Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 77

various shrubs and trees. The source water was stained with
acid fuschin to a concentration of about 1 g/L. This tracer
was either injected directly into the plant or exposed roots,
or the roots were immersed in a solution of water that
contained the dye. The path taken by the water as seen by
the tracer then was recorded at its position in cross sections
of the plant taken from above the tracer injection and immer-
sion site. Arrival in above-ground branches by the appear-
ance of dye in the cross sections provided evidence for
Waisel et al. (1972) to categorize the flow in trees as belong-
ing to two main types: spiral and straight ascent (Figs. 3.14a,
b , respectively). It also was observed that, in some cases, the
point injection of the water dispersed into a wider zone
around the circumference of the tree as the water ascended,
termed ring ascent. Ring ascent was revealed in some trees
to progress in either a clockwise or counterclockwise pat-
tern. Waisel et al. (1972) looked specifically at trees that

today are candidates for use at many phytoremediation
sites—species of the Populus genera, in this case, Populus
euphratica . They examined water and dye flow in a tree
from a wet site that reportedly had diffuse porosity com-
pared to a tree from a drier site having ring porosity. In both
cases, straight ascent that turned into ring ascent was noted.
The rationale behind the spiral pathway of water ascent
from the roots to the leaves is not directly intuitive. The
answer, however, is contained in the efficiency of the plant
world as influenced by selective pressures. Tree trunks and
stems can be envisioned as cylinders. The shortest path
between two points on a cylinder is described by a helix,
or spiral pathway. The pattern of flow also is related to the
number of leaves. As was first recorded by Leonardo da
Vinci, a correlation can be made between the cross-sectional
area of the branches and their attendant leaves and the cross-
sectional area of the trunk. This indicates that, in essence, the

Fig. 3.14 ( a ) Water ascending a tree in a spiral vertical pathway in the xylem beneath the bark. This will have implications for core collection and
analysis at contaminated sites, as discussed in Chap. 15. ( b ) Water ascending a tree in a straight vertical pathway in the xylem beneath the bark.