DEVELOPMENT OF PLANTS 245 



results two growing points which thus produce the equal forking 

 or dichotomous branching of the thallus, so characteristic of these 

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



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. 



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 

 remarkable series of transformations in the cells that are cut off 

 from the 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 



