4.1 Three lineages, one grade of body #
4.1.1 What they share, and how they may be related #
Liverworts (Marchantiophyta), mosses (Bryophyta in the strict sense), and hornworts (Anthocerotophyta) are the non-vascular land plants. “Bryophyte” in the broad sense means all three. The gametophyte is the lasting photosynthetic body. The sporophyte is unbranched and attached. There is no lignified xylem and no sieve-element phloem. Sperm must swim.
Older textbooks drew liverworts branching first, then mosses, then hornworts as sister to the vascular plants. Large phylogenomic datasets have more recently favoured monophyletic bryophytes as the sister group of vascular plants. The order inside the bryophytes is less secure and has changed before. No living bryophyte is treated here as a stand-in for the ancestor of a fern.
4.1.2 Size, water, and rhizoids #
Without lignified tracheids there is no woody conducting column. Many bryophytes are ectohydric: water moves over the surface, and the plant is active when wet. Polytrichum has hydroids and leptoids, which are not tracheids or sieve-tube elements. Dawsonia reaches about half a metre, the upper end of the plan. The sporophyte makes one sporangium and cannot branch. Mosses nevertheless cover tundra, bog, and forest floor. Sphagnum holds water in dead porose hyaline cells and builds peat that stores a large share of soil carbon, because waterlogging and acidity slow decay.
Rhizoids anchor. They are unicellular in liverworts and hornworts and multicellular filaments in mosses. They have no cap, no stele, and no root hairs. A root is a later sporophyte organ (Chapter 5), not a moss rhizoid enlarged.
4.2 Three body plans #
Thalloid liverworts such as Marchantia are dorsiventral sheets. Dorsal air pores are not stomata. Leafy liverworts, more numerous, often have three ranks of lobed leaves without a moss costa. Marchantia is unisexual, with gametangia on an antheridiophore or archegoniophore. Multicellular haploid gemmae in splash cups clone the gametophyte; which face becomes dorsal is set by light after landing. The sporophyte is foot, seta, and capsule. The capsule, with spores and spiral-walled elaters, matures while the seta is short. The seta then elongates by cell expansion and lifts the finished capsule, which often splits into four valves. Capsule first, stalk second: the reverse of a typical moss.
The moss one sees is the gametophore, a leafy stem, usually with a costa, grown from a bud on a protonema. In the Bryopsida an operculum falls and a peristome of hygroscopic teeth regulates spore release, bending with humidity, built of wall fragments in most genera and of whole dead cells in Polytrichum. Sphagnum has no peristome: drying raises pressure until the operculum blows off. The sporophyte is foot, seta, and capsule. The seta rises before the capsule is mature. A calyptra, the torn archegonial neck, is haploid gametophyte tissue sitting on the diploid capsule. It is not part of the capsule wall.
Hornwort gametophytes are thalli, often with Nostoc in mucilage cavities. Many cells have one chloroplast with a pyrenoid. The sporophyte is a horn with a basal meristem, green, with stomata, maturing from the tip down, still foot-dependent. It is the least dependent bryophyte sporophyte and it is not a proto-fern. Pseudoelaters aid dispersal.
4.3 The moss life cycle in full #
The study of The moss life cycle in full provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Angiosperm Life Cycle
4.3.1 Spore, protonema, gametophore #
Use Funaria or Polytrichum. A haploid spore germinates into a filamentous protonema: chloronema, then caulonema with oblique cross-walls. Buds on that filament become leafy gametophores. One spore can make a whole tuft, one haploid genet rather than a population of seedlings.
The gametophore is the dominant photosynthetic body. It is monoecious or, as in Polytrichum, dioecious. Sex is a property of the gametophyte. The sporophyte does not have a sex of that kind.
4.3.2 Gametangia, embryo, capsule #
Antheridia are stalked jackets of sterile cells around spermatogenous tissue. Moss sperm are biflagellate and swim. Polytrichum splash cups fling sperm-laden drops; arrival still requires a water film. Archegonia are flasks: neck-canal cells break down, one egg lies in the venter. Sperm are attracted to archegonia; these notes do not invent one universal attractant molecule. The zygote in the venter is the embryo, retained and fed. Without external water there is no sporophyte.
The embryo becomes foot, seta, and capsule. The foot absorbs; transfer cells with wall ingrowths enlarge the membrane at the junction. The seta lifts the capsule. Young sporophytes are often green and partly photosynthetic, and they remain dependent and unbranched. Sporogenous tissue around a sterile columella produces spore mother cells. Meiosis makes tetrads of haploid spores. The calyptra is lost, the operculum falls, and peristome teeth meter the release. There is no seed and no pollen.
Ploidy check: spores, protonema, gametophore, gametangia, gametes, and calyptra are haploid. Zygote, foot, seta, capsule wall, columella, and spore mother cells before meiosis are diploid. Meiosis is in the capsule, not in the antheridium. Fragmentation and gemmae clone the gametophyte. That is not apomixis, which concerns seeds (Chapter 9), and it does not make a sporophyte.
4.4 Key terms, summary, and self-check #
Key terms. Ectohydry; hydroid; rhizoid; protonema; gametophore; calyptra; peristome; foot; seta; capsule; elater; hyaline cell; gemma.
Summary. Liverworts, mosses, and hornworts keep a dominant gametophyte and an unbranched dependent sporophyte, and current molecular evidence generally makes them one clade sister to vascular plants. They stay small because they lack lignified vascular tissue and because the sporophyte cannot branch, though Sphagnum is ecologically large. The moss cycle runs from spore to protonema to gametophore, through swimming sperm and a retained embryo, to a foot, seta, and capsule where meiosis occurs. The calyptra is gametophyte. External water at fertilisation is not optional. Liverwort setae elongate after the capsule is mature; moss setae elongate before.
Self-check.
1. Why is a moss rhizoid not a root?
2. What is the calyptra, and how does its ploidy compare with the capsule?
3. Order these: capsule, protonema, spore, gametophore, zygote.
4. Why is a Marchantia air pore not a stoma?
5. When does the seta elongate relative to capsule maturity in a moss, and how does a liverwort differ?
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