44 GKOUPS OP TISSUES, OR TISSUE SYSTEMS 



formed dividing subsequently by both horizontal and 

 longitudinal partitions (as in Sphacelaria and many other 

 algse). 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- 

 „ , . . , „ , sues become differentiated. Some- 



FiQ. 18. — Apical cells of _ .. 



a seaweed (Sphacelaria), timeS, instead of the apiCal Cell 

 and a moss. 



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 

 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 



