74 



PLANT 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 further, so that they become almost or quite 

 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 the 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 number 



of roots, especially those 

 I ■ of dicotyledons and gym- 



jicr 



oosperms, the secondary 

 changes result in increasing 

 the diameter, sometimes 

 very greatly. Increase in 

 diameter comes about by 

 the formation of concentric 

 layers of new tissue in two 

 or more regions. The new 



Fig. 87. — Transverse section of the periphery of nfl ii „^^ ^„^j„^^j :„ ^„ u 



the root of ciusia, showing the formation of cells are produced in each 



periderm, ec, cells of cortex; a/>, the super- • i_ .1 



ficial cells of the root (suberized) ; /^r, the region by the resumption 



periderm, its inner cells (opposite per) actively r .• j • • • • 1 



dividing by tangential walls, its two outer layers, of active division in a layer 



//, suberized, its innermost layer, ph. the phello- r n i»_i_ 1 3 1 



derm. Highly magnified.— After Van Tieghem. Ot CellS Which had been 



temporarily inactive. This region is then called the cambium 

 or secondary merislem (see^f 77). The divisions which ensue 

 in these cells are in the main parallel to the surface of 



