CAPSELLA BURSA-PASTORIS. 239 



changes in the primary structure. Certain of the deli- 

 cate parenchyma cells lying between the xylem and 

 phloem elements undergo repeated division, producing 

 wood and bast tissue. The layer of cambium cells thus 

 begun on either side of the original plate of xylem soon 

 unites with its neighbor at the ends, and forms a closed 

 cambium ring. This ring has the properties of the 

 cambium layer of the stem, as in Pinus, and by means 

 of it the root is enabled to increase in thickness 

 to any extent. It does not, however, as in the 

 stem, produce its phloem exclusively on the outside and 

 xylem on the inside of the ring, but they lie side by 

 side in radiating lines, the number of these lines 

 increasing with the increase in circumference. 18 



The fundamental system in the stem of dicotyle- 

 dons is much more differentiated than is usual in mon- 

 ocotyledons. It is divided into an inner and outer 

 region by the fibro-vascular system, in the latter of 

 which various tissues may be developed, such as col- 

 lenchyma, fibrous tissue, etc. In the case of Capsella 

 the principal modification of the parenchyma of the 

 fundamental system is the development of the abun- 

 dant fibrous tissue (sometimes referred to scleren- 

 chyma), which embraces the xylem of the bundles 

 and arches between the phloem areas. In the fibro- 

 vascular system the chief characters of the dicotyle- 

 donous stem appear. The wedge-shaped bundles are 

 not scattered through the fundamental tissue, but 

 are arranged in a zone concentric with the surface of 



13 On the secondary thickening of roots see DeBary, Comp. Anat , p. 

 473 ; Goodale, Physiol. Bot, p. 113 ; VanTieghem, Ann. Sci. Nat., ser. 

 5, xiii, p. 185. 



