LOWER LAND PLANTS 



157 



BRYOPHYTA (Mosses, Horned Liverworts, and 

 Liverworts) 



Structure: generally tissue organization in two 

 generations, sporophyte and gametophyte; gameto- 

 phyte generation of one or two phases; single phase 

 is the simple to branched or leafy membranous 

 gametophyte present in horned liverworts (Antho- 

 cerotae) and most liverworts (Hepaticae); two phases 

 include a filamentous to membranous stage (proto- 

 nema) which buds one or more branched leafy stages 

 (gametophores) found in a few liverworts (Hepaticae) 

 and in the mosses (Bryales); gametophytes photo- 

 synthetic, normally simply organized of one or more 

 tissue layers into a variously organized flat mem- 

 branous sheet or into a branched stem-like structure 

 bearing "leaves"; gametophyte "roots" are simple, 

 short, hair-like structures; the gametophyte usually 

 is the larger and more conspicuous generation; sporo- 

 phyte may be partly photosynthetic, but is wholly or 

 partly dependent upon the gametophyte for its nutri- 

 tion, often complexly organized with about four dif- 

 ferentiated tissue layers into a specialized spore-form- 

 ing sporangium (capsule) that may or may not be 

 supported by a stalk that is joined to the gametophyte 

 by a foot, usually no more than a few inches long. 



Tissues: many types present, but no vascular 

 (xylem and phloem) tissues organized into veins as in 

 the vascular plants. 



.\utrition: typically photosynthetic, but sporo- 

 phyte is essentially a parasite upon the gametophyte. 



Reproduction: asexual generally involves gameto- 

 phyte fragmentation (mostly by death of parts con- 

 necting surviving parts), gametophyte production of 

 specialized areas along its margin, and gametophyte 

 formation of special structures called gemmae; no 

 spore formation; sexual involves sperms and eggs and 

 the production of spores; gametes usually form in 

 unlike male and female multicellular sex organs. 



Life cycle: diplobiontic, with alternation of genera- 

 tions well developed (Figure 10.1); mature sporo- 

 phyte sporangium (capsule) sheds spores, the spores 

 germinating into a gametophyte generation (if two 

 phases, protonema germinates into a gametophore); 

 mature gametophytes develop male and female sex 

 organs either on the same or difTerent plants; sperms 

 are shed and require water to reach the female sex 

 organ; fertilization produces a zygote which by mito- 

 sis forms first an embryo and then an adult sporo- 

 phyte; the inner layer of the sporophyte capsule, by 



meiosis, forms spores; generally moisture absorption 

 is necessary to create the pressure needed for the 

 capsule to open and shed its spores; spores typically 

 must be shed into water to be transported and to 

 germinate into a gametophyte (or a protonema which 

 buds oflf gametophores). 



Occurrence: normally found in moist areas, but 

 range from fresh-water situations to dry rocks; about 

 20,000 species. 



The bryophytes are sometimes treated as two 

 phyla, the Bryophyta, or mosses, and the Hepato- 

 phyta, or horned liverworts and liverworts. No mat- 

 ter which interpretation is preferred, it is generally 

 agreed that these plants originated from algae and 

 most likely from the Chlorophyta. Therefore, if two 

 phyla are recognized, each phylum is assumed to 

 have evolved from algae independently. 



The origin from the Chlorophyta is assumed for 

 various reasons, but especially because bryophytes 

 and gfeen algae share the same photosynthetic and 

 other pigments. The similarities can be appreciated 

 by comparing the Bryophyta and Chlorophyta synop- 

 ses. However, it should be noted that the Charophyta 

 also bear the same similarities plus sex organs, struc- 

 tures found in the Bryophyta but not in the Chloro- 

 phyta. In spite of this, the Chlorophyta still tend to 

 be favored as ancestors because of their generalized 

 nature, especially that of the primitive forms, and the 

 fact that the Charophyta, both living and fossil, 

 appear to be specialized. Experience would imply 

 that such specialized forms are not likely to be an- 

 cestral to other groups of organisms. Therefore, the 

 ancient Chlorophyta remain as the best possible an- 

 cestors of the Bryophyta, but this does not mean that 

 either the Charophyta or even other algae can be 

 exempt from ancestral contention. 



At one time the bryophytes, especially the horned 

 liverworts, were believed ancestors of the Tracheo- 

 phyta, or vascular plants. Although this theory is no 

 longer favored by most botanists (present information 

 tends to make the possibility of such a relationship 

 remote), one can find a definite similarity in the 

 gametophytes of these groups. 



If there is any close relationship between the phyla 

 of land plants, this evolution is unlikely to be from 

 Bryophyta to Tracheophyta. Rather, the fossil record 

 almost demands the opposite ancestry; fossils imply 

 that the tracheophytes are older than the bryophytes. 

 Moreover, there is some slight chance that tracheo- 

 phytes were ancestral to bryophytes. This chance is 



