HISTORY OF THE PLANT KINGDOM 



305 



te 

 \m\ 



constitutes the individual, and this cell must do all the 

 kinds of work of which the plant is capable ; it must pho- 

 tosynthesize, assimilate, excrete, grow, and divide to form 

 new cells. There is no setting apart of 

 one kind of cell for vegetative work and 

 another kind for reproduction. 



Progress toward higher types of plant 

 life is marked by the formation of rows 

 of cells with a specialized holdfast, as 

 in Ulothrix (Fig. 219). Another step for- 

 ward is marked by the appearance of 

 cell-division in two directions, so that the 

 thallus increases in length and breadth as 

 in Coleochcete (Fig. 220). 



Differentiation of a thallus into epider- 

 mis and spongy parenchyma cells within 

 is well shown in some of the simplest 

 liverworts, such as Hicciocarpus (Fig. 189), 

 and filaments, known as rhizoids, serve to 

 absorb water for use in nutrition. Higher 

 liverworts, such as Mar chantia (Figs. 190- 

 194), show more division of labor among 

 the cells of the thallus, and such liver- 

 worts as Frullania (Figs. 195, 19G) have 

 rhizoids, stems, and two kinds of leaves 

 besides curious pitcher-like appendages. 



387. Division of Labor among Vascular 

 Plants. — Ascending along the succession 

 of classes until those of the pteridophytes or vascular spore- 

 plants are reached, we find at this point a marked advance 

 in specialization of parts. Ferns, for example, are the first 

 forms represented in the scheme of classification (p. 210) to 



Fig. 219. Ulothrix, 

 * Green Alga. 

 (X 300.)- 



r, rhizoid cell, which 

 serves as a hold- 

 fast. 



