MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 16. BRYOPHYTES

Conclusions

Bryophytes represent a distinct plant division divided into three classes: Hepaticae (liverworts), Anthocerotae (hornworts), and Musci (mosses). Their pigments and reserve compounds show affinities with both green Algae and vascular plants. Like the latter, the sporophytes of certain hornworts and most mosses possess Stomata, while the leaves of some mosses and liverworts bear a cuticle. Although many bryophytes lack specialized vascular strands, absorbing Water directly via leaves and stems, others contain strands of hydroids (water-conducting Cells), and some also possess leptoids (food-conducting cells). Bryophytes and vascular plants presumably share a distant common ancestor.

All bryophytes exhibit a well-defined alternation of heteromorphic generations, with the gametophyte generation being dominant. The male (antheridia) and female (archegonia) sex Organs are multicellular and surrounded by sterile protective jackets. The archegonium contains a single egg Cell, whereas the antheridium produces numerous free-swimming biflagellate sperm cells. The sporophyte develops within the archegonium, but typically grows beyond it in mosses and hornworts while remaining attached to it. In mosses and some liverworts, the sporophyte is differentiated into a FOOT, a seta, and a capsule, or sporangium. Hornwort sporophytes are unique in possessing a basal meristem that continuously produces sporangial tissue over an extended period. The sporophytes of hornworts and mosses become nutritionally independent of the gametophyte, whereas those of most liverworts remain surrounded by gametophytic Tissues even at maturity, typically deriving all their nutrients from them. Upon germination, moss spores produce a filamentous or thalloid gametophyte (protonema) from which leafy gametophytes arise. Protonemas are known in some liverworts, but are absent in hornworts.

Appendix 1. Spore Dispersal in Liverworts

Liverworts utilize three distinct mechanisms for spore dispersal. The majority share a mechanism similar to that of Cephalozia, while some resemble Marchantia. A less common method is characteristic of Frullania (see p. 277).

Appendix 2. Insect-Mediated Spore Dispersal in Mosses

Among plants, angiosperms are renowned for their high degree of evolutionary advancement and specialized interactions with insects. However, one group of mosses has also evolved structural and physiological traits enabling them to enlist insects in the dispersal of their haploid spores. The parallelism between these mosses and angiosperm flowers is remarkable.

The unusual genus Splachnum, which exhibits such relationships, grows on dung or carrion, particularly when deposited in wet habitats such as bogs. These mosses form dense cushions up to 25 cm across, surmounted by capsules borne on thick, translucent setae. At the Base of the capsule, an expanded, brightly colored Structure known as the apophysis develops—ranging from purple and yellow to red—and can reach up to 2 cm in diameter. Unlike those of other species, the spores of these mosses are sticky and are dispersed by flies attracted by their vivid coloration and the odors emitted by the apophysis in some species (see p. 278).

Insects visiting these mosses may deposit eggs on fresh dung, thereby concurrently introducing plant spores and ensuring their dispersal. It remains unknown whether different species of Splachnum (up to six of which may co-occur in a single Location) selectively attract specific dung flies. This and other questions concerning this adaptive system are currently under investigation.