Alternation of Generation Between Mosses

¶ … Alternation of Generation between Mosses and Ferns

Literature Review Chapter Introduction

This section will be used to introduce the chapter which presents a review of the relevant scholarly and peer-reviewed literature to provide a general background concerning the alternation of generation between mosses and ferns and the specific life cycles for these organisms. A brief summary of the research will conclude this chapter.

Background Information about Mosses

Mosses are part of the phylum, Bryophyta. Mosses do not have vascular tissue, are invariably small and tend to grow in wet or very moist areas; there are about 14,000 known species mosses (Hanson 1991:28). Some moss species have commercial value, such as sphagnum peat moss, which is described by Arcand and Talbot as "a natural organic material created in very humid conditions when plant debris. Peat moss in its raw state is a spongy, fibrous, lightweight material whose color varies from light to dark brown depending on the degree of decomposition" (2000:36). Both mosses and ferns are dependent on the presence of lichens to help establish a foothold in rocky outcroppings where lichens form soil deposits (Oplinger, Halma and Halma 2006). According to Oplinger and his associates, "Only then can mosses and ferns assume a role in the succession process" (2006:24). Based on their analysis of the evolution of bryophyte sequences, Floyd, Zalawski and Bowman report that, "The ancestors of moss can be inferred to have inherited a single gene from an algal ancestor [and] the conducting tissues of mosses may be either analogous or homologous to those of vascular plants" (2006:373).

Background Information about Ferns

Ferns are commonly found in forest areas; these organisms have sporophytes that are typically delicate in appearance with broad leaves that are highly divided (Mohlenbrock 2003). Ferns that prefer moist, moss-covered soil on rock outcroppings include berry bladder fern and brittle bladder fern (also called fragile fern); both of these species form spores as well as creating asexual "bladders" which are pieces of tissue that are capable of giving rise to a new plant (Mohlenbrock 2003). By contrast, walking fern do not resemble many other types of ferns with leaves that are undivided and narrow (Mohlenbrock 2003). This species is capable of reproducing in such a fashion that it can "walk" across rocky outcropped surfaces that contain sufficient amounts of suitable soil (Mohlenbrock 2003). Other species of fern, including purple cliff-brake, two species of Woodsia, and the spike moss northern selaginella, prefer drier environs (Mohlenbrock 2003). Interestingly, farmers in China used burned ferns (primarily Dicranopteris dichotoma) as a crop fertilizer (Coggins 2002) and ferns are the most diverse component of the flora of the Southern Pole region (Falcon-Long, Cantrill and Nichols 2001).

Alternation of Generation in Mosses

According to McDaniel, Willis and Shaw, "Like all land plants, the moss life cycle consists of a multicellular haploid gametophyte generation that alternates with a morphologically distinct diploid sporophyte generation" (2007:2489). Likewise, Polunin reports that, "The mosses are all rather small plants in which the gametophyte, during the greater part (but by no means all) of its life, consists of a more or less upright stem bearing small leaves" (1960:51). The various stages of the life cycle of moss are described by McDaniel and his colleagues thusly: "The gametophyte generation consists of pollen and ovules, while the sporophyte generation is much larger and long lived. In contrast, the gametophyte is the dominant portion of the moss life cycle, and the sporophyte is ephemeral" (2007:2490). In addition, McDaniel, Willis and Shaw add that, "Recombinant haploid spores are produced in a single cross between two moss gametophytes" (2008:1425).