= Solution
\b[The habitat occupied by the conspicuous <plant> is only part of the life cycle: successful reproduction also requires suitable conditions for its other generation, dispersal and <fertilization>.] In <alternation of generations>, a multicellular <haploid> <gametophyte> alternates with a multicellular <diploid> <sporophyte>. The transitions in <ploidy> occur at <meiosis> and <fertilization>; growth within either generation is by <mitosis>:
$$
\begin{aligned}
\text{sporophyte }(2n)&\xrightarrow{\text{meiosis}}\text{spores }(n)\xrightarrow{\text{mitosis}}\text{gametophyte }(n),\\
\text{gametophyte }(n)&\xrightarrow{\text{mitosis}}\text{gametes }(n)\xrightarrow{\text{fertilization}}\text{zygote }(2n)\xrightarrow{\text{mitosis}}\text{sporophyte }(2n).
\end{aligned}
$$
Thus <spores> disperse and initiate a generation, whereas <gametes> fuse. A <plant> <gametophyte> normally produces <gametes> by <mitosis>, not by another reduction division. Retention and nourishment of the young <plant embryo> on the maternal <gametophyte> distinguishes the land-plant reproductive arrangement from simply releasing a naked <zygote> into the environment.
In <bryophytes>, the persistent, usually most conspicuous generation is the <gametophyte>. It bears <antheridia> producing motile <sperm> and <archegonia> containing eggs. Following <fertilization>, the <sporophyte> remains attached to the maternal <gametophyte> and obtains resources through that association. Although some <sporophytes> photosynthesize, this does not make them nutritionally independent. The unbranched reproductive axis terminates in a single <sporangium>; elevating it can improve <spore dispersal> without requiring the whole <gametophyte> to grow tall.
This organization helps explain both constraints and successes on land. <bryophyte> <gametophytes> lack the true <xylem> and <phloem> of <vascular plants>, and often depend strongly on surface water and short transport distances. <rhizoids> provide attachment and can contribute to uptake, but are not anatomically equivalent to a <root (botany)>. Their small stature and limited control of water loss restrict sustained activity during drying. However, many tolerate drying and resume activity after rehydration: <desiccation tolerance> and avoidance of drying are different strategies. Periodic metabolic inactivity can permit persistence in exposed habitats even where continuous growth would be impossible.
Reproduction imposes an additional bottleneck. Motile <sperm> need a water connection to reach an egg, so successful vegetative survival on a dry surface does not imply that <fertilization> can occur there at any time. Rain, dew and protected microsites can provide intermittent opportunities. Separation of male and female <gametophytes> can add a spatial limitation. Resistant <spore> walls containing <sporopollenin> protect a dispersal stage, but a dispersed <spore> must still germinate and establish, and the resulting <gametophyte> must reproduce. Some <mosses> and <hornworts> have <stomata> on their <sporophytes>; <liverworts> lack true <stomata>, and their air pores are not equivalent regulated valves. It is therefore misleading to assign all these lineages one identical water-control mechanism.
Early land-plant <evolution> was not a ladder in which living <mosses> became living <ferns>. The <Rhynie chert> preserves reproductive bodies interpreted as <gametophytes> with axial growth, conducting tissues and <stomata>, sometimes sharing structural characters with their associated <sporophytes>. These fossils document combinations unlike those of modern <bryophytes> and caution against projecting a tiny modern fern <gametophyte> onto every early lineage.<The Gametophyte Generation of Some Early Devonian Land Plants by Winfried Remy and others>[International Journal of Plant Sciences 154, pp. 35–58; anatomical evidence for gametophytes and proposed associations with sporophytes]{ref} Identification depends on reproductive structures and anatomy; resemblance alone cannot demonstrate that two isolated fossil bodies belonged to one life cycle. Branching <sporophytes> bearing several <sporangia> increased reproductive output per <fertilization> event, while the <evolution> of effective transport and support permitted greater independence from the maternal generation.
In most familiar <ferns>, the large leafy individual is an independently growing <sporophyte>, with a <root (botany)> system, <xylem>, <phloem>, a <plant cuticle> and <stomata>. These features support water acquisition, long-distance transport and regulated exchange, allowing larger stature and access to a broader range of terrestrial environments. The <sporophyte> produces <spores> in <sporangia>, often clustered into <sori>. In a typical <homosporous> fern, a <spore> grows into a small free-living <gametophyte>, commonly a photosynthetic <prothallus>. The young <sporophyte> initially develops on it but becomes independent. Therefore <sporophyte> dominance reduces one kind of dependence without eliminating the ecological requirements of the other generation.
The fern <gametophyte> commonly lacks the hydraulic and water-conservation capacities of the mature <sporophyte>; establishment, competition, drying and the water requirement for <sperm> movement can limit recruitment. Yet it need not be uniformly more drought-sensitive. Experimental drying and rehydration of tropical fern <gametophytes> revealed substantial variation in recovery, with differences associated with habitat and repeated drying cycles.<Ecological and evolutionary consequences of desiccation tolerance in tropical fern gametophytes by James E. Watkins Jr and others>[New Phytologist 176, pp. 708–717; controlled desiccation cycles and chlorophyll-fluorescence recovery]{ref} Such assays need hydrated controls and checks of subsequent growth or reproduction: recovery of <chlorophyll fluorescence> alone is not completion of the life cycle. The two generations can exploit different microsites, so habitat limitation is not simply the intersection of two identical environmental requirements.
There are important alternatives within the fern life cycle. <heterosporous> water ferns produce distinct <microspores> and <megaspores>, with reduced male and female <gametophytes> developing within the <spore> walls. This offers protection and changes provisioning, but it does not supply the delivery mechanism of a <pollen tube> that can remove dependence on an external water film. Nor does <adaptation> to an aquatic habitat demonstrate freedom from water for <fertilization>. Differences among <homospory>, <heterospory>, mating systems and vegetative persistence mean that a single generalized prothallus cannot describe every fern.
Developmental genetics confirms that the two generations are distinct developmental programs rather than automatic visual consequences of chromosome number. Removing <KNOX2> regulators in a moss produced gametophyte-like bodies from <diploid> embryos without intervening <meiosis>.<KNOX2 genes regulate the haploid-to-diploid morphological transition in land plants by Keiko Sakakibara and others>[Science 339, pp. 1067–1070; KNOX2 deletion and development of diploid gametophyte bodies]{ref} This does not reconstruct the historical ancestor, but demonstrates a mechanism by which the deployment of generations can evolve. \b[Across these lineages, increasing <sporophyte> autonomy broadened terrestrial opportunities, while establishment of the <gametophyte> and water-dependent <fertilization> continued to filter where complete sexual life cycles could occur.]
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