PREPARATION OF VEGETABLE TISSUE. 375 



work lining the duct, whicli is then said to be reticulated. The 

 continuation of the deposit, however, gradually contracts the 

 meshes, and leaves the walls of the duct marked only by pores 

 like those of porous cells (§ 227) ; and canals upon this plan, 

 commonly designated as dotted ducts, are among the most com- 

 mon forms of vasiform tissue, especially in parts of most solid 

 structure and least rapid growth (Fig. 162, 3). The " scalari- 

 form" ducts of Ferns (§ 218) are for the most part of the 

 spiral type ; but spiral ducts are frequently to be met with 

 also in the rapidly growing 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. 162. The 

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

 openings to be distinguished by the 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 or Mahogany, than in those of which 

 the fibres, being softer, can themselves be subservient to the con- 

 veyance of fluid. They are entirely absent in the Couiferse. 



234. The Vegetable Tissues, whose principal forms have been 

 now described, but among which an immense variety of detail 

 is found, may be either studied as they present themselves in 

 thin sections of the various parts of the plant under examination, 

 or in the isolated condition in which they are obtained by dissec^ 

 tion. The former process is the most easy, and yields a large 

 amount of information ; 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. (§ 227), can scarcely be 

 obtained except by slicing and grinding (§ 108) ; and these may 

 be mounted either in Canada balsam or in weak spirit. In 

 cases, however, in which the tissues are of only moderate firm- 

 ness, the section may be readily and most eifectually made with 

 the "Section-Instrument" (§ 107); 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. In certain cases, however, in which 

 even this compression would be injurious, the sections must be 

 made with a sharp knife, the substance being laid upon a slip of 

 glass. In dissecting the Vegetable tissues, scarcely any other 

 instrument will be found really necessary, than a pair of needles 

 (in handles), one of them ground to a cutting edge. The adhe- 

 sion between the component cells, fibres, &c., is often sufiiciently 

 weakened by a few hours' maceration, to allow of their readily 

 coming apart, when they are torn asunder by the needle points 

 beneath the simple lens of a dissecting microscope. But if this 

 should not prove to be the case, it is desirable to employ some 



