260 TIMBERS AND THEIR USES 



held in reserve in the cells of the medullary 

 rays. 



Let us, for the sake of greater clearness, 

 examine a stem in its early stages of develop- 

 ment, and trace the changes which take place 

 as the stem becomes older. The initial stages 

 of growth may best be studied at the growing 

 point of the stem, and the growing point is 

 situated in each bud with which the stem or 

 its branches terminate. 



At the extreme tip of the stem we find a 

 tissue, i.e. a collection of cells, amongst which 

 there is little or no differentiation ; one cell is 

 almost exactly like its neighbour. Under the 

 microscope a section of the growing point appears 

 to be composed of a dehcate honeycomb. The 

 simile is hardly apt, as we shall see when we have 

 studied these plant tissues more closely, but their 

 discoverer, more than two hundred and fifty 

 years ago, likened them to the comb of the 

 honey bee and called them cells, a name by 

 which they have been known to the present day. 

 These delicate cells are all bounded by cell walls 

 composed of cellulose, a substance composed of 

 carbon, hydrogen and oxygen and known as a 

 carbohydrate. Cotton wool is almost pure 

 cellulose. Each of these cells contains a sub- 

 stance known as protoplasm which is composed 

 of carbon, hydrogen, oxygen, nitrogen, sulphur, 



