62 PRACTICAL BOTANY. 



tissue in the following way. When fully developed the vessels 

 have lost their protoplasmic contents and their turgescence ; their 

 walls are unevenly thickened, at some points being thin ( = pits) 

 at others strongly thickened. If thin-walled tissue, the elements 

 of which are active and turgescent, abut on such a wall, it is 

 obvious that but slight resistance to the internal tension will be 

 offered at the pits, where the wall of the vessel is thin. As a 

 result the wall bulges at these points, and the cells encroach as 

 papillae upon the cavity of the vessel. Cell-divisions may occur in 

 these papillae, and the whole process be continued till the cavity 

 of the vessel is completely filled with a cellular tissue. 



Look in the longitudinal sections of the old stem of Helianihus 

 for instances of such encroachment of cells upon the cavity of the 

 vessel. Good results may be obtained from the old stem, or root, 

 of Cucurbiia, and from the stems of Robinia, or Vitis. 



5. The central Pith is composed of parenchymatous 

 cells, with thin walls consisting of cellulose (reactions) 

 slightly pitted : they have lost their protoplasmic cell- 

 contents in many cases, and especially near the centre 

 of the stem. Occasional resin-passages may be found 

 in the pith. 



* * Young Stem. 



IV. Cut transverse sections of a young stem, i.e. not 

 more than one-eighth of an inch in diameter. 



If the sections be cut from the hypocotyledonary stem, though 

 they will correspond in all important points to the following de- 

 scription, they will differ in some minor details ; e.g. hairs will 

 be absent, the bundle-sheath will be more obvious, &c. 



Mount in glycerine, and passing from the periphery 

 inwards observe successively under a low power 

 1. The Epidermis as before a single layer, with 



