166 THE STORY OF THE PLANTS. 



wood. And what is wood ? Well, it is an ex- 

 tremely hard and close-grained tissue, manufac- 

 tured by the plant out of its ordinary cells by a 

 deposit on their walls of thickening matter. This 

 process of thickening goes on in each cell until 

 the hollow of the centre is almost entirely filled 

 up by the thickening material, leaving only a 

 small vacant space in the very middle. The 

 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 con- 

 tents 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 simi- 

 larly 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 indi- 

 vidual 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 coni- 

 fers. In monocotyledonous trees, like the palms and 



