162 



PLANT MORPHOLOGY 



Gametophyte. The Marchantiales are characterized by a flat, dorsi- 

 ventral, thalloid gametophyte — with few exceptions ribbon-hke and 

 nearly always rather fleshy. It branches either dichotomously from the 

 apex or, less commonly, by means of adventitious outgrowths arising 

 apically or ventrally. In Riccia the thallus is small and, as a result of 

 repeated dichotomy, often grows in the form of a fan or rosette (Fig. 128). 

 In all the Marchantiales growth takes place by means of an apical cell 

 situated in an apical notch. It is of the cuneate (wedge-shaped) type, 



Fig. 128. Dorsal view of the gametophyte of Riccia nutans, showing sporophytes in the 

 grooves and scales arising from the ventral surface, X 3. 



Fig. 129. Reboulia hemisphaerica. A, longitudinal section of portion of growing region 

 of thallus with apical cell and developing air chambers, X 160; B, portion of upper region of 

 thallus, showing air pore and air chambers, X85. 



cutting off segments on four sides — above and below as well as left and 

 right (Fig. 129A). 



The gametophyte is of simple external form but exhibits a high degree 

 of internal differentiation, nearly always consisting of (1) an upper 

 epidermal layer; (2) a loose, green, dorsal region having one or more 

 layers of air chambers; (3) a compact, colorless, ventral region. The 

 epidermis, usually colorless or pale green and often with slightly thickened 

 walls, nearly always contains numerous air pores that communicate with 

 the air chambers. Air pores and air chambers are not developed in a 

 few genera {e.g., Dumortiera and Monoclea), their absence being a result of 

 reduction. 



