 
of moisture. Roots are positively hydrotropic in that they grow 
towards moisture. This is commonly the case with trees. If 
seedlings are placed in a perforated vessel full of damp’ moss, 
and the apparatus is then hung up, the primary roots will grow 
down as a result of positive geotropism, and make their way 
through the perforations. If the surrounding air is moist, the 
roots will continue to grow downwards, but if dry, they will bend 
up towards the damp moss. The tips of roots are also sensitive 
to contact, which causes them to curve. 
It is evident that all the above stimuli act in such a way as to 
fit the root for its share in nutrition. 
Note.—The ‘‘ epidermis ” of roots is but rarely morphologically equivalent 
to the epidermis of stems and leaves, but the term has been kept for the sake 
of simplicity. It would be more strictly correct to call this outermost coat- 
ing of roots the piliferous (7.e., hair-bearing) layer. 
