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.

dave huebert