240 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



lamin£e distinct. The thickening is greatest in bast tissue, and in 

 it also the secondary layers are most distinct, those of wood cells 

 being more homogeneous, and pores are also usually present. A 

 small wide-turning spiral band is seen near the bordered pores in 

 wood cells of yew, vine, mezereon, Vihurnum Lantana, &c., and 

 with narrower turns in the wood of Pinus picea. Broader spiral 

 bands are observable in the outer wood cells of the annual ring of 

 conifers, and the striation is more sharply defined by application 

 of nitric acid. When unlignified, as in Apocynacece and Ascle- 

 piadacecE, the iodine tests produce on the laminge the usual blue 

 reaction. 



III. — Tubular Cells. 



These also occur only in the vascular bundles, and belong partly 

 to the wood, partly to the bast portion. In form they are usually 

 elongated cylinders, and are distinguished from long parenchym 

 cells and from fibre cells by their oblique transverse partitions, 

 which endure only so long as they carry sap, but as soon as the 

 liquid contents become replaced by air, these partitions are re- 

 sorbed, or if they should remain, their structure is different from 

 that of the longitudinal wall. 



The perforation of the transverse partition takes place in various 

 ways ; in those vascular cells of the woody bundles in which the 

 partitions are horizontal or but slightly oblique, the opening is 

 usually a large bordered pore, as we see in the oak, ash, or beech, 

 and not unfrequently reticular as in Pteris aquilina. If the par- 

 titions are still more inclined to the longitudinal wall, we find many 

 round or longish pores, producing netted or scalariform perforation 

 as in Ephedra, Lonicera, Vihurnum, &c. 



Thickening of tubular cells occurs under various forms, but it 

 never attains to the extent met with in fibre cells. We have ring- 

 like thickening in the annular vessels of the medullary sheath of 

 Gymnosperms and Dicotyledons, and they may be seen adjacent to 

 the spiral vessels in the vascular bundles of monocotyledons and 

 ferns ; in the stems of maize, reeds, and Balsamina they are well 

 developed. 



Spirally thickened tubular cells, spiral vessels or Traclienchyma 

 are equally common in the medullary sheath of Dicotyledons, the 

 most internal cells having very wide turns of the spiral band, in 

 consequence of this part of the vascular bundle being in most 

 active growth, the spirals become more quickly drawn apart. The 



