104 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



that the twig is below the surface, and force air through 

 with the pump ; air-bubbles escape from the lenticels. 



116. Structure of Lime Stem. The Lime-tree (Tilia) 

 is taken here as a type in which the minute structure of 

 the woody stem may be studied, chiefly because its wood 

 is easily cut and its phloem is arranged in conspicuous 

 wedge-like strands ; other types e.g. Oak, Elm may 

 with advantage be used, however. The winter-bud of 

 Lime has few scales, but the girdle- scars can be found 

 on careful inspection of the twig. Starting from the 

 tip of a twig, cut out parts of each year's growth, and 

 make thin transverse sections of each. It is enough 

 to go back as far as the four-year-old portion in this 

 way ; then cut out about 1 cm. from a still older region, 

 slice from this a wedge-shaped piece including about one- 

 eighth of the circumference and extending right into the 

 pith, and cut sections from this wedge. 



117. T. S. Young Lime Stem. In the young cur- 

 rent-year twig, cut in early summer shortly after open- 

 ing of the bud, note (1) the epidermis, with cuticle ; 

 (2) cortex collenchyma ; (3) cortex parenchyma; 

 (4) an interrupted zone of sclerenchyma ; (5) phloem, 

 more or less broken up into masses by the expanded outer 

 ends of (6) the medullary rays, which extend through 

 (7) the cambium and (8) the xylem into (9) the pith. 

 In the cortex and pith note the conspicuous large mucilage- 

 containing cells and the small crystal-containing cells. 



In older portions of current-year twig, cut in late 

 summer or autumn, note that (1) the xylem and phloem 

 zones have increased in width, especially the xylem ; 

 (2) in passing outwards, the xylem elements after a 

 time diminish in width so that the part just inside the 

 cambium is compact and close in texture; (3) bands of 

 sclerenchyma have been developed in the secondary 

 phloem, alternating with bands of soft tissue the sieve- 

 tubes and parenchyma ; (4) the phloem is more distinctly 

 broken up into roughly triangular masses with the apex 

 outwards, alternating with the fan-like outer portions of 



