272 STRUCTURE OF RICCIOCARPUS 



or dichotomous branching of the thallus, so characteristic of 

 these plants (Fig. 179, A, B). The appearance of many of these 

 hepatics is suggestive of the algae. Especially is this true of the 

 aquatic Ricciocarpus and Riccia. 



(a) Structure of Ricciocarpus. An examination of the struc- 

 ture of one of these will show, however, that extensive changes 

 have been induced in even the simplest forms. The new stimuli 

 to which the terrestrial conditions expose them cause a remark- 

 able series of transformations in the cells that are cut off from the 



c 



FIG. 179. Forms of semiaquatic Marchantiales : A, Riccia, showing the 

 dichotomous branching of the thallus. B, Ricciocarpus. The sexual or- 

 gans are concealed in furrows that appear as radiating lines in the center 

 of the branches. C, diagram of a cross-section of a branch, showing the 

 male gametangia in the bottom of one of the furrows. 



apical cell. The upper cells of the thallus, as soon as they are 

 formed at the growing point, are exposed directly to the air and 

 light, and they develop chlorophyll and grow up into vertical 

 rows or plates just as you have already noticed in the palisade 

 chlorenchyma of the leaf (Fig. 180, A). At an early period these 

 rows of green cells become separated so that air spaces arise 

 between them, and thus the chlorophyll-bearing cells are brought 

 into direct contact with the atmosphere and enabled to carry on 

 photosynthesis to the best advantage. The terminal cells of 

 these rows enlarge considerably and form a rudimentary epider- 

 mis. It is also interesting to note that the direct contact of the 

 cells with the atmosphere results for the first time in the forma- 



