In the genus Polystichum, the fronds spring in tufts from a short slow-growing caudex. They are rigid, linear or lanceolate, and either pinnate, bipinnate, or tripinnate. The upper terminations of the fronds are sharp and spinous, while the pinnæ are auriculate at their base above, and oblique below. Circular sori are seated on the anterior branches of the parallel veins, and these are covered by circular indusia, opening all round and remaining attached by a short central stalk. The free venation and this peculiar form of the sorus and indusium are the distinguishing characters of the genus. Fig. 58 shows a sorus and indusium of Polystichum or Aspidium, which differ only in venation. One of the spinous, serrated pinnæ of the Holly fern, Polystichum Lonchitis, is represented in fig. 59; it is auriculate at its base above and oblique below, with its sori disposed in regular series on each side the mid-vein; these often become confluent at maturity. This beautiful Alpine fern, which is of a deep glossy green, has linear lanceolate, and simply pinnate fronds. The pinnæ are short, arranged alternately and obliquely on the rachis, and extend nearly to its base, which is rather densely clothed with reddish-brown chaffy scales. The other British ferns of this genus are bipinnate. P. aculeatum is rigid, but P. angulare is lax, and drooping. One of its varieties, named proliferum, is abundantly viviparous, producing small bulbils about the bases of the lower pinnæ and pinnules, which readily reproduce the plant. [Illustration: Fig. 59. Pinna of Polystichum Lonchitis.] All the preceding genera, from Lastrea inclusive, belong to the Aspidieæ, an enormous tribe, abounding in species, and almost conterminous with the old genus Aspidium. The Nephrolepis tuberosa and other species bear sub-translucent tubers on the rhizome. They are subterranean, ovoid, an inch and a half long, and filled with a nearly translucent mucus. The tubers have a circle of vascular bundles forming a sort of balloon, proceeding from a common base below, and converging to the apex.[73] Most of the species are tropical, but Lastrea and Polystichum include several European species, some of which are extremely variable. The Onoclea is a remarkable genus from the pinnæ being contracted into berry-like globes. Onoclea sensibilis is a handsome free-growing American species and appears to have been named sensibilis, from the particularly rapid withering of the fronds after being gathered.