40 



THE ELEMENTARY STRUCTURE OF PLANTS. 



are continuous tubes of considerable length (Fig. 57) ; but occasion- 

 ally they exhibit cross-lines at certain intervals, plainly showing that 

 they are made up of a row of cells placed end to 

 end, and becoming a tube by the obliteration of the 

 intervening partitions (Fig. 56). In Fig. 32 some 

 dotted ducts (one of them exhibiting oblique parti- 

 tions or ends) are shown in place among the woody 

 tissue. It is in the wood that they commonly 

 abound. Being of greater calibre than any other 

 cells or vessels found there, they form the pores so 

 conspicuous to the naked eye on the cross-section 

 of many kinds of wood, such as of Oak, Chestnut, 

 56 57 and Mahogany, as well as the lines or channels 

 seen on the longitudinal section. Their size, compared with that of 

 the wood-cells in the wood of the Plane-tree, is shown both in longi- 

 tudinal and transverse section, in Fig. 31, 32. 



59. Scalariform DllClS (Fig. 58, 59), differ from dotted ducts only 

 in the form of the markings, the thin spots being transversely elon- 

 gated instead of circular, and appearing like 



cross-bars, which have been likened to the 

 rounds of a ladder, whence the name. This 

 is the more striking when the ducts are pris- 

 matic (by mutual pressure) and the cross-bars 

 occupy nearly the whole length of each side, as 

 in Fig. 58. Ducts of this sort abound in the 

 stems or stalks of Ferns. The markings are 

 often spiral in their arrangement; as is shown 

 in Fig. 59, by the way the duct tears into a 

 band. Ducts of this and of the foregoing sort, 

 where the markings are thin places, have been 

 named by Morren and Lindley Bothrenchyma, 

 meaning pitted tissue. 



60. Reticulated, Annular, and Spiral Ducts (Fig. 60-65), on the 



other hand (called Tracheae, from their resemblance to the windpipe, 

 or rather to the tracheae or air-tubes of insects), have been distin- 

 guished by Morren and Lindley under the general name of Trachen- 

 chyma. In these the markings, at least in most cases, are thicker 



FIG. 56 Portion of a dotted duct from the Vine, evidently made up of a series of short cells. 



FIG 57. Part of a smaller dotted duct, showing no appearance of such composition 



FIG. 58. Scalariform ducts of a Fern, rendered prismatic by mutual pressure. 



FIG 59 Similar duct of a Fern, torn into a spiral band. 



