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Plant of the Week: Moss

Picture of Moss on rocks

GREEN -- Moss is at its best in the winter. (Image courtesy Gerald Klingaman)

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Latin: Mostly Leucobryum

Walking in the woods during an Arkansas winter leaves me looking for anything green. Most often I find myself zeroing in on clumps of moss, for they are always present and rewarding if you are willing to zoom in and appreciate nature on a Lilliputian scale.

Botanists classify the 12,000 species of moss as bryophytes. These are primitive plants with their origins tracing back about 470 million years, making them one of the first land plants. They didn’t conceive photosynthesis but ancient moss ancestors were the species that brought it from the tidal pools on the edges of earth’s oceans to dry land.

Mosses have leaves and stems and a rudimentary kind of root system, called a rhizoid, but it is mainly used for anchoring the plant in place, not taking up water and nutrients. Moss leaves are a single cell thick and lack a protective epidermis, making them dependent on the moisture content of their environment to maintain their turgidity. Because of this reliance on ambient moisture conditions they are most often found in moist, shaded locations. In the wintertime they flourish because the cooler temperatures reduce water loss and the humidity – a function of temperature and the amount of moisture in the air – often approaches 100 percent.

Picture of Moss on a lawn.

LAWN -- This moss lawn at the Missouri Botanical Garden in St. Louis must be swept often to keep leaves from killing out patches of moss. (Image courtesy Gerald Klingaman)

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Moss leaves, themselves often quite tiny, are spirally arranged around the tiny stems which contain the earliest known example of a vascular system. Mosses were long thought to not contain a vascular system but it is now known that theirs is in a non-lignified form, unlike the hardened tubular conductive tissue of higher plants.

Mosses usually grow perched atop a rock, board or the base of a tree. Water and nutrient essentials come in the form of rainfall or dew and wind borne debris. If the rainfall doesn’t come in a timely manner, the plant shrivels up and looks dead, only to revive quickly when rehydrated. Mosses are tough – some have been revived after storage for over a century in a dry herbarium cabinet.

But the biggest feature differentiating these primitive plants from higher plants is how they reproduce. They reproduce by spores like ferns but their mode of reproduction is so very different. In higher plants, including ferns, and in animals, the normal compliment of chromosomes in the cell nucleus is a paired set of two chromosomes (the 2n or diploid state), one each derived from the male and female parent. Only during reproduction does the chromosome strand separate into a single, independent state (the 1n or haploid state).

Mosses do it differently. The moss plant exists with single, unpaired chromosomes (the 1n state) and only unites with a compatible partner during reproduction to create the 2n phase. The 2n zygote then matures and undergoes meiosis before releasing 1n spores from the reproductive capsules.

Many mosses have worldwide distribution and are found on all continents. This speaks of their ancient linage with many species being around when the continents were merged as a single great land mass.

I’ve made several half-hearted attempts to identify my moss photos to the species level but find it difficult -- well, perhaps impossible. Photos, unless taken with great care and very close up, don’t record the kind of detail needed to identify mosses to the species level. To do so with accuracy one needs a good live specimen – preferably with capsules present, a well-illustrated regional field guide, a hand lens and a lot of patience.

Mosses have been a part of garden design in Japanese and Chinese gardens for centuries but it has only been in the past couple decades that moss gardening has taken root in this country. They can be grown on the ground like any groundcover but if they are in an area that gets covered with leaves or where weeds grow well, they usually will not survive. They are especially well suited for use between stepping stones, on north facing slopes or cool shaded nooks, but weed completion and leaf litter must be carefully watched.

Hand sprinkling will keep them sufficiently moist during most of the year but in summer months extra care will be needed to keep them from going dormant. The best choice of adapted moss is to find patches growing in your own garden and then transplanting it to similar environmental niches in the garden.

By: Gerald Klingaman, retired
Retired Extension Horticulturist - Ornamentals
Extension News - January 17, 2014