222 XVII. THE BOOT APEX. 



calyptrogen are, in virtue of their origin, arranged in straight 

 rows ; at first thin, they soon increase in height. At the apex 

 of the root-cap they become rounded ; finally separate from one 

 another and become disorganised (r). It is a peculiarity of the 

 Gramineae that their dermatogen becomes strongly thickened on 

 the outer side (c). This thickened outer wall is glistening, swells 

 strongly, and appears so much the thicker the longer the section 

 lies in water. At the lateral limits of the cells we see highly 

 refractive striae pass more or less deeply into this thickened outer 

 wall. These are the primary radial walls of the cells ; and the 

 older they are, the more deeply they always penetrate into the 

 thickened wall. This wall shows distinct lamination. The periblem 

 has rapidly increased the number of its layers by means of peri- 

 clinal divisions. Between its inner cell layers small intercellular 

 (interstitial) passages filled with air very soon appear, as is repre- 

 sented in our figure by dark lines (e.g. at i). The periblem forms 

 the cortex, the innermost layer of which constitutes the endo- 

 dermis. The plerome ends conically in a group of initials ; two 

 such initials can be seen in the longitudinal section which we have 

 figured. The plerome forms the central cylinder, or stele. The 

 differentiation of the large central duct or vessel in this central 

 cylinder can be traced up to the group of initials. The cells from 

 which this duct will be constructed are distinguished by their 

 greater breadth (a). 



Root-apex of Gymnosperms. The roots of Gymnosperms 

 show, in many respects, a peculiar organisation in the meristem 

 of their growing apex. We will study more closely Thuja occi- 

 dentalis. The cross-section through a fully-developed root re- 

 sembles that of the root of Taxus baccata (the Yew), excepting 

 that the roots of Thuja are usually tetrarch, i.e., have four 

 primary ligneous rays, or bundles. The median longitudinal 

 section through the apex of the root can be examined first in 

 water, and then after treatment w r ith Eau de Javelle. It shows 

 a sharply-defined plerome cylinder, which terminates in a few 

 initial cells, and is surrounded by a covering of periblem, some 

 twelve to fourteen cell layers thick. This latter passes over the 

 apex, and forms there its terminal initial layers of eight to 

 ten inner rows of cells, while the outer rows pass over into 

 irregularly-arranged, comparatively large cells. These large 

 cells extend to the apex of the root-cap, where they ultimately 

 pass out of union, and become disorganised. The root-cap of 



