11-14-2012, 02:35 PM
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Redchigh's mention of the crown and node is the key. What I have termed "substrate rooted plants" for the purpose of our profiles are plants that all have a crown from which leaves emerge upward and roots downward. There may be a tuber or rhizome at the crown.
Stem plants by contrast have no crown, but a continuing stem which will grow literally forever, and roots and leaves emerge along the stem from nodes. Side stems may emerge, especially if the stem is broken.
All plants, be they substrate-rooted or stem, assimilate nutrients through their roots and leaves. Some nutrients can only be taken up via the leaves [or in some cases the plants prefer this]--carbon, potassium, nitrogen (ammonium), oxygen, calcium, magnesium and sulphates.
The nutrients taken up primarily through the roots must be in the water, since plant roots can only assimilate nutrients from water.
The substrate rooted plants as redchigh mentions are usually amphibious bog plants in nature, spending half the year emersed and half submersed. It is during the emersed phase that they grow most prolifically and flower. Nutrients in the "soil" (which is often less like soil as we know it) dissipate into the water and are taken up via the roots. Things change a bit when these plants are submersed, which is one reason they lose their aerial leaves and develop often very different submersed leaf forms.