An entire leaf is called a frond, while further subdivisions are referred to as pinnae (first division), which grow along the main stem (called a rachis in ferns), and pinnules (subsequent divisions). Many ferns have long, creeping rhizomes that form intricate networks underground, and while the leaves may senesce and drop off due to age or cold weather, these rhizomes can persist indefinitely, sending up new leaves year after year. Most ferns have rhizomes, underground stems from which the leaves are produced (Figure 2). Fortunately, further sub-divided groups within ferns have shared traits that are easier to observe. This is opposed to the condition in seed plants in which the protoxylem also develops through the midpoints and center of the xylem strand in any given vascular bundle. According to Kenrick and Crane (1997), the mesarch (derived in the middle) protoxylem (protoxylem = the water-conducting cells that are the first to grow in a developing stem, the result of primary growth) in ferns is confined to lobes of the xylem strand (Fig. There is consequently only one anatomical feature that unites them, an inconspicuous trait that requires observing the development of vascular tissue in the stem. Partly because of their considerable age, ferns contain a high amount of diversity, with some groups that look nothing like the more common representatives we usually associate with ferns. There are separate sections that cover topics ranging from fern morphology, phylogenetic relationships, and the fern lifecycle, along with the important role gametophytes play in the biology of ferns. How did ferns become so diverse, and what are the secrets to their success? What traits do they share in common, and how are they different from other groups of plants? What follows is a short primer on the biology of ferns, starting at the beginning, with how ferns first originated and evolved into the plants we see in the present, making special note of some of the groups that went extinct along the way. With around 10,500 living species (PPG 1), ferns outnumber the remaining non-flowering vascular plants (the lycophytes and gymnosperms) by a factor of 4 to 1. Today, ferns are the second-most diverse group of vascular plants on Earth, outnumbered only by flowering plants. The diversity of ferns we see today evolved relatively recently in geologic time, many of them in only the last 70 million years. Groups like the Rhacophytales, which were possibly some of the earliest progenitors of ferns, the ancient tree ferns Pseudosporochnales and Tempskya, and the small, bush-like Stauropterids have all long ago disappeared. However, despite the venerable age of the group as a whole, most of the earliest ferns have since gone extinct. Recent divergence time estimates suggest they may be even older, possibly having first evolved as far back as 430 mya (Testo and Sundue, 2016). Ferns are one of the oldest groups of plants on Earth, with a fossil record dating back to the middle Devonian (383-393 million years ago) (Taylor, Taylor, and Krings, 2009).
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