HISTORY OF THE PLANT KINGDOM 305 
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 Coleochete (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 Ricciocarpus (Fig.189), 
and filaments, known as rhizoids, serve to 
absorb water for use in nutrition. Higher 
liverworts, such as Marchantia (Figs. 190— 
194), show more division of labor among 
the cells of the thallus, and such liver- 
worts as Frullania (Figs. 195, 196) have 
rhizoids, stems, and two kinds of leaves 
besides curious pitcher-like appendages. 
387. Division of Labor among Vascular 
Plants. — Ascending along the succession 
Fie. 219. Ulothria, 
a Green Alga. 
(x 300.) 
rT, rhizoid cell, which 
serves as a hold- 
ast. 
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 
