30.] 



CHAPTER II. THE TISSUES. 



149 



but are interrupted at the apex by the large apical cell. The 

 apical cell is, in fact, the initial cell for all the tissue-systems. 

 The apical cell undergoes division by walls formed parallel to 

 each of its (two or three) flanks successively, the segments thus 

 formed growing and dividing to form the tissues of the stem or 

 root. In tbe root the apical cell also undergoes divisions parallel 

 to its curved base. After the cutting off' of a segment the apical 

 cell grows to its previous size, so that the repeated segmentation 

 does not diminish the bulk of the apical cell. 



The most important morphological fact in connection with the 

 subsequent division of the 

 segments cut off from the 

 flanks of the apical cell is 

 that each such segment 

 is divided by a periclinal 

 wall into an external and 

 an internal half j from the 

 external half are developed 

 cells belonging to the cor- 

 tical ground-tissue, the 

 outermost layer of which 

 eventually acquires the 

 characters of an epider- 

 mis, though it is not the 

 morphological equivalent 

 of the epidermis of the 

 stem of Phanerogams, but 

 is only the external layer 

 of the cortex, being of 

 common origin with the 

 other cortical layers ; from 

 the internal half of the 

 segment is developed a portion of one or more plerome-strands, 

 and a certain amount of internal ground-tissue as well. 



The segments cut off parallel to the base of the apical cell of 

 the root (Fig. 114 c' ; Fig. 115 k) represent a true dermatogen, a 

 structure, which, as mentioned above, is wanting in the stem. 

 These dermatogen- segments grow and divide both anticlinally and 

 periclinally to form the root-cap (Fig. 114 c c ; Fig. 115 & n ) ; but 

 this many-layered epidermis only persists at the growing-point, 

 since it becomes entirely exfoliated as the parts grow older. 



FIG. 114. Diagram of growing-point of a Fern- 

 root. A A Anticlinal walls ; P P periclinal walls ; 

 s apical cell ; b b segments cut off from the flanks 

 of the apical cell; c' segment cut off the base 

 of the apical cell ; c c cells derived from previous 

 segments cut off from the base of the apical cell, 

 now forming the root-cap. (After Bower.) 



