132 



BOTANY 



PART I 



The growing points of the roots of Phanerogams, on the other hand, 

 have no apical cells. They consist of equivalent meristematic cells 

 that are frequently arranged in regular layers. 



The apex of a root of one of the Gramineae (Fig. 157) may be described as an 

 example. The stratified meristem, from which the permanent tissue of the root 

 arises, is separated into an outer layer of cells, the DERMATOGEN (d) ; a central 

 region formed of several layers which gives rise to the central cylinder of the root 

 and is called the PLEROME (pi) ; and into a number of layers between the derma- 



Fio. 156. Median longitudinal section of the apex of a root of Pteris cretica. 

 t, Apical cell ; A-, initial cell of root-cap ; k", root-cap, (x 240. After STRASBUROER.) 



togen and plerome which form the PERIBLEM. The dermatogen (d) and periblem 

 (pr) unite at the apex in a single cell-layer, outside of which lies the CALYPTROGEN 

 (k) or layer of cells from which the root-cap takes its origin. 



In many other roots, however (in the majority of Dicotyledons), the formation 

 of the root-cap results from the periclinal division of the dermatogen itself, which, 

 in that case, remains distinct from the periblem. In Gymnosperms, and in many 

 Leguminosae, the dermatogen, periblem, and calyptrogen are not marked out as 

 distinct regions. In roots, the plerome cylinder (pi) almost always terminates in 

 special initial cells. 



2. External Features of the Root. Behind the growing point 



