104 ELEMENTS OF STKUC'TURAI, BOTANY, 



now explain somewliat more in detail the nature of this 

 difference. Bearing in miud the fact stated in the pre- 

 ceding part of the chapter, that old and new parts differ 

 mainly in the shape of then- component cells and the 

 tov 'are of the cell-walls, it will be found that the dis- 

 tinction between Exogenous aad Endogenous growth 

 depends mostly u^ion the relative situation of the new 

 cells and the old oises — of the parenchyma and the 

 prosencliyma. 



1G9. Let us begin with the stem of a Dicotyledon, 

 Fig. 167 shows a section of a young shoot. The whole 



of the white part is cellular tissue, 

 the central portion being the pith. 

 The dark wedge-shaped portions 

 are fibro-vascular bundles, consist- 

 ing mainly of woody tissue, a few 

 vessels, easily recognised by their 

 larger openings, being interspersed. 

 '^" ■'^^' As the shoot becomes older, these 



bundles enlarge, and others are formed between them, 

 so that the radiating channels of c<^llular tissue which 

 separate them are in the end rc- 

 ducfd to much smaller compafs 

 than in the earlier stages of growth 

 (Eig. 168). The narrow channels 

 are the vteduUarij rai/s. The 

 cells of which they are composed 

 are flattened by compression. 

 Eventually, a riiuj of wood is Fig. i68. 



formed, the medullary rays intersecting it in fine lines, 

 a-s the sawed end of almost any log will show. Outside 

 the zone of wood is tlie Ixirk, wln'ch at first consists 

 altogetlier of celluhir tissue. As the season advances, 



