GROWTH IN LENGTH 77 



same distance apart, and will do so however long the 

 observations are continued. There are some trees (see 

 section 3) on the surface of which the leaf-scars persist 

 for many years, and we may, by direct comparison of 

 the old stems and young branches, see that the leaf- 

 scars are on the whole the same distance apart, and 

 therefore that there has been no longitudinal growth. 



This is an observation which you should not fail to 

 make for it is quite easy, and it is a clear proof of a 

 very important principle that a tree does not grow, as 

 is sometimes supposed, by a general lengthening of 

 the trunk and branches, nor by being pushed up out 

 of the ground from the roots, but only by the addition 

 of new pieces to the ends of the stem and branches 

 above. So that if a branch meets the stem at a 

 height, say of ten feet from the ground one year, 

 it will be at the same level the next year, and the 

 next, and for a hundred years if it lasts so long. It is 

 only the height of a tree as a whole that increases each 

 year, and it does so as a rule till death or accident 

 puts a stop to further growth. 



This mode of growth constitutes an important point 

 of difference between animals and plants, for animals 

 grow by enlargement of every part and organ, not 

 by addition to the ends of the trunk and limbs. 



2. Again, the branch or stem of an ordinary tree 

 consists of a hard woody core surrounded by a tough 

 bark, which is soft or hard and stringy on its inside, 

 and often rough and broken towards the exterior. 

 These two parts, the woody core and the surrounding 

 bark are entirely different in nature. Wood never 

 turns into bark, rior does bark turn into wood ; and 



