128 



BOTANY 



condition (Caltimarieae, Sigillarieae, Lepidodendreae) they first became 

 of general occurrence in the Phanerogams. In them the formation 

 of secondary tissues is almost exclusively confined to the roots und 

 stems ; secondary growth is met with in some foliar structures but 

 only in a slight form. 



Cambium ( 124 ). The primary and secondary meristems, the 

 activity of which gives rise to secondary growth, are given the name 

 of CAMBIUM. These tissues persist in a meristematic condition and 

 undergo successive divisions by walls parallel to one another, so that the 

 cells produced from them are arranged in radial rows. A similar mode 



of division characterises the prim- 

 ary meristem in the procambial 

 strands, which give rise to col- 

 lateral vascular bundles (cf. p. 

 113), and this might also come 

 under the term cambium. It is 

 advisable, however, to restrict 

 this term to meristems giving 

 rise to secondary growth. As a 

 rule the cambium forms new 

 tissue-elements on both sides, but 

 cases are not wanting in which 

 this takes place to one side only. 



As a rule 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 forma- 

 tion, 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 continuously given oft' 

 from the initial cells 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 which undergo secondary 

 growth are usually arranged in a circle. After the cambium in the 

 bundles begins its activity, a zone of tangentially dividing tissue, 



Fio. 137. Transverse section of a stein of Aristc 

 lochia Sipho 5 mm. in thickness, m, medulla 

 fv, vascular bundle ; el, xylem ; cb, phloem ; /c 

 fascicular cambium ; \fc, interfascicnlar cam 

 bium ; p, phloem parenchyma ; pc, pericycle 

 sk, ring of sclerenchyma ; e, starch -sheath 

 c, primary cortex ; cl, collencliyma in primary 

 cortex, (x 9.) 



