376 STRUCTURE OF PHANEROGAMIC PLANTS. 



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 libe- 

 rated 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 ; and it will then be found very easy to tear 

 apart the individual cells, ducts, &c., of which it may be com- 

 posed. These may be preserved by mounting in weak spirit. 



235. Structure of the Stem and Root. It is in the stems and 

 roots of Plants, that we find the greatest variety of tissues in 

 combination, and the most regular plans of structure ; and sec- 

 tions of these, viewed under a low magnifying power, are objects 

 of peculiar beauty, independently of the scientific information 

 which they afford. The Axis (under which term is included the 

 Stem with its branches, and the Root with its ramifications) 

 always has for the basis of its structure a dense cellular paren- 

 chyma; though this, in the advanced stage of development, may 

 constitute but a small proportion of it. In the midst of this 

 parenchyma we generally find fibro-vascular bundles ; that is, 

 fasciculi of woody fibre, with ducts of various kinds, and (very 

 commonly) spiral vessels. It is in the mode of arrangement of 

 these bundles, that the fundamental difference exists between 

 the stems that are commonly designated as Endogenous, and 

 those which are (more appropriately) termed Exogenous; 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 parenchyma ; whilst in 

 the latter they are arranged side by side, in such a manner as to 

 form a hollow cylinder of wood, which includes within it that 

 portion of the cellular substance which is 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, respec- 

 tively characteristic of those two great groups into which the 

 Phanerogamia are subdivided, namely the Monocotyledonous, and 

 the Dicotyledonous, will now be more particularly described. 



236. When a transverse section (Fig. 163) of a Monocotyledo- 

 nous stem is examined microscopically, it is found to exhibit a 

 number of fibro-vascular bundles, disposed without any regu- 

 larity 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 ducts, which are at once distinguished 

 by the size of their openings ; and these are surrounded by 

 woody fibre, and spiral vessels, the transverse diameter of which 

 is so extremely small, that the portion of the bundles which they 

 form is at once distinguished in transverse section, by the close- 



