Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 58

Fig. 3.5 The magnified structure
of a growing root tip ( left )in
relation to the root distribution of
a woody plant (Modified from
Kozlowski and Pallardy 1997).
Most water enters at individual
root hairs by diffusion along a
gradient in water potential from
wetted sediments ( lower right ),
the capillary fringe, or water table
( upper right ). Root hairs exist in
the piliferous area, located just
behind the growth zone of the
root tip. Other important water-
acquisition roots include those
that are below as well as above
ground, such as adventitious
roots.

woody plants in Hawaii, however, have roots that, although
similar in structure near the ground surface, become linear as
they grow through the open areas of old lava tubes. These
roots can be tens of feet long and grow through the moist
atmosphere of the lava tube. This structure also enables them
to act as conduits to deliver rainwater by gravity to the
growing root apex far below ground.
The downward growth of roots is aided by the presence in
the cells at the root tip of organelles in the cytoplasm called
amyloplasts. These amyloplasts contain grains of starch that
make the amyloplasts heavier at the bottom of each cell, and
provide orientation with respect to gravity. The position of
the amyloplasts is not fixed in the cytoplasm, so the
amyloplasts are free to move to the lowest position in the
cell as the whole root grows.
Root hairs form behind the root tip and are the main site
of water entry (Fig. 3.6 ). They are an extension of the
epidermal cells and arise from the secondary growth that
occurs after elongation. The root hairs are not more than
1/250 to 1/3 of an in. (0.01-0.8 mm) long. The density of
root hairs and their capacity for withdrawing water from the
ground is enormous. Some grasses have been calculated to
have almost 4,000 ft (1,219 m) of root hairs in 1 cubic in. of
soil.

Fig. 3.6 Adventitious roots on hybrid poplar cuttings formed during
shipment in plastic bags and elongated after immersion of the cut end
into water (Photograph by author).