DIV. 



MORPHOLOGY 



81 



branched filaments of cells, and serve to absorb water. Many of 

 these plants can absorb water by their whole surface. 



Fio. 91. Riccia fluitans. 

 (Nat. size. After SCHENCK.) 



" 



FIG. 92. Blasia pustila. r, rhizoid. 

 (x 2. After SCHKXCK.) 



When the thallus lies on the substratum it is usually dorsiventral 

 as in Lichens, and frequently has abundant chlorophyll only on the 

 upper side exposed to the light (Fig. 95). In such cases the rhizoids 

 are confined to the lower surface. 



In the Bryophyta, which are all rnulticellular, 

 the summit of the apical growing point is fre- 

 quently occupied by a single apical cell. 



In ribbon -shaped Liverworts, such as Metzgeria and 

 Aneura, as in some similarly-shaped Algae, the apical 

 cell is wedge-shaped (Fig. 94), and cuts off segments in 

 two or sometimes four rows. The segments in the former 

 case are cut off by oblique walls inclined alternately to the 

 right and left ; the four-sided apical cell in addition cuts 

 off segments above and below. By further division the 

 segments give rise to the body of the plant. The apparently 

 dichotomous branching of Liverworts with growing points 

 of this type can be traced back to the early delimitation 

 of a new apical cell in the acroscopic half of a young seg- 

 ment (Fig. 94 at b}. In the erect radially -constructed 

 thalli of the Mosses the apical cell has the form of a 

 three-sided pointed pyramid, and cuts off three rows of 

 segments. The young leaves of the Mosses grow at first by 

 a two-sided apical cell, but later have intercalary growth. 



FIG. 93. Plagiochila asple- 



The permanent tissues reach a higher level of nimdes with leaves over- 

 differentiation than in the Algae. This is con- 

 nected with the difficulties which the life on land 

 of the Bryophyta introduces. There is only 

 rarely a definite epidermis, though the superficial cells are covered by 

 a kind of cuticle. On the thallus of the Marchantiaceae, however, 

 an external layer of cells is clearly marked off from the underlying 



G 



lapping like the laths of 

 a Venetian blind. (Nat. 

 size. After SCHENCK.) 



