/ 



92 OMGANOGRAPHY. 



spots, which appearance is produced by the rupture of the vessels 

 proceeding to the leaves, by the fall of which organs the scars 

 are produced. 



Upon making a transverse section of a Tree-Fern it presents, 

 as we have already briefly noticed (see page 65), the following 

 parts : — On the outside a hard Tmd(fiff. 157, e), composed of dark- 

 coloured wood-cells covered externally by parenchyma. Within 

 this we find a mass of parenchyma, m, the cells of which have 

 thin walls ; this is analogous to the pith of exogenous stems. 

 In old stems this central parenchyma is destroyed, so that the 

 stem becomes hollow. Towards the outside of this parenchyma, 

 and just within the rind, we find the so-called wood; this con- 

 sists of simultaneous vascular bundles arranged in the form of 

 plates, which, when cut, have a wa^'y outline, v, v. These masses 

 of wood forming the fibro-vascular system have generally open- 

 ings between them, by means of which the parenchyma beneath 

 the rind and that of the centre of the stem communicate ; but 

 in other cases these woody masses touch each other at their 

 edges, and thus form a continuous circle within the rind. These 

 masses consist of simultaneous vascular bundles, the vessels of 

 which are chiefly scalariform in their character ; these are situ- 

 ated in the centre of the bundles, where they may be readily 

 distinguished by their pale colour {Jig. 157), and are surrounded 

 externally by layers of dark-coloured hardened wood-cells. 



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

 absence of branches arises from their having, like Palms, no pro- 

 vision for lateral buds : hence the cylindrical form of stem which 

 is common to them as with the stems of monocotyledonous plants. 

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

 Some Ferns, however, become forked at their apex (fiff. 182); 

 Fia 18'? which forking is produced by the division of the 

 ^' *'' terminal bud into two, from each of which a 

 branch is formed. Such branches are, however, 

 very different from those of exogenous stems, 

 which are produced 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 triink, 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 whence they are 

 derived. As Acotyledonous stems only grow by 

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

 i^erii. tion of that bud necessarily leads to their death. 



There is nothing in the internal structure or external appearance 

 of such stems by which we can ascertain their age. 



2. Buds and Eamification. — We have already stated (p. 64) 

 that the presence of leaves and leaf-buds is the essential charac- 



