366 THE MICROSCOPE AND ITS REVELATIONS. 



forms between the spiral and annular ducts, which show the derivation 

 of the latter from the former, are very frequently to be met with. The 

 spires are sometimes broken up still more completely, and the fragments 

 of the fibre extend in various directions, so as to meet and form an irreg- 

 ular network lining the duct, which is then said to be reticulated. The 

 continuance of the deposit, however, gradually contracts the meshes, 

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

 cells ( 356); and such canals, designated as pitted ducts, are especially 

 met with in parts of most solid structure raid least rapid growth (Fig. 

 249, 3). The ' scalarif orm ' ducts of Ferns ( 340) 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. 249. 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,* gene rally speaking, they are larger in woods of dense texture, 

 such as Oak or Mahogany, than in those of which the fibres, remaining 

 unconsolidated, can serve for the conveyance of fluid. They are entirely 

 absent in the ConifercB. 



364. 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 vari- 

 ous parts of the plant under examination, or in the isolated conditions in 

 which they are obtained by dissection. 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, etc. ( 356), can scarcely be obtained except by slicing 

 and grinding ( 192); and these may be mounted either in Canada 

 balsam or in Glycerine jelly. In cases, however, in which the tissues are 

 of only moderate firmness, the section may be most readily and effectu- 

 ally made with the 'Microtome ' ( 184); 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 sections must be made with a 

 sharp knife, the substance being laid on the nail or 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 adhesion between the component cells, 

 fibres, etc., is often sufficiently 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 other 

 method for the sake of facilitating their isolation. None is so effectual 

 as the boiling of a thin slice of the substance under examination, either 

 in dilute nitric acid, or in a mixture of nitric acid and chlorate of potass. 

 This last method (which was devised by Schultz) is the most rapid and 

 effectual, requiring only a few minutes for its performance; but as 

 oxygen is liberated with such freedom as to give an almost explosive 

 character to the mixture, it should be put in practice with extreme 



