THE INTEKNAL STRUCTURES OF PLANTS. 203 



see long, tapering threads overlapping each other, 

 something like Fig. 365, and called fibres. 



Vessels, or Ducts, as Prof. Gray prefers to call 

 them, are continuous tubes of considerable length, of 

 which the walls are never smooth, but 

 marked with dots, bars, rings, spirals, etc. 

 They are sometimes cylindrical, and some- 

 times tapering in form, and contract a lit- 

 tle from place to place along their length, 

 as seen in Fig. 366, where circles are 

 formed by the constriction. The meaning 

 of these constrictions may be gathered 

 from the following 



EXPERIMENT. Select from vegetable 

 pulp some of the stringy portion contain- 

 ing vessels, and pour upon it boiling water, sharpened 

 by a few drops of nitric acid. The vessels will break 

 up into fragments at the places of these circles. At 

 these points, also, you will find partitions across the 

 vessel, more or less perforated and broken, or mem- 

 branous folds, that may come from the breaking of 

 these partitions. Hence, it appears that a vessel is 

 formed from a row of cells, placed end to end ; the 

 partitions, which at first separated these cells, being 

 more or less completely removed. 



Vessels, or ducts, like cells, are named from the 

 markings on their walls. There are dotted, barred, 

 spiral, and annular vessels (Figs. 367-370). Fig. 371 

 represents scalariform ducts, so named from the lad- 

 der-like markings on their walls. 



Fibres, also, are produced from cells; they are 

 cells altered in certain ways. All vegetable tissue is 

 at first cellular, and it is by the elongation of cells 



