The easily visible and persistently photosynthetic bryophyte bodies described above are those of the haploid, gamete-producing generation. Because bryophytes release swimming sperm into the environment, their mode of fertilization also limits their size. Bryophyte sperm must swim through films of surface water or be transported by the splash of a raindrop. Sperm can travel in this manner only a relatively limited distance and only if water is present. A few mosses grow to be more than half a meter tall, but these giants of the bryophyte world are found only in the understory of very wet forests. In most bryophytes, the gametophyte grows to only a couple of centimeters in height.

Following fertilization, the diploid sporophyte develops while remaining physically attached to and, in varying degrees, nutritionally dependent upon the gametophyte. Recall that the diploid, spore-producing generation represents a new component of the life cycle that evolved as plants moved onto land. This sporophyte generation addresses the challenges plants face in dispersing offspring through the air. In most bryophytes, the unbranched sporophyte extends several centimeters above the gametophyte, greatly increasing the chances of a gust of wind dispersing spores through the air.

In mosses and liverworts, the sporophyte is short-lived, drying out after the spores are dispersed. However, in hornworts, the sporophyte can live nearly as long as the gametophyte because it can produce new cells at its base, similar to the way grass blades elongate (Chapter 32). Thus, bryophytes illustrate an important trend in the evolution of plants, an increase in the persistence of the spore-producing generation.

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