169
Germany. Spence ( 1967a ) opines that the
coarse unfavourable sediment of the exposed
shores is a major factor in this type of distribution.
However, it seems unlikely that the direct buffeting by wind and breaking waves is not equally
signifi cant.
It may be noted here that Phragmites australis
grows marginally with Scirpus in the water of the
lakeward side of the reed bed. Typha requires a
more protected habitat and is usually present
quite locally along the shore. In North America,
Typha latifolia is the most widespread of the
three species of the genus and the most tolerant
(McNaughton 1966 ). All species are composed
of populations which could be largely clonal and
are genetically adapted to specifi c environments.
It could, thus, be said that the emergent vegetation, generally, shows a dichotomy similar to
that exhibited by the hyphydate or submerged
fl ora. The soft water, unproductive lakes characterised by Lobelia and Isoetes usually have marginal vegetation with Equisetum fl uviatile, Carex
spp. and Eleocharis palustris ; but in more
evolved and eutrophic lakes, large reed beds of
Phragmites australis, Scirpus and Typha are
developed.
8.3.1.2 Floating-Leaved AM
There is a good deal of evidence of the regulatory
effect of disturbance in the zone of fl oating AM
(Pearsall 1921 ). Lakeward of the reed bed or
other marginal vegetation, the fl oating-leaved
vegetation generally shows evidence of zonation
depending on resistance to disturbance.
8.3.1.3 Submerged AM
These AM occur below the water surface. There
is a kind of depth zonation in lakes. In lakes with
exposed shores, the shallower zone is one of erosion. Below this, there is a neutral zone where
erosion is balanced by sedimentation. Further out
in deeper water, sedimentation generally prevails.
The actual amount of erosion may be small in
relatively mature basins. All the easily erodible
inorganic material has long since been removed.
However, the AM are themselves subject to the
eroding action. The plants of the erosional and
neutral zones are usually small rosulate species,
such as, Isoetes setacea and Ranunculus reptans .
Such plants are particularly characteristic of oligotrophic waters in lakes with exposed shores. It
is probable that they are primarily adapted to
obtaining a large part of their nutrients from the
sediments rather than the water, but they may
also be somewhat more resistant to violent water
movements than are trailing plants.
8.3.2 Distribution of AM
In some lakes, the distribution of various AM
species may depend on the nature of the sediment. This is most clearly seen around islands or
off the estuaries where the nature of the sediment
is locally different from what it would be at the
same depth elsewhere in the lake. In such cases,
the water plants tend to follow the pattern of sedimentation rather than bathymetry. It does not,
however, yet seem possible to categorise species
accurately by the physico-chemical parameters
of the sediments on which they occur. Very
organic sediments tend to be generally unfavourable. The unfavourability of a sediment may be
due to its reductive capacity, in which oxygen
may fall to too low a value in the vicinity of the
plant roots.
In many lake districts in the temperate northern hemisphere, unproductive lakes containing
characteristic vegetation, composed largely of
rosulate or isoetid plants, are known. If such
lakes are transparent, the euphotic zone extends
well below the depth range of angiosperms, so
that the plants found deepest in the lake are usually mosses of species of Nitella . The distribution
of plants in the shallower water is clearly partly
determined by erosion and sedimentation though
it is diffi cult to generalise from lake to lake
exactly how any particular species behaves.
Notwithstanding the above, there may be a
zone between that occupied by AM and that
occupied by the mosses with practically no vegetation. This may correspond to the maximum
slope of the basin in some cases. Concomitant
to above, in the more evolved basins, ordinarily
regarded as mesotrophic or eutrophic, vittate
species, particularly members of Potamogeton ,
8.3 Growth Forms and Depth Distribution
Germany. Spence ( 1967a ) opines that the
coarse unfavourable sediment of the exposed
shores is a major factor in this type of distribution.
However, it seems unlikely that the direct buffeting by wind and breaking waves is not equally
signifi cant.
It may be noted here that Phragmites australis
grows marginally with Scirpus in the water of the
lakeward side of the reed bed. Typha requires a
more protected habitat and is usually present
quite locally along the shore. In North America,
Typha latifolia is the most widespread of the
three species of the genus and the most tolerant
(McNaughton 1966 ). All species are composed
of populations which could be largely clonal and
are genetically adapted to specifi c environments.
It could, thus, be said that the emergent vegetation, generally, shows a dichotomy similar to
that exhibited by the hyphydate or submerged
fl ora. The soft water, unproductive lakes characterised by Lobelia and Isoetes usually have marginal vegetation with Equisetum fl uviatile, Carex
spp. and Eleocharis palustris ; but in more
evolved and eutrophic lakes, large reed beds of
Phragmites australis, Scirpus and Typha are
developed.
8.3.1.2 Floating-Leaved AM
There is a good deal of evidence of the regulatory
effect of disturbance in the zone of fl oating AM
(Pearsall 1921 ). Lakeward of the reed bed or
other marginal vegetation, the fl oating-leaved
vegetation generally shows evidence of zonation
depending on resistance to disturbance.
8.3.1.3 Submerged AM
These AM occur below the water surface. There
is a kind of depth zonation in lakes. In lakes with
exposed shores, the shallower zone is one of erosion. Below this, there is a neutral zone where
erosion is balanced by sedimentation. Further out
in deeper water, sedimentation generally prevails.
The actual amount of erosion may be small in
relatively mature basins. All the easily erodible
inorganic material has long since been removed.
However, the AM are themselves subject to the
eroding action. The plants of the erosional and
neutral zones are usually small rosulate species,
such as, Isoetes setacea and Ranunculus reptans .
Such plants are particularly characteristic of oligotrophic waters in lakes with exposed shores. It
is probable that they are primarily adapted to
obtaining a large part of their nutrients from the
sediments rather than the water, but they may
also be somewhat more resistant to violent water
movements than are trailing plants.
8.3.2 Distribution of AM
In some lakes, the distribution of various AM
species may depend on the nature of the sediment. This is most clearly seen around islands or
off the estuaries where the nature of the sediment
is locally different from what it would be at the
same depth elsewhere in the lake. In such cases,
the water plants tend to follow the pattern of sedimentation rather than bathymetry. It does not,
however, yet seem possible to categorise species
accurately by the physico-chemical parameters
of the sediments on which they occur. Very
organic sediments tend to be generally unfavourable. The unfavourability of a sediment may be
due to its reductive capacity, in which oxygen
may fall to too low a value in the vicinity of the
plant roots.
In many lake districts in the temperate northern hemisphere, unproductive lakes containing
characteristic vegetation, composed largely of
rosulate or isoetid plants, are known. If such
lakes are transparent, the euphotic zone extends
well below the depth range of angiosperms, so
that the plants found deepest in the lake are usually mosses of species of Nitella . The distribution
of plants in the shallower water is clearly partly
determined by erosion and sedimentation though
it is diffi cult to generalise from lake to lake
exactly how any particular species behaves.
Notwithstanding the above, there may be a
zone between that occupied by AM and that
occupied by the mosses with practically no vegetation. This may correspond to the maximum
slope of the basin in some cases. Concomitant
to above, in the more evolved basins, ordinarily
regarded as mesotrophic or eutrophic, vittate
species, particularly members of Potamogeton ,
8.3 Growth Forms and Depth Distribution