7CO MICKOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



sections must be made with a shnrp knife, the substance being laid on 

 the nail or on 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, &c., 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 exami- 

 nation either in dilute nitric acid or in a mixture of nitric acid and 

 chlorate of potassa. 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 caution. After being thus 

 treated, the tissue should be boiled in alcohol, and then in water ; 

 arid it will then be found very easy to tear apart the individual cells, 

 ducts, &c. of w r hich it may be composed. These may be preserved 

 by mounting in weak spirit. 



Stem and Boot. It is in the. stems and roots that we find the 

 greatest variety of tissues in combination, and the most regular 

 plans of structure ; and sections of these viewed under a low mag- 

 nifying power are objects of peculiar beauty, independently of the 

 scientific information which they afford. The axis (under which 

 term are included the stem with its branches, and the root with its 

 ramifications) always has for the basis of its structure a dense cellular 

 parenchyme ; though in an advanced stage of development this 

 may constitute but a small portion of it. In the midst of the 

 parenchyme we generally find fibro-vascular bundles, consisting of 

 w r oody fibre, with ducts of various kinds, and (almost always) spiral 

 vessels. It is in the mode of arrangement of these bundles that the 

 fundamental difference exists between the stems which are commonly 

 designated as endogenous (growing from within), and those which 

 are more correctly termed exogenous (growing on the outside) ; for 

 in the former the bundles are dispersed throughout the whole 

 diameter of the axis without any peculiar plan, the intervals between 

 them being filled up by cellular parenchyme ; whilst in the latter 

 they are arranged side by side in such a manner as to form a cylinder 

 of wood, which includes within it the portion of the cellular substance 

 known as pith, whilst it is itself enclosed in an envelope of the same 

 substance that forms the bark. These two plans of axis-formation 

 respectively characteristic of those two great groups into which 

 Phanerogams are subdivided namely, the Monocotyledons and the 

 Dicotyledons will now be more particularly described. 



When a transverse section (fig. 538) of a monocotyledonous stem 

 is examined microscopically, it is found to exhibit a number of fibro- 

 vascular bundles, disposed without any regularity in the midst of 

 the mass of cellular tissue, which forms (as it were) the matrix or 

 basis of the fabric. Each bundle contains two, three, or more large 



