Plant World Continued 



233 



Fig. 132. 



Arrangement of Secondary Tissues 

 in Roots and Stems. 



1. Peridem (bark). 2. Phellogen. 3. 

 Phelloderm (bark). 4. Phloem elements. 5. 

 Cambium. 6. Xylem elements. 7. Medullary 

 rays. Compare with Fig. 129. (After C. W. 

 Ballard's "Vegetable Histology." Courtesy of 

 John Wiley and Sons.) 



This causes each fibro-vascular 

 •bundle now to consist of xylem and 

 phloem elements, separated from 

 each other by a thin strip of cam- 

 bium. Such bundles, which have 

 been completed by the cambium, are 

 called complete fibro-vascular bun- 

 dles, while those not so completed 

 are known as incomplete fibro-vas- 

 cular bundles. (Fig. 134.) 



As the cambium continues 

 to grow constantly, the plerom 

 parenchyma becomes almost en- 

 tirley replaced by xylem. The new 

 hbro-vascular bundles develop in the broad primary medullary rays. 



The stem and root development differ somewhat. There are no 

 root hairs or root-caps on the stem. The primary stem epidermis often 

 possesses stomata (breathing pores), 

 while the root does not. The 

 parenchymal cells of the stem often 

 contain chloroplasts which the 

 parenchymal cells of the root never 

 do. Then, too, the root has no hypo- 

 dermis (mechanical tissue imme- 

 diately underneath the epidermis). 

 There is usually no endodermis in the stem, though there is in the root. 

 The plerom zone of primary stems differs considerably from that of 



primary roots both in the arrange- 

 ments and development of tissues. 

 All fibro-vascular bundles in the 

 plerom region of the primary stem 

 are complete, showing phloem, 

 xylem, and cambium elements 

 throughout their entire period of 

 growth. This means that the pri- 

 mary fibro-vascular bundles of the 

 stem are really equivalent to the 

 secondary bundles of the root. The primary stem structures, described 

 above, serve throughout the life of the plant only if such plant is an 

 annual. In perennials ( ), a better and 



more durable covering tissue must be developed. In these, the primary 

 epidermis is replaced by periderm tissues which have been produced by 

 a phellogen which in turn developed in the primary cortex. The peri- 



od 



Fig. 133. Diagram Showing the Method by 



which the Cambium Layer Produces Wood 



Cells on its Inside and Bark Cells on 



the Outside. 

 be, the cells of the bark; c, cambium cells; 

 xvc, the wood cells. 



Fig. 134. Completion of Fibrovascular Bundles. 

 F, Completed fibrovascular bundle. 1. 

 Xylem elements. 2. Cambium. 3. Phloem ele- 

 ments. (From C. W. Ballard's "Vegetable 

 Histology," Courtesy of John Wiley & Sons.) 



