178 



PLANT MORPHOLOGY 



of the Anacrogynae are thallose, but some are leafy. They include 20 

 genera and about 500 species. The principal genera, all widely distrib- 

 uted, are Riccardia, Metzgeria, Pallavicinia, Symphyogyna, Pellia, and 

 Fossombrotna. The largest genus, Riccardia, has over 100 species. 



Gametophyte. The Anacrogynae have a dorsiventral gametophyte 

 that is thalloid in most forms but more or less leafy in others (Fig. 146). 

 While tending toward a diversity of form, the gametophyte has remained 

 structurally simple, displaying little or no internal differentiation. In 

 fact, it is composed of compact, almost uniform tissue. The gameto- 

 phyte is sometimes unbranched, but generally branches dichotomously 



Fig. 145. Fossombronia cristida. A, longitudinal section through apical region of thallus, 

 showing the apical cell, a young archegonium, and a mucilage hair, X500; B, horizontal 

 longitudinal section of apical cell, X600; C, vertical longitudinal section of apical cell, 

 X600. {After Haupt.) 



or, in some forms, by means of ventral adventitious shoots. Growth 

 takes place through the activity of an apical cell that is prevailingly 

 dolabrate (hatchet-shaped), cutting off segments on two sides, alternately 

 left and right (Fig. 145). Rhizoids, usually formed in abundance, are all 

 of the smooth- walled type. Ventral scales are rarely present. 



The gametophyte of Pellia is one of the simplest in the Bryophyta. It 

 consists of a thin, flat thallus, wavy along the margin, and with a broad 

 indistinct midrib gradually passing into lateral wings composed of a single 

 layer of cells (Fig. 146A). Some of the species of Riccardia have a similar 

 thallus, while others have one like that of Metzgeria, with a narrow dis- 

 tinct midrib sharply marked off from the wings. A distinct midrib is also 

 present in Pallavicinia and Symphyogyna, but in both of these genera some 

 species have an entire or wavy margin, others a margin that is deeply 

 lobed (Fig. 146jB, C). Fossombronia represents an advanced condition, 

 the wings of the thallus being dissected into two lateral rows of leaf-like 

 segments, the midrib forming a stem. This series leads directly into the 

 Acrogynae, where differentiation of the plant body into a leafy axis 

 reaches its highest expression. 



