INTRODUCTION. 



II 



3. (Fig. 4), Plerome-cylinder and periblem sharply defined, the latter overlying 

 the apex of the plerome, and covered by a common initial layer for dermatogen 

 and root-cap. The divisions of the initial layer, which are parallel to the surface of 

 the bluntly conical apex, add on the one hand new cells to the root-cap above the 

 apex, and on the other hand renew the initial layer itself. As the distance increases 

 from the apex of the periblem, which by its growth in length is constantly advancing, 

 the divisions become rarer, arid at last cease. The last of these separates the 

 initial cell into two, one of which is added to the root-cap, the other to the 

 dermatogen as a permanent member of it. We may therefore say with Janczewski 

 that root-cap and dermatogen arise in this case from the calyptrogen-layer. The 

 cells of the dermatogen and root-cap, which owe their origin to the division just 

 described, divide further by walls perpendicular to the surface ; from each therefore 

 is produced a section of a layer 

 consisting of several or many 

 cells. In the root-cap each of 

 these sections is so arranged 

 relatively to the similar ones la- 

 terally next it, and to others 

 which have arisen above the 

 apex, as to form a conical hood 

 one layer of cells thick ] and the 

 whole root-cap is built up of such 

 hoods fitting one into another. 

 The cells of the sections of the 

 dermatogen undergo extension 

 perpendicularly to the surface, 

 in such a measure that each sec- 

 tion remains for a time consider- 

 ably less extended in that direc- 

 tion than its predecessor, which 

 is farther from the apex. The 

 surface of the dermatogen layer 

 therefore becomes narrower, as 

 the apex is approached, by steps 

 which occur at definite distances from it. Each step is covered by that section of a 

 layer of root-cap which originally corresponded to it, while the edge of the latter 

 abuts on the next lower, that is, the next older step. 



A not unimportant variety occurs, according to Janczewski, within the type of 

 differentiation in question. In the majority of plants which have been investigated, 

 the periblem consists at its apex of a single initial cell (Fig. 4) or of two such, 

 which lie side by side in one layer ; it is below the apex that it first increases to 

 several layers. But in one case, namely; Linum usitatissimum, the apex of the 

 periblem consists of two initial layers. One of these, the inner or lower of the two, 

 behaves as in the first case just described. The other, the outer one, belongs to a 

 layer of cells, which clothes the whole periblem. This, like the dermatogen, divides 

 only perpendicular to the surface, and therefore always remains a single layer. 



Fig. 4.— (210) Polygonum Fagopyrunt. Apex of root in median longitudinal 

 section, /c pericambiurn, the outer boundary of tiie plerome-cylinder; *; der- 

 matogen or epidermis, between^t and t periblem, h root-cap. 



