TISSUES OF PHANEROGA3IIA 



699 



leaving the walls of the duct marked only by pores like those of 

 porous cells ; and such canals, designated as pitted ducts, are 

 especially met with in parts of most solid structure and least rapid 

 growth (fig. 537, 3). The scalariform ducts of ferns may be re- 

 garded as a modification of the spiral ; but spiral ducts are fre- 

 quently to be met with also in the rapidly growdng leaf-stalks of 

 flowering plants, such as the rhubarb. Not unfrequently, however, 

 we find all forms of ducts in the same bundle, as seen in fig. 537. 

 The size of these ducts is occasionally so great as to enable their 

 openings to be distinguished by fche unaided eye ; they are usually 

 largest in stems whose size is small in proportion to the surface of 

 leaves which they support, such as the common cane or the vine ; 

 and, generally speaking, 

 they are larger in woods 

 of dense texture, such as 

 oak and mahogany, than 

 in those of which the 

 fibres, remaining uncoil - 

 solidated, can serve for the 

 conveyance of fluid. They 

 are entirely absent in the 

 Coniferce. 



The vegetable tissues 

 whose principal forms 

 have been now described, 

 but among which an im 

 mense variety of detail is 

 found, may be either 

 .studied as they present 

 themselves in thin sec- 

 tions of the various parts 

 of the plant under exami- 

 nation, or in the isolated 

 conditions in which they 



are obtained by dissection. FIG. 537. Longitudinal section of stem of Italian 

 The former process is the re ed : a, cells of the pith ; &, fibre- vascular 

 most easy, and yields a bu^le, containing 1, annular ducts; 2, spiral 

 r 7 j. ducts ; 3, pitted ducts with woody fibre ; c, cells 



large amount of mforma- O f the epiderm. 

 tion ; but still it cannot 



be considered that the characters of any tissue have been properly 

 determined until it has been dissected out. Sections of some of the 

 hardest vegetable substances, such as ' vegetable ivory/ the ' stones ' 

 of fruit, the ' shell ' of the cocoa-nut, &c., can scarcely be obtained 

 except by slicing and grinding ; and these may be mounted either in 

 Canada balasm or in glycerin jelly. In cases, however, in which the 

 tissues are of only moderate firmness, the section may be most readily 

 and effectually made with the ' microtome ; ' and there are few parts 

 of the vegetable fabric which may not be advantageously examined 

 by this means, any very soft or thin portions being placed in it 

 between two pieces of cork, elder-pith, or carrot. In certain cases, 

 however, in which even this compression would be injurious, the 



