98 ORGANOGRAPHY. 



within the rind. These masses consist of simultaneous vascular 

 bundles, the vessels of which are chiefly scalariform; these 

 are situated in the centre of the bundles, where they may be 

 readily distinguished by their pale colour {fig. 165), and are sur- 

 rounded externally by layers of dark-coloured hardened wood- 

 cells. 



We liave already stated that Tree-Ferns have no branches. 

 This arises from their having no provision for lateral buds : 

 hence the cylindrical form of stem which is common to them. 

 For tlie same reason also, they are rarely of great diameter. 

 Some Ferns however become forked at their apex {fig. 190) ; 

 Fiq. 190. ^'^^^ ^^ pi'oduced by the division of the terminal 

 bud into two, from each of which a branch is 

 formed. Such branches are, however, very dif- 

 ferent from those of Exogens, which are pro- 

 duced from lateral buds, for, as they arise simply 

 from the splitting of one bud into two, the 

 diameter of the two branches combined is only 

 equal to that of the trunk, and in all cases 

 where the stems of Acotyledonous Plants branch, 

 the diameter of the branches combined is only 

 equal to that of the axis from Avhencc they are 

 100 Fork d *^^^*^"^^^' -^^ Acotyledonous stems only grow by 

 stem of a TreV the development of a terminal bud, the destruc- 

 I'ern. tion of that bud necessarily leads to their death. 



There is nothing in the internal structure or external appear- 

 ance of such stems by which we can ascertain their age. 



All Acotyledonous Plants below the Mosses and Liverworts 

 have no true stems bearing leaves, as we have already seen 

 (see p. 64), but they simply form cellular expansions of various 

 kinds, to which the name of thallus is applied, hence they arc 

 termed Thal/ophj/fes or Thallogens ; while all plants producing 

 stems are called Cormophytes, and these are divided according to 

 their internal structure into three great classes called respectively 

 Exogens or Dicotyledons, Endogens or Monocotyledons, and 

 Acrogens or Acotyledons. The nature of the internal struc- 

 ture of the stem has been now fully treated of, and we proceed 

 to descril)e the buds whicli are situated upon tlic surfiice of 

 stems, and by means of wliich, as already briefly noticed, they 

 increase in height and form branches. 



2. Buds and Kamification. — We have already stated 

 (p. 70) that one of the main cliaractcristics by wliich a stem is 

 distinguished from a root is the presence of leaves and buds. 

 The leaves will lie particularly treated of hereafter, but we have 

 now to describe the jmrts of the stem from whence they arise, 

 and the nntin-c of buds. 



Leaves arc always developed at regular points upon the sur- 

 face of the stem, which arc called nudes or simply knots, and 



