AOKOGENOUS OE AOOTYLEDONOUS STEM. 



71 



to the meshes seen in the liber of Exogens (fig. 119). In these spaces 

 vessels of communication pass between the outer or cortical, and the 

 inner or central portions of the stem. 

 Prom the point where the vascular 

 bundles unite or anastomose, other 

 vessels are given off to supply the 

 fronds, ajad some pass into the ad- 

 ventitious roots, which are often pro- 

 duced abundantly on the outside of the 

 stipe (fig. 135 ra). 



The trunk of the Acrogen dififersfrom 

 that of the Exogen, by having its 



Fig, 136. 



« 



vascular cylinder penetrated by only 

 one kind of horizontal tissue, namely, 

 the vascular bundles belonging to the 

 fronds ; while the Exogen has in addi- 

 tion another horizontal tissue, namely, 

 meduUary rays, composed of cellular 

 tissue, and performing a totally difierent 

 function. 



The acrogenous stem in the young 

 state is solid, but it frequently be- 

 comes hollow in the progress of. Kg. 135. 

 growth, by the rupture and absorp- 



Fig. 135. Tree fem {AlsopJiUa perroteticma), of the East Indies. Stem or^tipe is 

 cylindrical, unbranched, and presents at its base, r a, a conical enlargement, formed by a 

 mass of adventitious roots. The leaves are terminal, and in the young state are rolled up 

 in a circinate manner. Fig. 136. Transverse section of the stem of a Tree Um(Cyatliea). 

 m. Cellular tissue, corresponding to pith, occupying the central part, s I, Vascular circle 

 composed of numerous irregularly-formed masses. /, Darlc-coloured woody or prosenchy- 

 matous fibres, forming the borders of the vascular masses, v v, Pale-coloured vessels, chiefly 

 scalariform, occupying the centre of the masses, p. Parenchymatous or cellular external 

 zone, communicating with the central portion, e. Hard epidermal envelope, occupying the 

 place of the bark. 



