THE STEM. 163 



protoplasm. They are formed by drawing out and 

 thickening the original cambium cells. Owing to their 

 shape, the ends dovetail into one another, and form an 

 extremely tough tenacious bundle of stringy fibres, 

 which sometimes protect the delicate phloem within 

 from being compressed, or may even assist in 

 strengthening the bark. 



The pericycle sometimes forms similar fibres, or it 

 may produce roots in those cases in which these are 

 found on the stem, as in the Ivy. The cambium, as 

 has been seen, forms every year new xylem on the inside, 

 and new phloem on the outside. The wood, or xylem, 

 formed in spring, contains many vessels, because at that 

 time there is a sudden great demand for water to supply 

 the developing twigs and leaves. In spring, also, all 

 the food material stored up in winter is transformed, 

 and made available for the cells, so that every life 

 process is at its most energetic stage. Towards the 

 end of the cambium's work for the year, the vessels 

 formed are very few, and the cells produced are smaller 

 and more thick walled. Thus the junction of the old 

 wood of 1 90 1, and the spring-wood of 1902 is visible 

 to the eye, and the so-called annual rings can be 

 distinctly traced in most common trees. They are not 

 easily seen in timber from tropical countries, where 

 wood formation may go on during the whole year. The 

 heart of the tree, i.e. the pith and the older xylem often 

 becomes dark and discoloured later on in life, because 

 resin and other substances accumulate in it. This 

 heartwood is not used to carry water. 



The cortex consists of living cells, which are inter- 

 mingled with many others specially set aside for secretion, 

 for strengthening, or as supporting fibres or strings. It is 

 in the cortex that astringent tannins, resins, spherical 



