606 THE STEMS OF CLIMBING PLANTS, 



containing single, lozenge-shaped, octahedral crystals. The 

 remains of a former cambium- ring are seen in the wood about 

 one-third of the distance from pith to cortex. A third cambium- 

 ring is seen forming in the bast, from which additional rings of 

 xylem and phloem will be created. 



6. Phloioeyela. — ViTis sterculifolia F.v.M., (Plate Ixiv., fig. 

 9). —The pith appears homogeneous. The medullary rays, 2-8 

 rows of radially arranged, brick-like cells, contrast clearly with 

 the murally arranged wood-fibres. The vessels are large, and 

 evidently capable of division. The medullary rays are as well 

 defined between the bast-masses as between the wedges of wood. 

 They increase in size towards the circumference of the transverse 

 section, and take a clavate outline. The bast is composed of 

 8-10 rows of thin-walled cells, with a terminal, almost circular 

 mass of sieve-tubes. Outside the stele, the remains of former 

 bast-masses, now sclerotic, form a strengthening layer in the 

 cortex. A few sacs containing raphides may be noted here and 

 there; others contain single, larger, flattened crystals, and others, 

 again, store starch-grains. 



7. Polyeyela. 



(a.) BOUGAINVILLEA SPECTABILIS JuSS., (PI. Ixiv., flg.lO). — The 



centre of the transverse section of the stem contains the largest 

 vascular bundles, scattered through the pith. The xylem and 

 phloem portions of each bundle are clearly defined, but the 

 cambium is usually marked by a scar, where it has torn under 

 the knife of the microtome. Outside these older and scattered 

 bundles, is a clearly defined ring of two years' growth of younger 

 bundles, normally dicotyledonous in general appearance, but 

 having alternate circles of wood, bast, wood, bast. J^etween any 

 two bundles of the outer ring are well defined medullary rays; 

 but, at the interior end of each ray, the cells composing it become 

 roughly polygonal, and merge into the pith-cells. When a new 

 cambium appears, outside the last ring of bundles, the cells of 

 the outer ends of the rays unite round the outer extremity of 

 each bundle, so as to isolate the ring previously formed b}^ a 

 definite wavy circle of parenchymatous tissue. The bast arising 



