STRUCTURE AND GROWTH OF ROOTS 129 



43. Structure and Growth of Roots. — Plants grow be- 

 cause their cells grow. New cells are formed, but the 

 formation of new cells does not alone cause growth. One 

 cell may divide into two, but the two together are at first 

 no larger than their parent was. Gradually they enlarge. 

 It is this cell enlargement rather than cell division which 

 causes growth. 



All living cells may enlarge, but not all living cells ap- 

 pear to have the power to divide. Comparatively few of 

 the cells of a plant appear to have this power. Tissue 

 composed of cells having this power is called meristem. 



So in considering the growth of roots we are interested 

 to find two things: (1) Where are those cells of the root 

 which have the power to divide ? (2) In what part of the 

 root does cell enlargement principally occur? 



(1) The meristem of roots we find in two places. We 

 find it in the center of the young root just behind the tip. 

 (See Figure 42.) We also find it in the older parts of the 

 roots near the edge of the stele. The meristem at the 

 tip forms those cells whose enlargement causes growth in 

 length and some growth in thickness. The meristem in 

 older parts forms cells whose enlargement causes growth 

 in thickness only. The growth resulting from the activity 

 of meristem in the older parts of plants is called secondary 

 growth; that resulting from the activity of meristem at 

 growing tips is called primary growth. In long-lived plants 

 secondary growth goes on year after year. As a result, the 

 wood of long-lived roots appears to be formed in layers, 

 like the wood of long-lived stems. A meristem which 

 causes secondary growth is called cambium. Cambium 

 occurs in stems more prominently than in roots, and in 

 connection with stems we shall study it in more detail. 



