APEX OF THE ROOT. 103 



2. That these rows of cells converge towards a point 

 at some distance below the external apex of the root. 

 This is the punctum vegetationis. 



3. Note the Procambium-cylinder, or formative 

 tissue of the vascular bundles, which pursues a longi- 

 tudinal course up the centre of the root. 



Examine with' a high power : and observe that 



1. At some distance from the apex a definite layer of 

 Epidermis covers the root externally. Follow this to- 

 wards the apex : at some short distance from it this single 

 layer splits into two : the inner is the Dermatogen, or 

 formative layer of the epidermis : the outer is the outer- 

 most layer of the Calyptra, or root-cap. Following the 

 dermatogen further inwards, it will be seen to split again 

 several times in succession : the dermatogen may be 

 traced as a continuous layer covering the inner tissues. 

 The layers thus thrown off externally from the derma- 

 togen form collectively the Root-cap, or Calyptra. 

 We have in this case a common formative layer for 

 epidermis and root-cap (cf. root of Maize, p. 120.). 



2. Between the procambium and epidermis lies a 

 broad band of formative tissue of the cortex, or 

 Periblem : follow this to the punctum vegetationis : 

 it is also a distinct continuous band, though reduced to 

 a single layer of cells at the apex. 



3. The Plerome, or central procambium cylinder, 

 may also be traced as distinct up to the apical 

 point. 



This type of arrangement of tissues of the meristem 

 may then be expressed thus : 



Calyptrogen \ a single layer of cells, i.e. epidermis and 

 Dermatogen j root-cap have a common origin. 



