5 2 THU PLANT CELL. 



bundles, xylem upon its inner aspect and phloem upon its outer 

 aspect, but, as will be readily understood, there is no protoxylem 

 or protophloem to be seen in these portions, as they are 

 secondary formations (see Figs. 30, 31, and 32, ifc). 



The fascicular and interfascicular cambium unite during the 

 first year's growth, and thus is produced a complete ring of 

 meristem in stem (or root) which gives rise to fresh annual rings 

 of xylem and phloem (see Figs. 31, 32, 33). The whole process 

 is known as secondary thickening. 



In roots, although the ultimate disposition of xylem, cambium, 

 and phloem is similar to that just described, the protoxylem and 

 protophloem alternate with one another, and are not situated 

 upon the same radial lines in the young root. 



Fig. 35. — Diagram of a Transverse Section through a Young Mono- 

 OOTYLEDONOUS STEM — e, Epidermis ; /, fundamental tissue ; h, fibro- 

 vascular bundles (black = xylem, dotted = phloem). 



In the Monocotyledons ;uid higher Ferns no persistent ring of 

 meristem analogous to the cambium of Dicotyledons exists, and 

 the fibro-vascular bundles are made up of xylem and phloem 

 formed early from certain rudimentary elements ; generally 

 speaking, in Monocotyledons, the phloem is found between th& 

 arms of a V-shaped mass of xylem (see Fig. 35) ; whilst in the 

 higher Ferns the phloem surrounds a centrally situated mass of 

 xylem in each separate bundle. Thus the bundles of JMonocoty- 

 ledons and Ferns are termed " closed " bundles, in contradistinc- 

 tion to those of Dicotyledons and Conifers, which are known as 



