82 ELEMENTS OF PLANT ANATOMY. 
families. The single cell has one outer surface wall and either 
two or three inner walls surrounded by the cells of the growing 
end. Where there are two inner walls, the new 
walls arise alternately, first on one, and then on the 
: ¢ other side of the cell and parallel with the existing 
inner walls. Where the outer or surface wall is 
three-sided, the other three walls are enclosed in the 
Fig. 39. growing end, making a triangular pyramid, and the 
new partition walls arise so that each is parallel with the third 
before the last, therefore at an angle of 120° with the last wall. 
From the unequal rapidity of growth 
of the segments, the meristem cell or cells 
lie in a sinus at the growing end of the 
thalloid forms. The normal branching 
takes place in this sinus, the last segment a 
cut off becomes a new apical cell and ae 
by its rapid growth pushes aside the ee 
original one and thus a dichotomous divi- 
sion is brought about. The simplest forms 
show no other differentiation of tissues than the possession of 
Fic. 40. 
a midrib of several layers, while the thallus proper is composed 
of only one layer of cells. In the higher forms, there is an 
epidermis with organs corresponding to the stomata of higher 
plants. The thallus of the higher forms consists of several lay- 
ers of cells composing different sets of tissues, which resemble 
those of similar functign in higher plants. There is a distinct 
epidermis which covers both sides of the thallus, but its strue- 
ture on the upper, is unlike that on the under side. Numerous 
epidermal cells of the under surface are extended into rhizoids, 
which fasten the plant to the soil and furnish it, in part, with 
food. On the upper surface the connection between the epider- 
mis and the tissues below is interrupted in such a manner as to 
leave regular air spaces between them. <A single stoma opens 
into each of these spaces, which are partly filled with assimi- 
