52 PLANT LIFE. 



originate exclusively in the stem, and consequently are 

 not leaf-traces or prolongations of the leaf-bundles. The 

 course followed by bundles in the stern is various, but may 

 be reduced to three types, (i) The leaf-traces combine 

 in the stem to form a single axial or central bundle ; this 

 is the least usual arrangement, occurring in a few ferns 

 and aquatic flowering plants. (2) The numerous bundles 

 of each leaf on entering the stem remain isolated at first, 

 grow towards the centre and downwards for some distance, 

 then become thinner and bend outwards ; finally coalescing 

 at a point considerably below that at which they entered 

 the stem. This arrangement is characteristic of the group 

 of flowering plants called Monocotyledons, and is most 

 highly developed in palms. (3) The comparatively few 

 bundles from each leaf bend downwards soon after entering 

 the stem, and grow downwards parallel to each other, 

 forming a broken ring about equidistant from the centre 

 and the circumference of the stem, thus separating the 

 previously . existing fundamental tissue into two portions: 

 that included within the ring of bundles is called pith or 

 medulla ; the portion exterior to the ring is the cortex, which 

 is bounded on the outside by the epidermis. Those 

 portions of fundamental tissue lying between the isolated 

 fibro-vascular bundles forming the ring are called medullary 

 rays, and are continuous with the pith and cortex. This 

 mode of arrangement is characteristic of the two groups of 

 flowering plants called Gymnosperms and Dicotyledons. The 

 above types are not sharply defined, but connected by 

 intermediate forms. 



A typical fibro-vascular bundle consists of two kinds of 

 permanent tissue; the wood or xylem, and the bast or 

 phloem ; and in the greater number of cases they are arranged 



