STEMS OF MONOCOTYLEDONS 137 



vascular bundles from that met with in dicotyledons. Instead 



of being arranged in a single ring, they appear scattered in 



several irregular circles throughout the ground 



tissue (Figs. 69 ,and 70). Usually the cortex is 



very narrow and inconspicuous and a distinct pith 



is rarely present. The bundles are common to 



leaf and stem as in dicotyledons, but on entering 



from a leaf they bend gradually inwards to near 



the middle of the stem, and then generally curve 



outwards again, finally joining other bundles 



near the outside of the stem. ve^^ec^o^W 



In addition to these differences, measurement stcm°'"(Thre^e%i'^s 

 shows that the older parts of such stems which °""^^' "^-J 

 have ceased to grow in length are no thicker than the young parts 

 near the tip ; that is to say, the stems of most monocotyledons 

 do not increase in thickness when once they have ceased to grow 

 in length. This incapacity for growth in thickness is due to the 

 fact that the vascular bundles do not possess a cambium tissue, nor 

 is such meristem developed in the ground tissue except in a few 

 special cases which we cannot deal with here. Vascular bundles 

 in which no cambium is present are known as closed bundles. 



In most grasses the vessels of the wood of each bundle are 

 few in number, and in transverse section appear arranged in 

 the form of a V (Figs. 70 and 71) ; the vessel nearest the centre 

 of the stem is annular, the others being spirally thickened. 

 Tracheids are not uncommon and thin-walled wood-parenchyma 

 is always present. 



The bast which lies between the free limbs of the V-shaped 

 wood consists entirely of sieve-tubes and companion-cells. 

 The ground-tissue immediately surrounding each bundle is 

 generally thick-walled and gives mechanical support and pro- 

 tection to the soft parts of the bundle. Similar thickened ground- 

 tissue in larger or smaller amount is met with beneath the 

 epidermis, the rest being thin-walled tissue. 



