66 



AGRONOMY 



The way in which the bundles are arranged in stems 

 makes it possible to separate the flowering plants into two 

 very natural groups. In one, called 

 the monocotyledonous group, these bun- 

 dles are scattered throughout the cen- 

 tral pith. A cornstalk or an asparagus 

 stem is a good example of this. In 

 the other, known as the dicotyledonous 

 group, the bundles are arranged in a 

 FIG. 42. Cross section circle. The sunflower or any of our 

 of young dicotyledon forest trees illustrates this type. The 



stem showing the circle dicot ledong are further distinguished 

 of nbrovascular bundles * 



by the presence of a ring of cambium, 



which cuts through each bundle in the circle and separates 

 the two sets of tubes. The part of the bundle inside the 

 cambium is the wood and its tubes are ducts; the part out- 

 side the cambium is bast, or phloem, and its tubes are called 



FIG. 43. Part of a cross section of year- 

 old basswood twig 



b, bark ; pa, parenchyma ; ph, phloem ; 



c, cambium ; med, medullary rays ; 



w, wood ; p, pith 



FIG. 44. Section of oak wood 



showing the annual rings and 



medullary rays 



sieve tubes. Water and food materials pass upward through the 

 ducts, but elaborated food is transported downward through 

 the sieve tubes. The wedges of pith that extend outward 



