GROWTH. 45 



stituent cells also, so that various tissues fibrous, woody, 

 vascular, or epidermal are formed; and thus it comes 

 about that in tjie 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 different, 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 Boots. 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 growing 

 root at equal distances apart, say l-8th inch, and the 



