ORGANS OF NUTKITION. 97 



liiivc been iilreiuly described. (Sec psit^c 73.) The vessels 

 fbiitid ill the viiscular bundles of the Lyc(>iM»diacea; arc spiral, 

 and in those of the K(|uisetacejc annular All Acotyledonous 

 steins ;;ro\v by additions to their apex, and hence the term 

 Acroyenous or sununit growers, which is ai)i)lied to them. 



In tlic Ferns (Kilic(!s), we have the Acroj^enous stem in the 

 hij^hest (U't^rci! of (levelo]»nicnt. 'J'hose Avhich are indi;;c,noiis 

 to this country are but insij;niflcant specimens of such plants, 

 for in th(^m the stem nuTiily runs along the surfa(;c of the ground, 

 or burrows bciuiath it, sending u\> its leaves, or fronds as they arc 

 commonly called, into the air, which die down yearly (Ji/j. LOS). 

 In warm regions, and more csjiccially in the tropics, we find 

 n\n-h jilants in the highest degree of development. Here the stem, 

 called th(! candexuv stipe, rises into the air to the height of fifty 

 or sixty feet or more; (//r/. 1.')'.)), bearing on its summit a tuft of 

 foliage. In their gcn(!ral appearance ex- 

 tcnuiUy th(!S(^ Tree;- Ferns have great resem- 

 blance to Monocotyledonous trees, not oidy 

 in bearing their foliage like them at their 

 summits, but also in producing no lateral 

 braiu'hcs, and being of uniform diameter 

 from near their base to their sununits. The 

 outside of the stem of a Fern is marked 

 with a nund)(!r of scars, which have a more 

 or Icssrhoniboidal outlin(! (Ji(/. 189). The 

 surface of these scars present little hard- 

 ened projections, c, or darker-coloured 

 spots, which ai»i)carance is pro<luc(!d by 

 the rupture of the vessels proceeding to 

 the leaves, by the fall of which organs the ^'ftairrem. m'lx'rkcTcx-' 



scars are J)roduccd. temaUy by rliomlx.idal 



Upon making a transverse section of a r/klcoSi-cd See' 

 Tree- Fern it pniscnts, as we have already tionH, c. 

 briefly noticed (see i)age 71), the following parts: — On the 

 outside a hard rind {Jig. le-O, e), conqjosed of dark-coloured 

 wood-cells covered externally by parenchyma. Within this 

 we find a mass of parenchyma, »/, 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 beconuis hollow. Towards the outside 

 of this cellular mas.s, and just within the rind, we find the 

 wood consisting of simultaneous vascular bundles arranged 

 in the form of plates, which, when cut, have a wavy outliru;, v, v. 

 These nuisses of wood forming the fibro-vascular system have 

 generally oi»enings between them, by means of which the 

 parenchyma beneath the rind and that of the centre of the 

 stem comnmnicate ; in other cases these woody masses touch 

 each other at their edges, and thus form a continuous circle 

 11 



