Category: 03. Ferns

In certain fern genera, such as spleenworts (Asplenium), wood ferns (Dryopteris), and holly ferns (Polystichum), hybridization between species (interspecific crossing) may be so frequent as to cause serious taxonomic problems. Hybridization between genera is rare but has been reported between closely related groups. Fern hybrids are conspicuously intermediate in characteristics between their parents, and simple dominance of single characters is unusual. Occasionally, when…

The study of chromosomes, hybrids, and breeding systems has revealed much of value in understanding ferns. The chromosomes of ferns tend to have high base, or x, numbers, ranging from approximately 20 to 70, with the majority between 25 and 45. The familiar genus Osmunda, for example, has x = 22, Pteris 29, Asplenium 36, Dryopteris 41, Botrychium 45, and Pteridium 52. Ophioglossum reticulatum has 1,440 chromosomes, the highest number of any organism known…

Approximately one-third of fern species have paraphyses of one type or another. These are sterile hairs or scales intermixed with the sporangia, and they are, like indusia, believed to perform a protective function. Paraphyses usually are hairs or modifications of hairs that arise among the sporangia or on the sporangial stalk or capsule. In various genera of ferns,…

Protection of the sporangial cluster from exposure, drying, and other hazards is accomplished in various ways, such as by the formation of the sori in grooves or pockets or by the production of various forms of covers. One is the so-called false indusium, a rolled-over leaf margin under which sporangia form and mature. The true indusium…

Hand in hand with the reduction in size of single sporangia are seen more and more complex aggregations of sporangia known as sori. The meristematic area—the region of new cell growth—that produces them may continue its activities over a number of weeks, producing sporangia of all ages, older ones being pushed aside as new ones…

The spore cases, or spore-producing structures, in ferns range from globose sessile (nonstalked) organs more than 1 mm (0.04 inch) in diameter down to microscopic stalked structures, the capsules of which are only 0.3 mm (0.01 inch) in diameter. The former are known as eusporangia and arise from several cells, the latter as leptosporangia and arise from a…

The fern leaf, or pteridophyll, differs from the “true leaf” (euphyll) of the flowering plants in its vernation, or manner of expanding from the bud. In most ferns, vernation is circinate; that is, the leaf unrolls from the tip, with the appearance of a fiddlehead, rather than expanding from a folded condition. It also differs in its venation, which usually…

At the tissue level the leaf blades have certain differences from those of other plants, but the same general picture prevails. There are an upper and a lower epidermis, the latter with many stomata (microscopic pores). In between, the mesophyll is usually composed of cells with large intercellular spaces. In thicker leaves the upper mesophyll is composed of palisade cells—elongated…

Fern leaves vary in the relationship of the petiole, or leaf stalk (often referred to as stipe in ferns), to the blade (the expanded part of the leaf). Many strap-shaped leaves essentially have no petiole and are described as sessile; broad, ovate, or triangular leaves commonly have a pronounced leaf stalk, called a stipe, and are…

Generally, the patterns of the leaf veins, or vascular bundles (which can be seen readily by holding the specimen up to a strong light), are pinnate, and the veins are free; that is, they all diverge and never coalesce, either along their sides or at the ends. Nevertheless, there are numerous fern groups in which netted, or…