ELM STEM 97 



starch : they are elongated in a radial direction. Note 

 that they have special cambium-cells, differing in form 

 from the ordinary cambium (compare Fig. 9, A). 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 winter). Tissue of this nature 

 merges gradually into the central tissue with thin walls 

 (lignified and pitted), and no protoplasm. Mucilage 

 cells occur here and there. 



There is considerable variety in the structure of the pith of 

 woody plants : the Elm is an example in which the peripheral 

 thick-walled tissue remains active, while the central thin-walled 

 part is inactive : in certain other plants these tissues are mingled 

 together throughout the pith, and this may be very well seen in 

 Rosa or Rubus : here the thick-walled actively living tissues 

 form a reticulum, the meshes of which are filled with thinner- 

 walled inactive cells. 



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

 and smooth the cut surface with a razor. 



The age of a twig may be judged externally by counting back- 

 wards the annual increments of growth from the apex. The 

 limits of each annual increment of growth may be recognized by 

 the close 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 with 

 the geometrical centre. Externally to this lies 



2. The xylem, which is here a broad yellowish 

 band, clearly marked off into a succession of concentric 



H 



