The short answer to your question is that
mosses (there are many species) were around before
vascular systems evolved. Some of their relatives
evolved into vascular plants like our modern
ferns. As you know, once they had tubes to carry
water and nutrients up, and carbohydrates down,
they could get a lot taller than mosses.
You seem to be wondering, "If those tubes are
so great, why didn't mosses get them?" The answer
is that natural selection does not work that way.
You don't get something just because it would be
helpful.Mutation (change in DNA) is a random
process. If you're lucky enough to get a helpful
mutation, you're more likely to leave more
offspring than your neighbor, but getting a good
mutation is just luck.
Does the lack of a
vascular system mean that mosses aren't
successful?Not at all. Mosses have been around
for over 300 million years. That's a pretty good
track record. They can't live everywhere and do
everything that a vascular plant can do, but they
do some things better than vascular plants, like
living in cool, moist, acidic environments that
don't get a lot of sunlight.
Can you think
of any other plants or animals that are pretty
much the same now as they were when dinosaurs were
roaming around the earth?Would you say they are
more or less successful than a species that is
relatively new? How would you decide to measure
"success?"Thanks for asking,