Small, Green and Easily Overlooked

Mosses are small, green and easily overlooked or mistaken for other plants.  Spanish moss is a flowering plant. Reindeer moss is really a lichen.  Club moss is actually a relative of the ferns.  There are more than 12,000 species of real mosses and they grow on every continent including Antarctica. It is said that moss grows on the north side of trees but don’t count on that to save you if you are lost in the woods. 

A patch of moss in a lawn, Salisbury, NC

Smallness is the key to moss success. Mosses are modern-day echoes of the first primitive plants that colonized the land.  They crawled from the sea, or perhaps a warm little pond, 470 million years ago and covered the land with green fuzz.  The first land animals came in the wake of the primitive moss-like plants.  These early land animals left no fossils but they did leave fossil tunnels in soil similar to what millipedes make today.  

Back to smallness.  There are no 100-foot mosses, not even in the fossil record.  This is because mosses lack xylem and phloem, the vascular tissues of plants that transport water and nutrients.  Mosses stayed small because they have no means to move water to the top of a 100-foot plant. They rely on diffusion to move nutrients about the plant.  Lack of vascular tissue also limits size in another way.  Trees attain their great height because of their woody trunks, and wood is mostly xylem, the water transporting tissue. 

Ceratodon purpureus with
sporophytes growing from the
gametophyte

Mosses lack true roots, stems and leaves.  They have structures that are very leaf-like called phyllids.  Phyllids are usually one cell layer thick, flat, green and carry out photosynthesis.  The moss “stem” is the axis and the root-like structures are called rhizoids.  The rhizoids, axis, and phyllids are all made of cells that have only one copy of each chromosome, and make up the gametophyte generation. The gametophyte generation produces eggs and sperm, the gametes.   

Mosses go through another stage to complete their life cycle. In early spring gametes fuse and the result of fertilization produces the sporophyte.  The sporophyte generation is made of cells with two copies of each chromosome and grows out of the top of the moss.  The sporophyte has a stalk and atop that stalk is the capsule, the spore producing structure.  Since the gametophyte generation produces gametes the sporophyte generation must produce spores.  The spores are microscopic and when released from the capsule float through the air.  If a spore happens to land in a suitable environment, a new moss gametophyte grows.   

The marvelously named Entodon seductrix,
the seductive entodon

The first moss-like plants that came onto land had a dominant gametophyte.  As land plants  evolved the sporophyte generation became more important.  Ferns, conifers and flowering plants all have dominant sporophytes that make vascular tissue.  This allows these vascular plants to become much bigger than mosses. 

So, in the spring look for the sporophytes peeking out of the mosses and know they are completing a life cycle that has been turning since long before vertebrates came onto the land. 

Ceratophyllum purpureus