GROWTH, AY 
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- 
