224 



XVII. THE ROOT APEX. 



root-cap; lastly, the plerome (pi), the termination of which is 

 not quite clear with a low power. We are inclined to imagine 

 that the upper part of the plerome is 

 bulkier than it really is, because the inner- 

 most layers of the periblem, bordering on 

 the plerome, are devoid of intercellular 

 spaces, and therefore (as is shown in the 

 figure) appear just as clear as the plerome 

 cylinder. In the oldest parts of the section 

 the plerome cylinder appears surrounded 

 by a layer of red cells, which, as comparison 

 with the cross-section shows, indicates the 

 endoderrnis filled with red cell-sap. As we 

 approach the apex these endodermic cells 

 become unrecognisable. Vascular trach- 

 eides (s) also appear in the older parts of 

 the plerome cylinder. The bright-looking 

 column (c) traverses the apex of the perib- 

 lem. Upon this impinge laterally the air- 

 containing periblemic layers, which extend, 

 however, completely neither to the plerome 



nor to the surface of the root. The last 

 FIG. 88. Longitudinal . 

 section through the root- is composed of large brown cells. 



T^r ,"a?t Branching of fioofe-The roots of Con- 



organised cells; pr, periblem; iferge will serve to make us acquainted with 

 pi, plerome ; e, endodermis ; 



*, spiral vessels; c, periblemic the method ot branching oi roots in general, 



column ; k, root-cap (x 26). j n tfae examina tion of the roots of Thuja 



occidentalism it will strike us that they bear their lateral roots in 

 four or in three vertical rows. We readily prove by cross-sections 

 that three rows of lateral roots indicate a triarch, i.e., with three 

 xylem rays, four rows a tetrarch, central cylinder. If we now 

 prepare a cross-section through a root at the place of insertion of a 

 lateral root, we shall see that the lateral root projects from one of 

 the xylem rays. Its point of origin, indeed, is in the pericycle, 

 opposite to a protoxylem group, and as the xylem rays run in 

 straight lines in the central cylinder, the arrangement of lateral 

 roots in straight rows is at once accounted for. 



Apical Cell of Pteris. We will now study one more type of 

 root, in which the growing apex elongates by means of an apical 

 cell. This apical cell is always a trilateral pyramid, and the 

 co-ordination of the segments formed from it is constant. We 



