The giant leather fern (Acrostichum danaeifolium) is native to Florida and occurs elsewhere in the tropics outside the United States. According to the U.S. Department of Agriculture, the plant is hardy in plant hardiness zones 8b through 12b. As with so many things in nature, there are exceptions to the rules, but the giant leather fern generally follows a well-documented set of stages in its life cycle.
The sporophyte stage of the giant leather fern is the most conspicuous stage in its life cycle. It’s the stage we recognize as the fern itself. The mature giant leather fern sporophyte -- as with ferns in general -- features a rootstock or rhizome, with roots growing into the soil and a stalk or stipe growing upward. The “giant” part of this fern’s name is fitting since it is indeed large, growing to 6 to 10 feet tall and occasionally taller. The fronds, which can be either sterile or fertile, are divided into many leaflets. Fertile fronds have small spore cases, or sporangia, on the undersides of their leaflets. These can become so dense as to cover the entire underside of the leaflets with a cinnamon-colored growth.
The giant leather fern sporangia produce spores that are released and dispersed when mature. While all the cells of the mature sporophyte, including the sporangium itself, contain the full complement of chromosomes, the spores contain only half the genetic complement of the mature fern. The significance of this becomes more evident in another stage of the life cycle. The tiny spores are the beginning of a second phase in the fern's life cycle. Given suitable conditions of moisture and temperature, the spores germinate and grow into a small stage of the plant known as a prothallus or gametophyte.
Although tiny and inconspicuous compared to the mature giant leather fern, the fern’s gametophyte is virtually a separate plant unto itself. It is anything but giant, but is nevertheless critical to the fern's life cycle. The gametophyte is, in a sense, the major reproductive phase of the giant leather fern. It gives rise to two important structures: antheridia and archegonia. The antheridia are the male reproductive structures and they produce sperm, sometimes called spermatozoids; the archegonia are the female reproductive structures and they produce eggs. The sperm swims to the egg when conditions are wet enough.
Fertilization and Maturation
Even though the sperm and egg develop on the same small prothallus or gametophyte, a given egg has a reasonably good chance of being fertilized by sperm from a different individual giant leather fern. This helps ensure cross-fertilization and genetic diversity, rather than the genetic sameness that would result from self-fertilization. Once fertilization takes place, a new sporophyte begins growing from the gametophyte. The union of sperm and egg -- each with half the genetic complement -- restores the genetic status to full in the new sporophyte. As the sporophyte grows and matures, the tiny gametophyte withers. In this way, a new fern comes into existence and completes one turn in the life cycle of the giant leather fern.
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