''Dawsonia'' is a genus of

acrocarpous The Bryopsida constitute the largest class of mosses, containing 95% of all moss species. It consists of approximately 11,500 species, common throughout the whole world. The group is distinguished by having spore capsules with teeth that are '' ...

moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta ('' sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and ...

es. ''Dawsonia'', along with other members of the order Polytrichales, are taller than most mosses and have thicker leaves. Their

sporophyte A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote ...

s have conducting systems analogous to those of

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

s. '' Dawsonia superba'' is found in New Zealand, Australia and New Guinea. '' D. longifolia'' is found in the

Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Gui ...

Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federal constitutional monarchy consists of thirteen states and three federal territories, separated by the South China Sea into two regions: Peninsular Malaysia and Borneo's East Mal ...

, and

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

. There is uncertainty as to whether ''D. superba'' and ''D. longifolia'' are actually distinct species.

Etymology

''Dawsonia'' was named in honor of Dawson Turner (1775–1858), distinguished cryptogamist and friend of Robert Brown, who named the genus in 1811.

Description

Height

Moss gametophytes lack internal transport tissues, which, coupled with the absence of

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

s, leads to the water-loss characteristic of

bryophytes The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants ( embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited ...

. As bryophytes can only grow when hydrated, the lack of conducting tissue restricts most mosses, even in relatively wet habitats, to a low stature. However, ''Dawsonia'' (along with other genera in the Polytrichales order) reaches heights comparable to those of vascular plants. Polytrichales are acrocarpous mosses – they have vertical stems with terminal reproductive structures, with the

growing vertically (along the same axis as the

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the ...

stem). The tallest moss in the world is ''D. superba'', which can have a stem up to tall.

Leaves

The leaves of ''Polytrichum'' and ''Dawsonia'' (and related moss) differ from those of most mosses, which are only one or two cells thick. The Polytrichaceae have

lamellae Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology), a papery rib beneath a mushroom cap * Lamella (botany) * Lamella (surface anatomy), a plate-like structure in an animal * ...

– upright sheets of small, photosynthetic cells on the upper surface of the leaves with a function analogous to the

mesophyll A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...

cells of vascular plant leaves. They increase the surface area of

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

s available for CO₂ uptake, while at the same time maintaining layers of moist air between lamellae, reducing water loss. Lamella margins have a surface wax layer which prevents water from flooding into the interlamellar spaces.

Stems

The stems of Polytrichales have conducting systems which are analogous to the

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

and

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...

of vascular plants. The water-conducting tissue is the hydrome, made up of elongated cells known as

hydroids Hydroids are a life stage for most animals of the class Hydrozoa, small predators related to jellyfish. Some hydroids such as the freshwater '' Hydra'' are solitary, with the polyp attached directly to the substrate. When these produce buds ...

. Unlike the xylem of vascular plants, there is no secondary thickening of cell walls, as bryophytes lack

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...

. The phloem analogue in Polytrichales is the leptome, made up of leptoids, they are similar to sieve cells. Hydrome and leptome are well-developed in Polytrichales, and also appear in a number of other bryophytes.

References

* *

External links

Interactive, virtual slide cross-section of ''Dawsonia''
{{Taxonbar, from=Q288827 Polytrichaceae Moss genera