THE STEM 



103 



Fig. 119. — 'riaiisvcrso section of a 

 very young stem of burdock, showing fibro- 

 vascular bundles not completely united 

 into a ring : e, epidermis ; c, primary cor- 

 tex ; /, a ring of fibrovascular bundles ; 

 p, central cylinder of parenchyma. 



e, Fig. 119; (2) the primary cortex, c; (3) a ring of fibro- 

 vascular bundles, /; and (4) a central cylinder of paren- 

 chyma, p. In some specimens there will be a fifth region, the 

 pith, which will appear in 

 the section as a white cir- 

 cular spot in the center of 

 the parenchyma. 



In specimens a little older 

 than the one shown in Fig. 

 119, a narrow circular line 

 will be seen running through 

 the ring of bundles nearly 

 midway between their inner 

 and outer extremities, con- 

 necting them into an un- 

 broken circle around the 

 central cylinder. This is 

 the camhiu7n layer, which supplies the vascular region with 

 materials for new growth, and thus enables dicotyl stems to 

 increase in diameter by the successive addition of fresh 

 vascular rings from year to year. 



Examine in the same way a vertical section, and find the 

 parts corresponding to those shown in Fig. 119. Make en- 

 larged sketches of both sections, labeling the various parts 

 observed. 



ii6. Minute structure of a dicotyl stem. — Place suc- 

 cessively under a high power of the microscope thin trans- 

 verse and longitudinal sections of the stem just examined, or 

 such other specimen as the teacher may provide. Bring one 

 of the fibrovascular bundles into the field, and try to make 

 out the parts shown in Figs. 120 and 121. The corresponding 

 parts in the two sections are indicated by the same letters. 

 Notice the cortex, R, on the outside and the pith, M, on the 

 inside ; between these, the cambium, C, the xylem, or woody 

 tissue, included between the radiating lines X, and the newer 

 tissues composing the phloem between the lines P. The 



