SECT. I 



MORPHOLOGY 



139 



As a vule a single persisting initial layer is present in the cambium from which 

 on one or both sides the rows of cells take their origin. In many cases in which 

 the cambial activity is confined to one side, according to J. C. Schoute, the 

 original initial cells are used up in the tissue formation, and new initial cells arise 

 from adjoining cells of the ground-tissue. 



Growth in Thickness of the Stem in Gymnosperms and Dicoty- 

 ledons. — The cambium of the open vascular bundles of Gymnosperms 

 and Dicotyledons, which exhibit a growth in thickness, commences 

 its activity almost directly after the formation of the primary tissue. 

 The primary meristem remaining between the xylem and phloem of 

 the bundle continues its active 

 growth as the cambium. Its 

 cells are full of protoplasm and 

 continue to divide by means of 

 tangential and occasionally radial 

 walls. The new cells thus con- 

 tinuously given off from the initial 

 cells toward the xylem and 

 phloem sides of the bundles 

 undergo another tangential divi- 

 sion, or several, before attaining 

 their definite form as elements 

 of the xylem or phloem portions. 



The vascular bundles of Gym- 

 nosperms and Dicotyledons which 

 undergo secondary growth are us- 

 ually 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 tlie 

 primary medullary rays between 

 uniting with 

 cambium 



Fk;. 147. — Transverse section of a stem of Aristo- 

 lochia Sipho 5 mm. in thickness, m, Medulla ; 

 /(', A-ascular bundle ; d, xylem ; ch, pliloem ; fc, 

 fascicular cambium ; ifc, inteifascicular cam- 

 bium ; 'p, phloem parenchyma ; pc, pericycle ; 

 sfc, ring of selerenchyma ; e, starch-sheath ; c, 

 primary cortex ; cl, coUenchyma in primary 

 cortex, (x 9.) 



ring. 



the original bundles, and, 

 the cambium in the bundles, forms a complete 

 This cambium ring is thus composed of two 

 distinct forms of meristematic tissue ; for while the cambium of the 

 bundles or the FASCICULAR CAMBIUM consists of primary meristem 

 (p. 107), the connecting zone of interfascicular cambium is of later 

 development, and is consequently a secondary meristem (p. 107). 

 A cross-section of a young stem of Aristolochia Sipho, with the cambium 

 ring in process of formation, is represented in Fig. 147 ; in Fig. 148 

 a single bundle of the same cross-section, more highly magnified, 

 shows the fascicular cambium with the interfascicular cambium to 

 either side in a condition of active division. Within the bundle 

 may be seen two large vessels (m"), in a still incomplete state ; while 

 in the adjoining primary meaullary rays the cells Avhich gave rise to 



