52 



PLANT LIFE. 



produce the whole thallus. The center of the thalhis is 

 generally thicker than the wings, and forms a sort of central 

 rib {B, fig. 60). 



60. The shoot. — In the greater number of liverworts the 

 mature vegetative body is a shoot, which is differentiated 



Fig. 61. 



Fig. 60. — Surface view of growing apex of tliallus of RIetzgeria /iircata just after 

 forking. «, primary apical cell; 1^, secondary apical cell of branch ; t-, the wing- 

 tissue between a.xis and branch outgrowing the apices. j5, the midrib. Magnified 

 160 diam. — After Kny. 



Fig. 61. — Diagram showing origin of branch in li/etzgeria /incala. a, primary 

 apical cell from which the segments right and left bounded by hea\'y lines have 

 been cutoff. All have undergone further division. In the right-hand one the latest 

 cell-walls have been so placed as to form a wedge-shaped cell, l\ which becomes 

 the apical cell of a branch. Its early formation gives the (false) appearance of 

 dichotomy. — After Kny. 



into stem and leaves (figs. 62, 63). Even in such a body 

 the dorsiventral character is well marked. The stem is a 

 filiform a.xis of uniform cells, bearing three ( rarely more or 

 fewer) rows of leaves, of which the two dorsal rows 

 are the larger, while the under leaves are much smaller, even 

 to being inconspicuous or wanting. These leaves are super- 

 ficial aggregates, consisting of uniform cells richly supplied 

 with chloroplasts, as are also the outer cells of the stem. 

 Their form is very \'aried and often of great beauty. They 

 are always sessile and are usually crowded so closely as to 

 overlap each other more or less, and hide the axis com- 

 pletely (fig. 63). 



