CHAP, i.] PLANT ARCHITECTURE. 45 



down the stem. Grasses, sedges, lilies, and palms are 

 typical Monocotyledons, and in the palms the mono- 

 cotyledonous stem structure reaches its maximum of de- 

 velopment, the bundles on entering the stem from the 

 leaves at first curve towards the centre, then grow down- 

 wards for some distance, eventually curving back towards 

 the periphery and anastomosing by their tapering ends 

 with lower and older bundles, as shown in fig. 13. In 

 palms the stem is cylindrical and terminated by a bud, 

 the only one possessed by the tree ; at a very early age 

 the stem acquires its full diameter, and henceforth the 

 terminal bud gives origin annually to about the same 

 number of leaves, which consequently supply an equal 

 number of fibro-vascular bundles that only proceed for 

 a short distance down the stem, the lower portion, as 

 it is left behind by the apical bud, being unable to 

 increase in diameter on account of its bundles being 

 closed. 



(2) Dicotyledons. In the dicotyledonous stem a ring 

 of detached vascular bundles, each consisting of an ex- 

 ternal phloem and an internal xylem portion, appears 

 in the fundamental tissue about midway between the 

 centre and the circumference of the stem ; that portion 

 of fundamental tissue lying outside the ring of vascular 

 bundles, and bounded externally by the epidermis, is 

 called the primary cortex, the central portion surrounded 

 by the vascular ring is the medulla or pith, while those 

 portions of fundamental tissue that pass between the ring 

 of isolated vascular bundles and connect the pith with the 

 primary cortex are called medullary rays. The xylem 

 and phloem elements of the fibro-vascular bundles do 

 not lie in contact with each other as in Monocotyledons, 



