GROWTH. 47 



comes about that in the fully developed root the internal 

 structure and the arrangement of the tissues are different 

 in the great majority of cases from those of the stem, 

 those of the stem from those of the leaf, and so on dif- 

 ferent, that is to say, in so far as the arrangement of the 

 elementary cells and tissues go, rather than as far as the 

 cells themselves and their modifications are concerned. 

 But while there is this difference in the structure of the 

 adult leaf, stem, and root respectively, all the time these 

 organs retain their active faculty of growth there remains 

 a portion of the cellular tissue in its original unmodified 

 condition the cells ready to divide and multiply and so 

 bring about the growth of the organ. This portion is 

 called the " cambium "or " meristem." So far as growth 

 in length is concerned, there are certain special points 

 where subdivision of cells is most active. These are 

 called the "growing points." At these places the cells 

 divide rapidly, each cell remaining small, and not, as 

 elsewhere, greatly extending its size by interstitial 

 growth. 



Growth of Roots. The growing point of a root, so 

 far as its length is concerned, is comprised within a 

 small area just above the extreme tip, the extreme tip 

 itself being, as previously stated, covered by a little cap 

 shed off from the skin of the root and serving as a shield 

 to it in its progress through the soil. 



That the growth in length takes place over a very 

 small area adjacent to the tip of the root is proved by a 

 very simple observation. If marks be made on the grow- 

 ing root at equal distances apart, say one-eighth inch, 

 and the progress of growth be watched from day to day, 

 then it will be found that while the uppermost marks 

 remain equi-distant, those near the tip become more or 

 less widely separated. This experiment is easily carried 

 out with a hyacinth growing in a glass vase, or by allow- 



