44 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
formed dividing subsequently by both horizontal and 
longitudinal partitions (as in Sphacelaria and many other 
algee). More often, we find that the apical cell is a three 
sided pyramid, the convex base of the pyramid being 
the apex of the shoot. Successive cells are cut off from 
the three sides and the segments 
thus produced divide by various 
partitions so as to produce the mass 
of meristem cells from which the tis- 
Tre. 18.—Apieal cells of SUCS become differentiated. Some- 
a jeaweed (Sphacelaria), times, instead of the apical cell 
cutting off three rows of segments, 
it produces only two or in other cases, four. 
58. In most of the Flowering Plants, a group of cells 
VY 
is found at the apex of the stem or root instead of one — 
cell, these giving rise, by their division, to the mass of 
meristem. This group of apical cells, or the single apical 
cell with the cells derived from it, is called the growing 
point. 
59. We can usually distinguish three different tissue 
regions at or a short distance back from the growing 
point of higher plants. At the outside we find a single 
layer, the epidermis, which consists of cells that divide 
only by walls perpendicular to the surface. When this 
layer has an initial cell or cells distinct from the inner 
layers the portion near the apex is often spoken of as 
the dermatogen. The next region is spoken of as peri- 
blem, and may consist of one or several layers of cells 
surrounding the centrally located plerome. These two 
regions may have separate sets of apical cells or the dis- 
tinction may occur only some distance from the apex. 
In most roots, the apex is covered by the root cap, a 
mass of cells produced by the periclinal division (i.e. 
by walls parallel to the surface) of a layer of cells outside 
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