Re: Why do roots exist?
Several different types of experiments have been carried out in attempts
to answer this question. The first type, first tried almost 100 years
ago, asked the question "Do rooted aquatic plants grow better with a
nutrient rich substrate or with a sand substrate and a nutrient rich
water column." The data clearly indicated that rooted aquatic plants,
though they will grow on sand with nutrients supplied in the water
column, grew far better with nutrients supplied through a rich substrate.
These experiments have been repeated many, many times since with many
different types of rooted aquatic plants and the data consistently show
that plants grown on substrates outgrow those grown on sand with
nutrients supplied through the water column.
The second question was "Which nutrients can be supplied exclusively from
the sediment and which must be supplied via the water column." The data
clearly indicate that P and N can be supplied from the sediment and that
S and micronutrients may also be supplied exclusively from the sediment
(the data for N and P is much more extensive). The only nutrients which
are needed in the water column are Mg, K, Ca and of course CO2. These
consistent for several different types of rooted macrophytes on many
different types of sediments.
The third question was "Which nutrients actually are supplied via the
roots from the sediment." This typ of experiment is much more difficult
to carry out but the evidence indicates that N and P are obtained by
rooted aquatic plants from the sediment, even when readily available in
the water column (this includes genera such as Elodea and Myriophyllum
which have small root:shoot ratios).
The fourth question is "Which nutrients can be supplied exclusively from
the water column." As far as I know this remains unanswered as it is
extremely difficult to manipulate the nutrient content of saturated soils.