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BOTANY 



endodermis appear as dark spots (Fig. 126), but in a tangential section 

 as sinuous lines. The stele or CENTRAL CYLINDER of the stem contains 

 vascular bundles (Fig. 125, cv), which, in the Equisetaceae and some 

 other Pteridophytes, as well as in the Gymnosperms and Dicoty- 

 ledons (Fig. 125), are arranged in a circle, whereas in Monocotyledons 

 (Fig. 124) they are irregularly distributed. In all these cases the 

 xylem portion of the vascular bundle is directed towards the centre, 

 and the phloem portion away from the centre of the stem. That part 

 of the peripheral tissue of the central cylinder lying outside of the 

 bundles is called the pericycle (pc) or pericambium, and is the special 



Fig. 126. — Transverse section of an adventitious root of Allium Cepa. c, Primary cortex ; e, endo- 

 dermis ; j), pericycle ; a, annular tracheitis ; sj>, spiral tracheids ; sc and sc*, scalariform vessels ; 

 <', phloem, (x 240.) 



seat of new growths. If the bundles are arranged in a circle (Fig. 

 125), that part of the central cylinder enclosed by them is the pith or 

 medulla (m), and the tissue between the different bundles the primary 

 medullary RAYS. In the case of scattered bundles (Fig. 124), a dis- 

 tinction between medulla and medullary rays is no longer possible, 

 and the whole tissue surrounding the bundles must then be considered 

 as corresponding to the primary medullary rays. The division of the 

 tissue systems in the stems of the higher plants into epidermis, primary 

 cortex, and central cylinder, brings with it a corresponding division of the 

 fundamental tissue into the fundamental tissue of the primary cortex 

 and the fundamental tissue of the central cylinder. Wherever there is 



