Botany Photo of the Day

Conduction of water in bryophytes (mosses, liverworts and hornworts) is broadly classified in three ways: ectohydric, mixohydric and endohydric.

Ectohydric bryophytes lack any form of specialized cells for internal conduction. Instead, water is conducted externally, typically through capillary action. Reliance upon external conduction, I'm certain, is correlated with height of these organisms – they tend to be the shortest of the bryophytes.

Today's photograph of an unidentified bryid moss is an example of a mixohydric bryophyte. While some water is conducted externally, these organisms have a measure of internal cell specialization, such as hydroids or stereids, that give them the ability to move water and nutrients within its tissues. It is not bona fide conducting tissue, i.e., it does not form an interconnected system throughout the entire body of the plant (including leaves or entire length of the stem), but it does represent a rudimentary way to transport water internally and decrease reliance on external moisture for growth and reproduction.

The third grouping, endohydric bryophytes, is represented by this BPotD on Polytrichum juniperinum. In these plants, an internal conducting system exists that is analogous to the conducting system of vascular plants. By no coincidence (as is mentioned in the Polytrichum link), this grouping contains the tallest of bryophytes reaching over a half metre in height.

In all three broad groupings, it is important to note that water uptake tends to be from atmospheric moisture (suggested in today's photo) absorbed via the leaf or stem cells as opposed to uptake in moisture from the substrate (compare with vascular plants, which often uptake moisture via roots in soil). For both ectohydric and mixohydric bryophytes in particular, this means that water is absorbed near the top of organism and passed downward to subtending tissues, the inverse of the way one typically thinks about water uptake in plants.

Photography resource link: For inspiration, the photomicrography of Hans Van Rafelghem.