BRYOPHYTA 183 



tiales and Sphaerocarpales, become four-lobed just previous to the forma- 

 tion of tetrads (Fig. 149^, F). The capsule wall is two or more layers of 

 cells in thickness, or only one layer by resorption of the inner layer at 

 maturity. Annular thickenings are generally present on one or both 

 layers, but are constantly absent in a relatively few forms, such as Pal- 

 lavicinia and Symphyogyna. Dehiscence of the capsule is nearly always 

 effected by splitting into four valves. 



2. ACROGYNAE 



In the "acrogynous" Jungermanniales archegonia may arise from seg- 

 ments of the apical cell, but sooner or later the apical cell itself becomes 

 an archegonium. The terminal position of the archegonia and sporo- 

 phytes is in marked contrast to their dorsal position in the Anacrogynae. 

 Practically all members of the group are leafy. The Acrogynae are a 

 well-defined assemblage, comprising 130 genera and about 5,000 species. 

 Notwithstanding its size, the group is fairly uniform in regard to general 

 morphological features. Some of the largest genera are Nardia, Plagio- 

 chila, Lophocolea, Radula, Porella, Frullania, Cephalozia, Scapania, and 

 Lejeunia. 



Gametophyte. The gametophyte of the Acrogynae is a dorsiventral, 

 branching, leafy axis. Only a few genera are thalloid and even these 

 produce leafy fertile shoots. Herherta and a few other genera have an 

 erect stem bearing three rows of similar, radially arranged leaves. The 

 other Acrogynae have a prostrate stem bearing two rows of dorsal leaves 

 and generally a row of ventral leaves (Fig. 151A, B). The ventral leaves, 

 which are reduced, are known as amphigastria. In a number of genera 

 amphigastria are not present. The dorsal leaves overlap and are gen- 

 erally bilobed, the lobes being unequal in size or, in a few cases (e.g., 

 Lophocolea), equal. The leaves nearly always consist of a single plate of 

 cells and, with rare exceptions, are without a midrib. The stems are 

 composed of essentially uniform tissue. Rhizoids are usually abundant 

 on the lower side of the stem. They are chiefly anchoring in function, as 

 much water absorption takes place directly through the leaves. 



In practically all the Acrogynae the gametophyte grows by means of a 

 tetrahedral apical cell (Fig. 15 IC). This has the form of a triangular 

 pyramid, cutting off segments on three sides. The two rows of dorsal 

 segments give rise, in part, to the dorsal leaves and the single row of ven- 

 tral segments, in part, to the amphigastria. Branching in the Acrogynae 

 is varied but most commonly is monopodial, with a main axis and lateral 

 branches. Vegetative propagation is well developed. Often branches 

 break off and give rise to new plants. One-celled or two-celled gemmae 

 are frequently borne on the margins or at the apices of leaves, while 

 multicellular discoid gemmae are produced in some forms. 



