98 PRACTICAL BOTANY 



rings; these, as a rule, correspond in number to the 

 years of the twig (annual rings). 



3. The cambium lies at the outer limit of the 

 xylem, but it will hardly be recognized as a definite 

 band of tissue under a simple lens, since it is a very 

 narrow zone : its position may frequently be recognized 

 by the rupture of the tissues, the walls of the cambium 

 being thin and easily broken. Outside this is - 



4. The phloem, which is a much narrower band 

 than the xylem, and is also marked off, though less dis- 

 tinctly, into concentric rings of equal number. Outside 

 this lie 



5. The cortical tissue and cork, which are usually 

 of insignificant bulk, compared with that of the vascular 

 tissues : in some cases, however, there is an unusually 

 great development of cork, which then appears ex- 

 ternally as longitudinal projecting plates. 



Note the medullary rays. Some of these (primary 

 rays) may be traced the whole distance from pith to 

 cortex; others (secondary rays) only part of that 

 distance. The latter have been entirely formed by the 

 cambium. 



It will be found useful to examine transverse sections 

 of other stems also, e.g. that of Eicinus : the three- or 

 four-year-old stem of the Lime (Tilia) is a remarkably 

 good one for showing the arrangement of tissues after 

 secondary growth in thickness ; but the soft wood is not 

 a good type for the study of the more minute details. 

 The general plan of the process of secondary thickening, 

 and the relation of the secondary tissues to the primary 

 arrangement, are made clear by means of the diagrams 

 A, B t of Fig. 8. 



