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Phascum Hedwig, 1801.

Phascum cuspidatum is the only species of the genus in California. It is so small that it is seldom collected without sporophytes. When sporophytes are present, only Acaulon offers a source of confusion. Both these genera have erect leaves that so completely enclose the sporophyte as to hide most or all of it. Numerous differences between the two genera can be noted. The costae of Phascum are terete and several times as thick as the adjacent lamina, and the abaxial stereid band is obvious in a cross-section. In contrast, Acaulon has costae that are not much thicker than the leaf laminae, and stereid cells are usually absent in a section of the median leaf. Both genera have concave leaves with recurved margins, but the recurvature of Phascum often extends to near the leaf apex. Phascum has a strongly excurrent costa that, at least distally, appears to be keeled in the dry leaf. The laminal cells of the two genera differ markedly in that Phascum has predominantly quadrate cells arranged in regular longitudinal rows; those of Acaulon are predominantly rhomboidal, and the linear arrangement of those cells is not obvious. Phascum is typically distinguished from Acaulon by the papillosity of its leaves. That is a good distinction, but the uninitiated may misinterpret as papillosity the oil bodies that appear abundantly in the leaves of both genera. It is best to place an entire plant on a slide under the microscope so that the leaves can be inspected in profile thus revealing the papillae.
see key to Acaulon Etc.

Species included:
Phascum cuspidatum Hedwig

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