VIII.] 



BOTANY. 



35 



future outer bark, and that between the fibro-vascular 

 bundles is the future silver-grain of wood. The 

 fibro-vascular bundles are in part future inner bark 

 and in part future wood, and consist of wood-tissue 

 (Par. 7) mixed with vascular tissue (Par. 9) towards 

 the centre, and of bast-tissue (Par. 8) towards the 

 circumference. Of these components of the vascular 

 bundle the bast-tissue forms the inner bark; the 

 wood and vascular tissues form the wood of the plant. 

 Such is the origin of outer bark, inner bark, wood, 

 pith, and silver-grain. 



A cross-cut of a one year's old twig of lime shows 

 the same arrangement of tissues as the flax; but 

 whereas the flax stem dies the same year as that 

 in which it is formed, the lime twig lives through the 

 winter, and is added to during the following summer, 

 increasing thus in thickness. 



FIG. 17. Transverse section of vas- 

 cular bundle from stem of a Dicoty- 

 ledon. 



FIG. 18. Transverse section of stem 

 of a Dicotyledon. 



55. This increase of thickness is caused by new 

 tissue being added between the bast and wood formed 

 in the previous year. This new tissue consists at first 

 of soft, tender, cellular tissue, produced by the growth 



