Mosses and liverworts are both non-vascular plants. This means that they lack the conducting tissue found in the veins of higher plants. Higher plants contain xylem and phloem cells which enable them to transport food and water throughout the plant structure. However, due to their thin tissues, small size, the need for a moist or watery environment to help with fertilization, they are restricted as to where they can live. The environment they can be found in is one of low light and dampness. If the area is too dry, they can become dehydrated and they can perish.
I believe the answer to that question lies in the fact that mosses and liverworts have an entirely difference composition and makeup.
These types of plants lack the usual characteristics of other woody or flowering plants. They lack stems, deeply lobed or segmented leaves, a vascular system, ovules, and seeds. Instead, they reproduce by means of small spores. The spores send out filaments which grow into a matted, clump-like carpet similar to thin, green felt. The mat eventually produces specialized cells that form distinctive male and female parts. You can observe these as little antenna-like projections rising above the clump or mat. A full reproductive cycle will produce more spores, which in turn propagate the species.
Most mosses rely on the wind to disperse their spores, but some species produce special leaves and branches, which when broken off, can reproduce with being fertilized.
Mosses can live on rocks, trees, wood, and by the sides of streams. Wherever they occur, mosses require moisture to survive because of the small size and thinness of tissues, lack of cuticle (waxy covering to prevent water loss), and the need for liquid water to complete fertilization. Some mosses can survive dessication, returning to life within a few hours of rehydration.
This, then, is the foremost reason why it is restricted in size and habitat.
Both xylem and phloem are vascular tissues found in a plant. Xylem is a tubular structure which is responsible for water transport from the roots towards all of the parts of the plant. Phloem is also a tubular structure but is responsible for the transportation of food and other nutrients needed by plant. Xylem imports water and minerals while Phloem transports water and food. Xylem exists as non-living tissue at maturity, but phloem is living cells.
Xylem: Hard wall cells transport water and mineral nutrients
Phloem: Relatively soft -walled cells transport organic nutrients
"Hardness and softness" is a function of the amount of lignification and extractive content of the individual cell walls not there location in the tree.
- It is a dead complex permanent tissue. Sapwood is xylem and mostly alive
- It is the principal water conducting tissue of vascular plants.
- Xylem consists of tracheids , vessels , xylem parenchyma and xylem fibres. Also ray parenchyma Also the xylem of softwoods (Gymnospermae) do NOT contain vessels.
- It is a living complex permanent tissue. Inner phloem is alive. Outer phloem is dead.
- It is the principal food conducting tissue found in vascular plants. Actually only the first .2-.7mm of the phloem is functional in food transport. The rest is non-functioning (ref. Esau's Plant Anatomy, 3rd ed. 2006, Chap.14) also (ALFIERI, F. J., and R. F. EVERT. 1968. Seasonal development of the secondary phloem in Pinus. Am. J. Bot. 55, 518-528.)
- It consists of sieve cells , sieve tube elements , phloem parenchyma and phloem fibres. There are also phloem ray cells.
Xylem consists of the inner heartwood (the dead part) and the outer portion called the sapwood (the mostly living part.) Xylem "wood" from the Greek xylon. The actual transport of the water and minerals from the roots (sap) is carried out by the outer portion of the sapwood (nearest the bark).The largest percentage of the sap is transported in the first few growth rings.
that is probably why mosses are restricted, for the amount ofxylem and phloem has to be fixed in an area, depending on the habitat.