74 



FLAA'T LIFE. 



loss of the external layer of cells, from which the root- 

 hairs arose. The sloughing off of this layer of cells carries 

 with it the hairs themselves and exposes the next inner layer 

 of cells, which had before become slightly altered so as to be 

 rather impervious to water. Upon their exposure, this altera- 

 tion proceeds fin-ther, so that they become almost or ()uite 

 incapable of being penetrated by the soil-water to which they 

 may be exposed. It follows from this that it is only the 

 younger part of the root, that is, the portion which has not 

 undergone secondary changes, which is capable of absorbing 

 water. In many roots this is tlic only change which occurs. 

 In a greater number certain tissues become thick-walled, so 

 that the root is also strengthened. 



In a few instances, the root-cap is cast off from the tip. 

 This, however, only occurs when the growth in length of the 

 root is permanently stopped. 



86. 2. Internal secondary changes. — In a large luunber 



of roots, especially thos'; 

 ' ' ot dicotyledons and gym- 

 nospcrms, the secondary 

 changes result in increasing 

 the diameter, sometimes 

 very grcath'. Increase in 

 diameter comes about liy 

 tlie Ibrniation of concentric 

 layers of new tissue in two 

 or more regions. The new 



Fig. 87.— Transverse sfitidii .,f the periplierv of ,, '' 1 j ■ , 



the root of Cliisia, showing the formation of Cells are prOiJUCCd in each 

 periderm, t'c, cells of cortex ; «/, the super- ■ 1 i 



licial cells of the root (suberized) ; f,cr, the region b\ the resumption 

 periderm, its inner cells (opposite /t-r) acti\-ely ,- ■ i- ■ ■ 



dividing by tangential walls, its two outer layers, 01 actne (.l]"\]S10n in a layer 

 //,suberized,its innermostIayer,///,the phelhi- r ii 1 ■ 1 , , , ' 



derm. Highly magnified. —After Van Tieglieni, 01 CCllS Whlch had bceil 



tempoi-anly inactive. This region is then ( ailed the raiiihmm 

 or secondarv nu'iis/ciii (see •^ 77). The divisions ^^■hich ensue 

 in these cells are in tlie main iiarallcl to the surlace of 



COC^ 



