TISSUES OF ANG10SPERMS. 447 



bium cells by the formation of transverse partitions is very 

 clear in this figure. 



547. In the trees and shrubs of cold climates, or of 

 those in which there is one annual period of growth, fol- 

 lowed by a period of rest or the cessation of growth, the 



FIG. 327. 



Pig. 326. A tangential section of the cambium region of Cytisus Laburnum, a, 6, 

 c, d, cambium eclN enclosing the section of a medullary ray ; ft, /i, trache'ides belong- 

 ing to the xylem. x 145.- After De Bary. 



Fig. 327. Tangential section of the inner phloem region of the same stein as Fig. 

 32G. , ,*, sieve vessels ; m, section of a small medullary ray ; the remaining parts 

 of the figure are phloem parenchyma, x 145. After De Bary. 



processes described above take place each year, giving rise 

 thus to an annual layer of xylem (wood) outside of the pre- 

 viously formed xylem cylinder, and an annual layer of 

 phloem (bark) inside of the phloem cylinder. In the wood 

 these layers are generally quite well marked, and in cold 

 climates they enable us to determine with accuracy the age 



