MORPHOLOGY 



121 



layers of thin-walled cells full of protoplasm. Of these cambial 

 layers the middle one is termed the initial layer; and from it 

 proceeds the development of new tissue elements. Its activity consists 

 in a continued division by means of tangential and occasionally radial 

 walls. The new cells thus continuously given off toward the xylem 

 and phloem sides of the bundles experience another tangential division 

 before attaining their definite form as elements of the xylem or phloem 

 portions. The vascular bundles of Gymnosperms and Dicotyledons 

 capable of secondary growth are usually arranged in a circle. After 

 the cambium in the bundles begins its activity, a zone of tangentially 

 dividing tissue, called the interfascicular cambium, develops in the 

 primary medullary rays between the original bundles, and, uniting 

 with the cambium in the bundles, forms a complete cambium ring. 

 This cambium ring is thus composed of two distinct forms of meri- 

 stematic tissue ; for while the cambium of the bundles or the FASCI- 

 CULAR CAMBIUM consists of primary meristem (p. 90), the connecting 

 zone of interfascicular cambium is of later development, and is con- 

 sequently a secondary meristem (p. 90). A cross-section of a young 

 stem of Aristolochia Sipho, with the cambium ring in process of 

 formation, is represented in Fig. 135; in Fig. 136 a single bundle 

 of the same cross-section, more 

 highly magnified, shows the fas- 

 cicular cambium in a condition 

 of active division. Within the 



bundle may be seen two large / , ^ js^a \^ iom> TffiferiY 1 \ • j-. 

 vessels (m"), in a still incom- / J i 3K/^> ^flS-^rT 



plete state; while in the ad- lf'j_ij /vvV±a\~ i' 



joining fundamental tissue the lil;Sp B ' ;/ ~\0^rTiXLlsl 



cells which give rise to the in- \U" i "3\~, ^f^ /TV . 7 



terfascicular cambium may be ||,iCJ>\^g7 ^^ Y0? ' 14— c 

 plainly distinguished. All the 

 tissue arising from the inner 

 side of the cambium ring goes 

 to form the WOOD, while that 



produced On the Outside is ]Tjg. 135. —Transverse section of a stem of Aristo- 



termed BAST The Vascular lochia Sipho 5 mm. in thickness. )ii, Medulla ; 



„ ,' j £ ,1 fv, vascular bundle ; vl, xylem ; cb, phloem ; fe, 



portions Ol the WOOd form tne fascicu]ar cambium; i/c, interfascicular cam- 



"WOOD STRANDS, the sieve por- bium ; p, phloem parenchyma ; pc, pericycle ; 



ring of sclerenehyma ; c, starch - sheath' ; 

 c, primary cortex ; cl, collenchyina in primary 

 cortex. ( x 9.) 



tions within the bast the BAST 

 STRANDS. By the activity of 

 the interfascicular cambium, the 

 primary medullary rays are continued throughout both the wood 

 and bast. As the wood and bast strands enlarge, secondary 

 medullary rays are developed from the fascicular cambium. In 

 one direction the secondary medullary rays terminate blindly in the 

 wood, and in the other in the bast ; the later they develop, the 



