114 SECONDARY THICKENING. 



Illustration : The ROOTS of ORCHIDACEAE, CYPEBACEAE, or 



LlLIACEAE. 



PREPARATION : Make transections of the young and old roots 

 of any of these plants, and note the changes that may have 

 occurred. DeBary, pp. 360-362, 618; Annals of Botany, Vol. 7, p. 21. 



II. Dicotyledons : The changes in the primary structure of 

 Dicotyledonous roots usually take place soon after the arrangement 

 of the primary tissues, and continue during the activities of the 

 plant. 



Illustration : Roots of various sizes from the RANUNCULACEAE 

 and SAPINDACEAE. 



PREPARATION : Treat the material as directed for herbaceous 

 and woody stems. 



Compare the sections of different ages, and observe that the 

 secondary thickening has brought about the following changes: 



1. Formation of a cambium ring, by the longitudinal division 

 of cells on the axial side of the Phloem areas, and the extension of 

 the same laterally to their union with the next areas, over the ends 

 of the xylem rays. VanTieghem, Ann. Sci. Nat., 5 ser., Tom. XIII, 

 p. 185, pi. 3, 4, 8. 



2. Irregular outline of the cambium rimj corresponding to 

 to the sinuses between the xylem areas. 



3. Disappearance of these sinuses in the older roots, and the 

 nearly circular outline of the cambium rimj. 



4. After these changes, the thickening corresponds to that in 

 the stem. 



5. In the mature roots; the wood (xylem), bast (phloem), and 

 alternating with these plates of parenchyma, the medullary rays. 

 DeBary, p. 473. 



VASCULAR SYSTEfl OF LEAVES. 



The arrangement of the VASCULAR SYSTEM in the leaf, with its 

 various modifications, or variations in the different groups of 

 plants, presents a subject too extended to be treated in this brief 

 course. 



Read carefully, Strasburger, pp. 160-169 ; DeBary, pp. 296- 

 307, 372-373 ; Goodale, p. 155. 



In structure the FIBRO- VASCULAR BUNDLES are much the same 



