The Bryoflora of Martha’s Vineyard
by Norton G. Miller, Ph.D
The three plant groups known as mosses, liverworts, and hornworts comprise what are commonly called bryophytes. They are the most primitive land plants known on Martha’s Vineyard. Until recently all three groups were classified together under one botanical name, Division Bryophyta (Phyla Bryophyta, Hepatophyta, and Anthocerophyta of Margulis and Schwartz, 1998). However, newly acquired knowledge about their genetic diversity, together with detailed morphological information, indicates that mosses, liverworts, and hornworts are better treated in three phyla, one of which, Bryophyta, continues to be used only for mosses. Fundamental differences of evolutionary importance separate them. The earliest known bryophyte, a liverwort, dates to the Middle/Late Devonian transition, perhaps 380 million years ago, though most specialists in fossil plants believe bryophytes originated at least 50 million years earlier in the Silurian period. Like other land plants, bryophytes evolved from one of the groups of green algae.
Mosses, liverworts, and hornworts all share an identical life cycle in which the plant is green (photosynthetic) and functions in reproduction to generate male and female gametes inside separate jacketed sex organs. Sometimes these are together on the same plant and sometimes on different ones. The union of a swimming male gamete and a stationary egg initiates what develops into the spore-producing component of the cycle, a leafless sporophyte, which remains attached to the plant where the egg formed. Because a bryophyte’s chromosome number is halved during spore formation, sporophytes have a double set of chromosomes, whereas cells of the gamete-forming plant, which arise from spores, have only one set. Bryophytes are the only land-inhabiting plants in which the conspicuous green organism consists of haploid cells (one chromosome set). These characteristics of the cycle establish many of the biologically significant aspects of the bryophyte mode of life, including spore formation and a need for water for sexual reproduction to occur.
Given the significance of water for completion of a bryophyte’s life cycle, it is perhaps paradoxical that bryophytes can live in places that in fact are exceedingly dry, for example, boulders, tree bark, and scorched sand plains. This is possible because bryophyte metabolism shuts down rapidly and enters a state of dormancy as drying proceeds. When rewetted, the plants spring quickly back to life. Rain, if of sufficient amount and duration, provides the water needed for sexual reproduction most places on land.
But it has to be wet at exactly the right time. Therefore, bryophytes sometimes do not regularly develop sporophytes. While the main reason for this is fickle weather, male and female plants can be too far apart (sometimes even on different continents) to allow gametes to join no matter how wet it may be. All bryophytes also reproduce new plants from bits of original living tissue or by special vegetative structures that are analogous to offsets, bulbils, and tubers found in various flowering plants. Walking on bryophytes may break plants into pieces, but each fragment has the potential to produce a new population.
The bryophyte flora of Martha’s Vineyard is ecologically interesting and surprisingly diverse. Bryophytes have been found to be most numerous on ravine slopes, along streams, in wetlands, and on the bark of deciduous trees in both lowland humid woodland and drier upland forest. White oak and red maple bark are where to look for moss and liverwort epiphytes, which are found in mats near tree bases and at scattered places upward for six to eight feet. Some specialized bryophytes colonize bare places in lawns and open fields. Others are associated with human activities such as gardening, road maintenance, and building. Vineyard soil and granite boulders provide acidic habitats in abundance. Calcareous sites, on the other hand, are few, and largely consist of construction-related concrete and mortar. Therefore, bryophytes typical of calcareous habitats are decidedly rare on Martha’s Vineyard.
Although inadequately surveyed, the bryophyte flora of the Vineyard appears to consist almost entirely of species otherwise found southward along the Atlantic Coastal Plain and places inland. Few northern species have been found so far. It is of great botanical interest that some recently discovered Vineyard bryophytes have not been documented to occur north of Virginia and nearby southern states. Vineyard populations of them are the northernmost known. How, why, and when they became established on Martha’s Vineyard is not certain, but possibly the island’s equable climate and a diversity of suitable habitats provide part of the answer.
The bryophyte flora of Martha’s Vineyard now is known to consist of 122 moss, 43 liverwort, and one hornwort species. Hilferty (1969) cited records for only 13 mosses, and an additional 27 species were documented by Anderson et al. (1997). The most common mosses and liverworts I have found so far are listed here, together with brief notes on habitats. Vouchers for all of my collections are on deposit at the New York State Museum.
Crum and Anderson (1981) and Schuster (1969–1992) are the standard identification manuals for mosses and liverworts, respectively. Schofield (1985) contains a well-illustrated summary of bryophyte structure and classification, and Shaw and Goffinet (2000) is a collection of contemporary articles about various bryological topics under current study. Most bryophytes do not have common names, but with a little effort their botanical binomials become familiar. Remember how quickly children master the technical names of dinosaurs, and forge ahead!
Mosses, Phylum Bryophyta
There are about 10,000 species of mosses, and many live in tropical environments. Numerous and usually different species are present in temperate climates, and more species of mosses occur on Martha’s Vineyard than liverworts. Because most people know what a moss is when shown one, there is wide appreciation for this group of plants. However, it is fair to issue a warning: not all plants with “moss” in their names belong to Phylum Bryophyta. Some are lichens (reindeer moss), others algae (“moss,” the slippery coating on damp soil and rocks) or flowering plants (Spanish moss). Some mosses form conspicuous carpets visible at a distance, and some are hidden in the sense that one needs to search for them close to the ground or in other habitats where they grow.
Many mosses have erect stems and leaves arranged in spirals. In others, the stem is parallel to the surface on which it grows, and the plants are flat or nearly so, because, as growth proceeds, secondary branches and leaves become oriented in compressed leafy sprays. Acrocarpous mosses are the first group, pleurocarpous ones the second. Some pleurocarps are called feather or fern mosses because the plants resemble miniature fern fronds and are flattened like feathers.
The spore-producing structure of a moss (the sporophyte) is attached to the leafy plant and consists of a slender leafless stalk (seta) that ends in a capsule in which spores arise. Capsules of many mosses terminate in a lid that eventually falls off the capsule to expose a peristome of small tooth-like structures consisting of cell wall remnants that flex inward and outward as they lose or gain water, thereby facilitating spore dispersal. Moss sporophytes are often green and photosynthetic. In contrast, those of liverworts are colorless and translucent when mature and have a black capsule that splits open into four valves or irregularly at the time of spore release. Differences in the moss and liverwort sporophytes are very clear and evidence of the distinctness of these two bryophyte groups. Liverwort sporophytes are objects of unique beauty, and it is unfortunate that they require a sharp eye to find and observe.
A moss begins development when a spore germinates. Soon after, a branched network of filaments grows over the substratum. This stage, the protonema, is sometimes conspicuous, especially on soil, and it is apt to be called an alga unless a search is made for leafy plants, which are produced from bud-like branches of the protonema. Mosses with persisting protonema are often found on patches of bare soil in lawns and fallow fields. Look for them in West Tisbury on grounds of the Polly Hill Arboretum.