The Adiantieæ form an exceedingly numerous and mostly tropical group of ferns, in which the rhizome is creeping or globose. Their sori are linear or oblong, straight or curved, and growing on the under-side of the edges of the leaf, which are folded over with them, and thus serve as an indusium. Adiantum has a free venation, but Hewardia, a related genus, has a reticulated venation. The Adiantieæ have linear, oblong or lunate sori, seated on the margin of the leaf. Of the seventy species of Adiantum, only A. pedatum and A. Capillus-Veneris, the first American, the latter British, are inhabitants of a cold climate. The A. Capillus-Veneris, which is universally distributed in warm latitudes, is believed to have migrated in ancient times to the mild and damp south-western counties of England and Ireland. The Adiantum Capillus-Veneris, or Maiden’s Hair, has a slender, black, scaly, creeping, and branching rhizome, from the extremities of which spring lax tufts of fronds growing from a few inches to a foot high. The stems with their alternate branches and branchlets are slender, hair-like, and of a blackish purple tint. The capillary branchlets bear at their extremities thin, bright, but glaucous green, wedge-shaped leaves, serrated at their edges. The fibro-vascular bundles which traverse the stems and branches, on reaching the leaves, spread into a palmate venation, which terminates at the margin in bifurcations; and upon these, in the fertile leaves, roundish sori are placed, and covered by the transverse oblong folds of the edge of the leaf, as in fig. 67. These delicately graceful ferns flourish almost exclusively in damp shady crevices of rocks under the spray of cascades, and still more luxuriantly in deep tropical forests, where the air is loaded with warm vapour. [Illustration: Fig. 67. Adiantum Capillus-Veneris.] The Acrosticheæ are among the most remarkable of ferns in having the whole under-surface of the frond indiscriminately covered with naked sori, and in a few species the upper-surface also. They are situated on the veins and veinlets, from whence they extend into the interstices of the fertile fronds, which are sometimes much contracted. The species are almost all tropical or subtropical. A New Zealand species of Lomariopsis climbs high trees by means of its stout rooting caudex, and has different leaves on different parts of the plant.