ANATOMY OF TISSUES. 87 
but may live on through the life of the plants arising from it, 
in which case part of it becomes rhizoid-like in character ; and 
in some few instances it continues to live and remains the most 
important part of the plant. 
The moss stem, which develops from this organ, grows, in 
almost all cases, by means of a tetrahedral apical cell, the outer 
or free wall of which is parallel to the substratum. The new 
walls formed in this cell generally follow the law before men- 
tioned, that is, each one is parallel to the third last before it, 
while in each segment cut off in this way division takes place 
by the formation of several new walls. The inner cells so 
formed go to make up the stem, while the outermost one 
becomes a wedge-shaped, meristem, or apical cell of a leaf. 
The leaf, however, develops by apical growth for a short time 
only, the apical cell soon ceases to divide, and intercalary 
growth near the base of the leaf produces the greater part of its 
tissue. The stem becomes a perfectly radial organ with lateral 
appendages or leaves. From its lower part the outer cells grow 
out into rhizoids which have the function of roots. The order 
of branching is seldom regular. In the genus Sphagnum there 
is one branch for every four leaves, but in the majority of cases 
there is no apparent law for the order or position of the 
branches ; they cannot be called axillary, as they not unfre- 
quently occur between the leaves. 
The tissues of the stem are more simple in character than 
those of the thallus of the hepatic. There is no distinct epi- 
dermal system, but on the other hand, there is in some of the 
higher forms a decided separation into fibrous and ground 
systems. The central portion of such stem consists of a single 
strand of thin-walled, long, cambium-lke cells, with short radial 
and tangential diameters. In one genus, Polytrichum, there is 
a number of these cambium-like strands. From these cells 
there is a more or less gradual change into the parenchymatic, 
thicker-walled cells forming the several layers of the rind. 
