Environmental Engineering Reference
In-Depth Information
informationmust be fitted together like the pieces of a puzzle. Only if all of
the pieces fit is the identification convincing. Distinguishing between var-
ieties within the same species can be very laborious, evenwhen seed-heads
are available. Sometimes only a DNA test can provide conclusive results.
Besides seed-heads, structures used for grass identification include
the
sheath
,
collar
,
ligule
,
auricles
,
vernation
and shape of the
leaf tip
.
The sheath can be described as the lower part of the leaf that is
attached to the node. It may be split along its full length from the collar
to the node, split partway and closed for the rest of its length, or closed
for its entire length (Fig. 1.12). The sheaths of most warm-season grasses
are completely split, so the sheath is often not a very helpful characteristic
for identification. An exception is carpet grass (
Axonopus compressus
(SW.)
P. Beauv.), which has a flattened sheath that distinguishes it from most
other warm-season species.
The collar is located at the junction between the leaf blade and the
sheath. In many species the collar is absent or microscopic in size. In
others it is quite visible and has distinctive characteristics like shape or
colour. Either absence or presence of a collar can provide information
useful in the identification process (Fig. 1.13).
The ligule grows from the collar area on the inner side of the leaf. It
may be absent, take the form of a thin membrane, a fringe of hairs or a
combination of the two. Most tropical turf-grasses have ligules, but
sometimes they are too small to be seen without a magnifying glass.
A ligule, if found, can be critical to identification (Figs 1.14 and 1.15).
Split with
overlapping margins
Closed
Split
Fig. 1.12. Three basic types of sheaths. (Drawing by R. Castro.)
