Thursday, December 29, 2011

Why are there no moss trees?

The land plants can be grouped into two broad categories:  vascular plants and non-vascular plants (better known as bryophytes).  Most of the plants we encounter in everyday life -trees, shrubs, flowering plants, ferns, etc., are vascular plants, while mosses, liverworts, and hornworts are non-vascular bryophytes, and quite humble in their habits.   

Vascular plants, such as this giant Sequoia,
 can get quite tall because they
possess xylem for upward water transport
 You may have heard the word “vascular” in reference to the vessels of our own circulatory system, and vascular tissues in plants perform a similar function. They consist of parallel sets of pipes: xylem vessels for conducting water upwards, and phloem tubes for moving dissolved foods from one part of the plant to another. Without such tissues, in particular the xylem, water cannot rise very high, and significant upward growth is not possible (see my posting "how does water get to the top of a redwood tree?").  Vascular plants have them, and bryophytes obviously don’t. 
Bryophytes cover every available surface
in the temperate rain forests of Washington
Mosses mostly do not exceed more than
a few centimeters in height. The leafy portion
of the plant produces gametes, and a fertilized egg
then develops into the spore-producing plant, which is
just the stalk and sporangium.  From W.H.
Brown, the Plant Kingdom, 1935.

Most bryophytes are prostrate, forming mats on the soil, trunks and branches of trees, rocks, and sometimes tombstones. Those that grow more upright are typically only a few centimeters high, but there are some giants among them that tower above their cousins to dizzying heights of about half a meter!

Liverworts lie flat, except for their reproductive structures.  From A. W.
Plant Morphology, 1953. 
Hornworts are similar to liverworts, but
with different form of reproductive
structures.  From G. M. Smith, Cryptogamic
Botany, 1938.
It seems strange at first that there are no larger mosses.  Nearly every group of organisms has both large and small members.    A common response, even in botany textbooks, is that mosses can't get any taller because they don't have vascular tissues.  More technically it is said, they "failed" to evolve lignin, a resin-like material that strengthens the walls of xylem vessels. 

Xylem vessels serve the same function as drinking straws.  Water is sucked up through them by the force of transpiration.  Imagine a straw made of silk.  It would collapse under the slightest suction and be useless.  The same applies to unreinforced cell walls.

I would suggest that if mosses really needed xylem, it would have evolved.  But we don't even have to pursue that argument because as it turns out, bryophytes couldn't get any taller even if they "wanted to," and it's not for lack of lignin or motivation.

For the real reason, we have to recall that plants have alternation of generations of gamete-producing plants (gametophytes) and spore-producing plants (sporophytes) (see my posting on "the Truth about Sex in Plants").   There are two alternate forms of every sexually-reproducing plant, one that produces spores and one that produces gametes. One is usually large and long-lived, the other small, short-lived, and generally unnoticed. 

In bryophytes, the main plants - the green mats that spread and live for many years - are the gamete-producing generation, just like their algal ancestors.  They cannot  get very tall, because their ultimate task is to release sperm cells and position eggs to receive them. Sperm cells can swim only a short distance but must reach an egg on another plant - a difficult proposition for fragile cells produced on a tree top.  Sperm cells produced on a large  gametophyte tree would be left literally "high and dry."

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Tree ferns are vascular plants, and
 their spore-producing generation is the main plant
that can get quite tall.
The spore-producing plant of a moss, its sporophyte, is a small, ephemeral structure that remains attached to the parent plant - just a slender stalk and a single sporangium.  It gets as tall as it can without toppling over or placing excessive demands on the gamete-producing plant - a few centimeters at most. 

But suppose that tiny spore-producing plant of the moss were to sprout its own roots and start growing on its own.  Then it could get as tall as it wants, because there is an advantage to dispersing spores from greater heights.  Well something like that did happen in the ancestors of the vascular plants, and their spore-producing generation became the dominant conspicuous one, inventing lignin and xylem as a means to become ever taller.  Voila, trees!

The gamete-producing generation of the fern resembles
that of a liverwort, but is even smaller and very short-lived.
From A. W. Haupt, Plant Morphology, 1953.

So in bryophytes, which are indeed well-adapted to creeping around in the shade, the gametophyte is the dominant plant, while the sporophyte is tiny, but in the tall-growing vascular plants, the sporophyte is the dominant plant, while the gametophyte is tiny.