THE DICOTYLEDONOUS STEM 



99 



111 the centre of the stem is the pithy stele composed of 

 parenchyma cells. The libro-vascular bundles are also separated 

 by masses of thin-walled cells called the medullary rays (Fig. 

 74), and the entire structure is enclosed in an epidermal mass 

 of cells known as the bark. In soft stems of the herbaceous 

 plants, geranium, etc., the bundles are separated by thick medul- 

 lary rays while in hard, woody stems of the tree, the rays are 

 very thin. In the stems or trunks of young trees, the bundles are 



Fig. 73. — Sieve tissue. 

 '4. — Cross section of dicotyledonous woody stem, showing annular rings, medullary 

 rays and bark. 



few and the rays thick, but the number of bundles increases and 

 the rays become thinner and thinner with age. If we cut across 

 a large stem, i.e., the trunk of a tree, we can readily recognize 

 the small point in the centre which we call the pith or stele, 

 the colored heart-wood, the sap-wood, and the bark. We will 

 also note the great number of circles known as the annular 

 rings. (Fig. 74.) In many trees, each ring represents one 

 year's growth. We will also observe a large number of lines 

 which radiate from the centre and are known as medullary rays. 

 (Fig. 74.) In young stems these rays are composed of pareii- 



