74 PRACTICAL BOTANY. 



b. Xylem-fibres or Wood-prosenchyma, elements 

 with much smaller cavity, little or no protoplasm, and 

 thick walls. 



c. Xylem-parenchyma, recognised by the presence 

 of a protoplasmic body, and (at all events in autumn) 

 of starch grains. The cells of this tissue are usually 

 grouped round the vessels, and often form bands con- 

 necting two consecutive medullary rays laterally. 



The cells of the Medullary rays are in the xylem 

 thick- walled (lignified) and pitted ; they have proto- 

 plasmic contents and starch. They are elongated 

 radially. Note that they have special cambium cells, 

 differing in form from the ordinary cambium. In the 

 phloem the cells are thin-walled (cellulose), and have 

 plentiful protoplasm. 



8. The Pith. In the peripheral part the cells have 

 thick, lignified, pitted walls, and a protoplasmic body 

 with starch (at least in autumn). Tissue of this nature 

 merges gradually into the central tissue with thin 

 walls (lignified and pitted) and no protoplasm. Muci- 

 lage cells occur here and there. 



III. Cut a four-year-old twig of Elm transversely, 

 and smooth the cut surface with a razor. 



Note, the age of a twig may be judged externally by counting 

 backwards the annual increments of growth from the apex. The 

 limits of each annual increment of growth may be recognised by 

 the closer aggregation of the scars of the leaves or scales at those 

 points. 



Examine with a lens, and observe : 

 1. The Pith, which occupies the organic centre of 

 the stem. [Its position does not, as a rule, coincide 



