56 



ACROGENOUS OB ACOTYLEDONOUS STEM. 



originally by the bases of the leaves, which remain for a long time 



attached to the stem. 



103. The vascular bundles are formed 

 simultaneously, and not progressively 

 as in the stem already noticed; and 

 additions are always made in an up- 

 ward direction. The stem then is 

 formed by additions to the summit,, 

 and by the elongation of vessels already 

 formed; hence the name Acrogenous, 

 (oLx.^, summit). The leaves unite by 

 their bases to form the stem, and the 

 arrangement of their vessels is traced 

 into it. The vascular system is of 

 greater density than the rest of the 

 tissue, and is usually distinguished by 

 the dark colour of the pleurenchyma 

 (fig. 117 /), which surrounds the paler 

 vessels in the centre (fig. 117 v v). 

 The vascular bundles do not follow a 

 straight course, but unite and separate, 

 leaving spaces between them, similar 

 to those seen in the liber of Exogens 

 (fig. 101). 



Fig. 116. Tree fern (Alsophilaperrotetiana), of the East Indies. Stem or stipe is cylindrical, 

 unbranched, and presents at its base, r a, a conical enlargement, formed by a mass of adventi- 

 tious roots. The leaves are terminal, and in the young state are rolled up in a circinate 

 manner. 



Fig. 117. Transverse section of the stem of a Tree fern (Cyathea). m, Cellular tissue, corre- 

 sponding to pith, occupying the central part, z I, Vascular circle composed of numerous irregu- 

 larly-formed masses. /, Dark-coloured woody or prosenchymatous 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, often communicating with the central 

 portion, e, Hard epidermal envelope, occupying the place of the bark. 



