113 SECONDARY T 



5. The isolation of the bundles, which still retain their indi- 

 viduality, on account of the large Medullary Kays. 



6. That the stem has nevertheless been thickened, after the 

 usual mode in Dicotyledons, i. e., by the addition of a new lajer, in 

 a uniform manner, to the bundles. 



Compare the sections of an Oak or Beech twig where the indi- 

 viduality of the bundles is not so well preserved. Sachs' Text 

 Book, p. 129 ; Vines' Text Book of Bot., pp. 1G7, 191 ; Goo.lule. 

 p. 137. Make sketches of the transection in Preparation Second 

 to illustrate the changes which have taken place in the thickening 

 process. 



II. In Honocotyledonous Stems. 



Illustration: STEMS of Smilax Mspida. Sections from (1) 

 one year old, (2) several years old. (Fig. 28.) 



It is to be noted that there is no cambium ////// in Endogens, 

 and the bundles being closed, i. e., without permanent cambium, 

 are not able to increase, by secondary growth, the size of the stem. 

 Compare the bundles in the stems of the two preparations and note 

 the changes, if any, tjjat have occurred during the development of 

 the plant. In many Monocotyledons there exists a ring of meris- 

 tematic tissue in the cortex, just beneath the epidermis, where new 

 closed bundles are formed, thus enabling the stems to increase in 

 size. 



Vines' Text Book, p. 202; Goodale, p. 135; DeBary, p. 618; 

 Annals of Botany, Vol. VII, p. 21. 



Make transectious of the stems of I* inn* which shall include 

 those from one to several years' growth. Note the changes that 

 have taken place in the thickening of the stem. 



Strasburger, pp. 115, 116; Vines' Text Book, p. 193; DeBary, 

 pp. 461, 567. 



The study of the development of Fern Steins will not be 

 included in this course. DeBary, p. 623. 



Secondary Thickening in Roots. 



I. Honocotyledons: As in the case of MONOCOTYLEDONOUS 

 STEMS, so with most of their roots, no true cambium ::<>n<' is found, 

 but the roots retain their primary differentiation throughout their 

 existence, except that the tissue may become firm by age. 



