I would have to agree with Paul, if your plants have roots then they
arent hornwort. The radicle (root tip) never develops in this genus.
Having said that, however, they do have modified shoots that will penetrate
a soft mud sediment and anchor the plant. These 'shoots', which have
been called rhizoids, are whitish and slender compared to the
above-ground shoot. They still have recognizable leaves, however, that
are arranged in whorls along the stem. These underground shoots develop
in the spring from a node of the previous years stem and grow down into
the substrate. About 20 years ago a man by the name of Dale Toetz looked
at Ceratophyllum and found that it could absorb ammonia from these
underground stems and transport it to the rest of the plant. This
suggests that the underground stems function like roots in many ways.
I checked my references on mycorrhizal associations in aquatic plants and
could find no mention of Ceratophyllum, so you should check that out
further Paul ... it would be an interesting observation since Ive never
heard of mycorrhiza forming associations with stems (not to say that my
knowledge is complete on this topic).