104 ELEMENTS OF PLANT ANATOMY. 
genetic connection between the vascular cryptogams and the 
gymnosperms. Some botanists carried this so far as to claim 
that the meristem cells at the apex of all phanerogamic stems 
might be traced back to a single apical cell. This, however, 
has never been accomplished in a single instance, and the gen- 
erally accepted theory of the arrangement of the meristems is 
that known as Hanstein’s, to which reference has already been 
made. In order to show more clearly what is meant by this 
theory and what are the real facts known at the present day, 
two examples may be described, one taken from the vascular 
cryptogams, the other from the phanerogams. 
The manner of growth from a single apical cell is at first 
very simple. It divides, making two cells, one called a seg- 
ment which is added to the cells of the axis, and another which 
retains the form, character, and position of the original apical 
cell and again divides, cutting off a new segment cell, which 
also enters into the structure of the axis. The Equisetum stem 
furnishes an example of this class. Its apex consists of a three- 
sided pyramidal cell, which cuts off segments by walls parallel 
to the three sides of the pyramid. In the tissues of the grow- 
ing region of the stem apex which are derived from these seg- 
ments, there may be distinguished two classes of walls with 
reference to their direction. These are periclinal, or walls 
parallel with the surface of the stem; and anticlinal, or those 
at right angles to the surface and so to the periclinal walls. 
The outer layer of cells, which later forms the epidermal system, 
does not extend over the apex of the stem above the origin of 
the leaves. On the contrary periclinal walls are formed in the 
outer layer of that portion of the apex which is above the leaves. 
Within the outer layer there is no separation of the cells into 
different layers. It can only be said that from the cells lying 
near the central portion come the pith cells, while from those 
lying near the periphery come the epidermal layer and the zones 
containing the bundles, (Fig, 44.) 
