life cycle and reproduction

Plants have a strange and unique life cycle called alternation-of-generations. In all plants there are two “adult” phases: a haploid generation (n) and a diploid generation (2n). Typically, one of these phases is dominant over the other. In most plants, the diploid generation (the “sporophyte”) is dominant over the haploid generation (the “gametophyte”) but in bryophytes, the haploid generation is dominant. This represents a strategy that was likely present in the earliest land plants that arose between 510 and 630 million years ago.

The generalized bryophyte life cycle is shown below. The fusion of sperm and egg give rise to a diploid sporophyte. The sporophyte is so named because it gives rise to spores (literally, “spore plant”). Inside the capsule, meiosis occurs in the sporangium to give rise to dozens or even hundreds of spores. These germinate and form protonema, eventually becoming mature haploid gametophytes. The female gametophyte gives rise to eggs in the archegonium, while the male gametophyte produces sperm in an antheridium.

There are many variations on this generalized life cycle.  For example, above is the life cycle of a species characterized by having male and female gametophytes, but gametangia (antheridia/archegonia) need not be segregated to separate individuals. When a species is characterized by individuals with both types of gametangia, it is called monoicous (n. monoicy). When a species is characterized by individuals that are either male or female, it is called dioicous (n. dioicy). There are further variations of these two kinds of sexual strategies, but we will only discuss one.

Among land plants, a unique sexual strategy exists in mosses alone, called phyllodioicy (adj. phyllodioicous). This is a modification of the dioicous condition, wherein males and females occur as separate individuals, but males grow to a dwarf size on the female plant. Some species are facultatively phyllodioicous, and males may grow in the absence of a female and reach a normal size. In other species this condition is obligate, with males only growing to a dwarf size on female plants (Une 1985; Hedenäs and Bisang 2011). The ecology and evolutionary implications of this system remain largely unknown, but some studies suggest it may have evolved as a mechanism to reduce the problem of short fertilization distances. Other names for this condition include nanandry, pseudoautoicy, pseudomonoicy, and others.

Asexual reproduction is an important aspect of bryophyte reproduction. Bryophytes may reproduce asexually by a number of means. Some species produce specialized propagules (brood bodies), while others may asexually reproduce by budding (gemmae) when a small part of the gametophyte breaks off (Glime, J.M. 2017). In asexual reproduction, all progeny are clones of the original plant. Some species produce specialized structures to aid in the dispersal of gemmae. These “gemmae cups” are produced on the upper surface of some thalloid liverworts such as Marchantia and Lunularia. If you look closely, the individual gemmae are visible inside the cup-shaped structure in the image below.