We continue our study of bryophytes in Kingdom Plantae. We have already seen how bryophytes are eukaryotic, have chlorophyll a and b as well as xanthophylls and carotenoid pigments for photosynthesis, store starch, and have pectin-cellulose walls. These characteristics are common for ALL members of Kingdom Plantae. All bryophytes have a sporic life cycle that is oogamous. Bryophytes are very similar to each other and fundamentally similar to flowering plants.
In hornworts, as for all other bryophytes, the dominant phase of the life cycle is the haploid gametophyte.
The sporophyte is parasitic (dependent!) on the gametophyte.
The hornwort gametophyte thallus is not much different from a thallose liverwort. The thallus is a one-cell-thick sheet at the margins and forms a pad of cells closer to the middle of the thallus. The thallus is irregular in outline and sometimes "frilled" at the margins. Beneath the thallus the lower epidermis anchors itself with rhizoids. Hollow areas of the central pad region may house symbiotic cyanobacteria. These are likely providing a particular mineral nutrient for the hornwort...which nutrient?
One interesting fact about many hornworts is that the gametophyte cell has a single chloroplast. Whether that is anamorphic or plesiomorphic or whether it represents anagenesis or cladogenesis or whether this is a parallelism or convergence is an interesting question.
Archegonia and antheridia are partially or completely embedded in the upper surface of the thallus in the padded central region. Both have sterile jackets which open in response to free water. The sperm are chemotactically attracted to the egg across a film of water trapped on the surface of the pad. It is my understanding that many of the hornworts are homothallic (monoecious!) and so syngamy may occur between an egg and sperm from the same parent. Is there any recombination possible in that mating system? Think clearly about it and tell where in the rest of the life cycle it may occur!
The zygote and resulting young sporophyte is initially dependent on the gametophyte for nutrition. The archegonium neck proliferates at the base of the seta and around the foot of the sporophyte. The seta outgrows the neck, however, and emerges into the sunlight. It begins to do its own photosynthesis. It becomes independent of the gametophyte!
The sporophyte epidermis is cutinized and includes stomata. The cortical layer inside carries out photosynthesis.
Surrounding hydroids and leptoids in the center of the seta, are cells which become the sterile jacket and the sporocytes. The latter undergo meiosis to produce spores. As the seta matures, it splits open longitudinally to shed the spores from within.
But one truly amazing aspect of the sporophyte is that, just above the foot, the seta is meristematic and can keep making new seta for a long period of time...even after most of the gametophyte has disintegrated! The sporophyte then is more-or-less indeterminate in growth! This is the "horn" of the "hornwort." Because the seta keeps growing from the base, the tip of the seta cracks open and sheds its spores while more spores are being produced in the growing seta beneath. The number of spores which hornworts can produce is similarly indeterminate!
The presence of a photosynthetic (less parasitic) sporophyte, its indeterminate growth, and indeterminate reproductive capacity are characteristics that we find in vascular plant sporophytes. In other words, these are apomorphic character states! The developing independence of the sporophyte of hornworts foreshadows what we shall see in the rest of this course. The more-primitive-than-mosses gametophyte and its reduction in "dominance" again foreshadows what we shall see in the vascular plants. Ultimately, in flowering plants, the gametophyte becomes the parasitic generation and is reduced to just a few cells!
So as we look at hornworts, a legitimate question about their taxonomic placement in a phylogeny becomes interesting!