|Vascular plants, such as this giant Sequoia,
can get quite tall because they
possess xylem for upward water transport
|Bryophytes cover every available surface
in the temperate rain forests of Washington
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
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."
|Tree ferns are vascular plants, and
their spore-producing generation is the main plant
that can get quite tall.
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.