232 



GENERAL BIOLOGY 



Fig. 132. Arrangement of Secondary Tissues 



in Roots and Stems. 



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

 Phelloderm (bark). 4. Phloem elements. 6. 

 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 

 growing constantly, the plerom 

 parenchyma becomes almost en- 

 tirely replaced by xylem. The new 



fibro-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 Fig . 133 . Diagram showing the Method by 



do. Then, too, the root has no hypo- wh ^ s th e n Srt^^f B^^L^ 



dermis (mechanical tissue immedi- bc the cells J^ES??'*. cambium ceih: 



ately underneath the epidermis), vac, the wood ceils. 



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 primary 

 fibro-vascular bundles of the stem 

 are really equivalent to the sec- 

 The primary stem structures 



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). 



bundles of the 



ondary bundles of the root, 

 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- 

 derm of stems is often ruptured and cast off as the inner tissue expands. 

 This does not occur in roots. When such casting off takes place, the 



