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Summary

The gametophytes of certain species of Haplomitrium and of Takakia have organs that may be called roots. These normally grow downward, are almost colourless and, unlike flagella-shoots, have no leaf rudiments. They may have up to two orders of branching; they have no root-caps but are covered by mucilage if grown in dry conditions.

The axes of most species have a central cylinder of elongated cells. In the aerial shoots these cells generally appear ‘empty’ like the hydroids of mosses. The leaf cells have an outer layer which is relatively impermeable to water and solutes. Shoots of Haplomitrium mnioides are able to maintain their water balance more effectively than those of Plagiochila asplenioides under marginally drying conditions. The endohydric habit of Haplomitrium and Takakia appears to confer little advantage in the field because it is not coupled with a protoplasmic resistance to desiccation.