THE STEM AND BKANCHES. 181 



thickening matter, which consists for the most 

 part of carbon and hydrogen, is built up there 

 by the protoplasm of the cell itself : but as soon 

 as the process is quite complete, the protoplasm 

 emigrates from the cell entirely, and goes to 

 some other place where it is more urgently 

 needed. Thus wood is made up of dead cells, 

 whose walls are immensely thickened, but whose 

 living contents have migrated elsewhere. 



In large perennial stems, like those of oaks 

 and elms, a fresh ring of wood is added each 

 year outside the ring of the last growing season. 

 This new ring of wood is interposed between the 

 bark (of which I shall speak presently) and the 

 older wood of the core or heart, which was 

 similarly laid down when the tree was younger. 

 In this way, the number of rings, one inside 

 another, enables us roughly to estimate the age 

 of a tree when we cut it down ; though, strictly 

 speaking, we can only tell how many times 

 growth in its trunk was renewed or retarded. 

 Still, as a fair general test, the number of rings 

 in a trunk give us an approximate idea of the 

 age of the individual tree that produced it. 



The principle is only true, however, of the 

 great group of dicotyledonous trees, such as 

 beeches or ashes, as well as of the pines and 

 other conifers. In monocotyledonous trees, like 

 the palms and bamboos, the stem does not 

 increase in quite the same w r ay from within 

 outward, and there are therefore no rings of 

 annual growth to judge by. Palms rise from 

 the ground as big or nearly as big at the begin- 

 ning as they will ever be in the end ; and though 



