156 



XI. THE DICOTYLEDONOUS STEM WOODY TYPE. 



parenchymatous cells adjoining them (p) which mostly contain the 

 abundant starch, but on the inner margin of the bast-fibres are 

 sometimes crystallogenous ; and the wide-cavitied sieve-tubes (v) 

 with their companion-cells (c). These companion-cells are in 

 most cases cut off singly and obliquely from a side of the sieve- 

 tube, and hence are usually bounded by a bast-parenchyma 

 cell or a medullary-ray cell. Where the section has chanced 

 upon a sieve-plate (r*), this is recognisable from its strong 

 refraction, its sieve-pores, and its yellow-brown colour under 

 the action of the iodine. In Tilia also we can determine that 



FIG. 60. Cross-section through the bast of Tilia parvifolia (alcohol material), v, 

 sieve-tubes, at v* exposing sieve-plate ; c, companion-cell ; />, bast-parenchyma ; k, 

 crystallogenous cell ; /, bast-fibre ; v, medullary ray (x 540). 



at a little- distance from the cambium the sieve-tubes and com- 

 panion-cells are empty. Traversing the bast the medullary- 

 ray cells are at once recognisable from their arrangement and 

 contents. 



In the radial longitudinal section, which we also examine 

 in iodine-glycerine, we first of all note the big vessels and 

 tracheides, which besides the bordered pits upon their walls, 

 also have thin spiral bands as an innermost thickening layer. 

 The pitted ducts and the tracheides are connected by every 

 intermediate form, and only the circumstance whether the end 



