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''Adiantum viridimontanum'', commonly known as Green Mountain maidenhair fern, is a rare fern found only in outcrops of serpentine rock in New England and Eastern Canada. The leaf blade is cut into finger-like segments, themselves once-divided, which are borne on the outer side of a curved, dark, glossy rachis (the central stalk of the leaf). These finger-like segments are not individual leaves, but parts of a single compound leaf. The "fingers" may be drooping or erect, depending on whether the individual fern grows in shade or sunlight. Spores are borne under false indusia (rolled flaps of tissue) at the edge of the subdivisions of the leaf, a characteristic unique to the genus ''Adiantum''. Until 1991, ''A. viridimontanum'' was grouped with the western maidenhair fern, ''A. aleuticum'', which grows both in western North America and as a disjunct on serpentine outcrops in eastern North America. At one time, ''A. aleuticum'' itself was classified as a variety (''A. pedatum'' var. ''aleuticum'') of the northern maidenhair fern, ''A. pedatum''. However, after several years of study, botanist Cathy Paris recognized that ''A. aleuticum'' was a distinct species, and that some of the specimens that had been attributed to that taxon (group of organisms) were a third, hybrid species intermediate between ''A. pedatum'' and ''A. aleuticum''. She named the new species ''A. viridimontanum'' for the site of its discovery in the Green Mountains in Vermont; it has since been located in Quebec and in one site on serpentine in coastal Maine. ''A. viridimontanum'' is difficult to distinguish from its parent species in the field. It can generally be separated from ''A. pedatum'' by the shape of the ultimate segments (the smallest divisions of the leaf), and by its habitat on thin, exposed serpentine soils rather than in rich woodlands. It more closely resembles ''A. aleuticum''; however, the stalks of the ultimate segments and the false indusia are longer and the spores larger. Due to its limited distribution and similarity to other ''Adiantum'' species within its range, little is known of its ecology. It thrives on sunny, disturbed areas where ultramafic rock is covered with thin soil, such as road cuts, talus slopes, and asbestos mines. Individual plants seem long-lived, and new individuals only infrequently reach maturity. It is one of four species endemic to serpentine in eastern North America and is considered globally threatened due to its habitat restrictions. ==Description== ''Adiantum viridimontanum'' is a medium-sized, deciduous, terrestrial fern, about wide and high. Its fronds range from in length from the base of the stem to the tip. Like many ferns, the frond of ''A. viridimontanum'' is divided into a series of leaflets, known as pinnae, and the pinnae are further divided into pinnules. The shape of the frond in ''A. pedatum'', ''A. aleuticum'', and ''A. viridimontanum'' is very similar. They are usually described as having a rachis that forks into two branches, which curve outwards and backwards. Several pinnae grow from the outer side of the curve of each rachis branch, with the longest pinnae located closest to the fork of the rachis. The fingerlike pinnae are pinnately divided into short-stalked pinnules. However, this interpretation of the frond architecture (pedately divided into pinnae, then pinnately divided into pinnules) presents a problem: no other species of ''Adiantum'', nor any other member of the Polypodiaceae ''sensu lato'' (the family in which ''Adiantum'' was once included) has a forking rachis. In fact, these species are not pedate, but pseudopedate. What appears to be a fork in the rachis is in fact the junction between the rachis and a basal pinna. That basal pinna makes up one of the two curving branches; the rachis runs straight up the first fingerlike segment on the other branch, while the remainder of that curving branch is made up of the other basal pinna. Both basal pinnae are further divided and subdivided to create the other fingerlike segments. Therefore, even though they appear structurally similar, the longest and most central fingerlike segment represents the tip of the frond, pinnately divided into pinnae (the first level of division of the frond), while the two shorter fingerlike segments immediately on either side of it are pinnae, pinnately divided into pinnules (the second level of division). Each fingerlike segment thereafter represents a level of division one greater than the one that precedes it. Therefore, the final, pinnate subdivisions of each fingerlike segment may be referred to as "ultimate segments" to avoid the technical inaccuracy of calling them pinnules. The rhizome shows little branching, with intervals of 4.0 to 7.5 mm between nodes. It measures 2.0 to 3.5 mm in diameter. The rhizome and the stipe (the part of the stem below the leaf) have bronze-colored scales. The stipe and rachis range from chestnut brown to dark purple in color and are glabrous; the stipe is about 2 to 3 mm in diameter while the rachis is smaller, 1 to 2 mm. The basal pinnae are from three to seven times pinnate (due to the pseudopedate structure of the blade), while the apical parts of the blade (and the corresponding segments of the basal pinnae) are once-pinnate. The penultimate segments of the blade (the apparent "pinnae", or fingerlike segments) are typically lanceolate in shape. The overall arrangement of the penultimate segments ranges from drooping and fan-shaped on plants growing in the shade to funnel-shaped on plants growing in full sun; under the latter conditions, the segments stand stiffly erect. The ultimate segments of the divided blade (the apparent "pinnules") are borne on short, dark stalks of 0.6 to 1.5 mm, with the dark color often spreading into the base of each segment. They are long and obliquely triangular, the basiscopic margin forming the hypotenuse. The tip of the segments is typically acute, but entire (not pointed). They measure from 9.5 to 22.5 mm in length and 4.2 to 7.5 mm in breadth, the average length being about 2.5 times the breadth. Their tissue is herbaceous (firmly leafy) to chartaceous (parchment-like) in texture, and bright green to bluish-green in color. As in other members of ''Adiantum'', the glabrous leaves shed water when young. Under shady conditions, the ultimate segments lie within the plane of the blade, but tend to twist out of the plane when grown in the sun. The acroscopic margins of these segments are lobed, with narrow (less than 1.0 mm) incisions lying between lobes. In fertile segments, these lobes are recurved to form false indusia beneath the leaf. These are transversely oblong, from 2 to 5 mm in length and from 0.6 to 1.4 mm in width. The sporangia (the fern's spore-bearing structures) are borne on the underside of the leaf beneath the false indusium, a trait found in all members of ''Adiantum'' and not in any species outside it. The sori are round, and are found on veins ending in the false indusium, below the veins' ends. The spores are tetrahedral to globose, yellow in color, and measure 41 to 58 micrometers (μm) in diameter (averaging 51.4 μm), on average larger than other species in the ''A. pedatum'' complex. Spores appear in the summer and fall. The species has a chromosome number of 116 in the sporophyte. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Adiantum viridimontanum」の詳細全文を読む スポンサード リンク
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