CHAP. I.] 



PLANT ARCHITECTURE. 



47 



but are separated by a thin zone of ftmdamental -tissue 

 that has received the name of cambium, and which by 

 rapid cell-formation adds periodically to the xylem and 

 phloem elements of the bundle, which thus increases in 

 size from year to year. Bundles furnished with cambium, 

 and thus capable of adding to their substance periodically, 

 are said to be open. 



In perennial plants increase in thickness of the stem 

 commences with the activity of the cambium situated 



Fig. 15. Horizontal section through 

 stem of a Dicotyledon, the melon, in- 

 cluding one fibro-vascular bundle, 

 which is completely surrounded by fun- 

 damental tissue composing the follow- 

 ing parts : M, pith ; BM, medullary 

 rays ; PC, primary cortex ; the wedge- 

 shaped bundle consists of L, the phloem, 

 followed internally by the xylem, the 

 two being separated by a thin band of 

 cambium. The large circles in the 

 xylem correspond to the sections of 

 vessels. .E, the epidermis. (Magnified.) 



between the phloem and xylem of each vascular bundle, 

 by repeated bipartition of the cells of the cambium a 

 mass of tissue is formed that becomes differentiated into 

 the elements of phloem and xylem respectively. Owing 

 to the position of the cambium, it will be observed that 

 the new elements of the xylem or wood will be added to 

 the outside of the already existing portion, whereas the 

 additions to the phloem will be on its inner surface, but 

 very much more material is periodically added to the 

 xylem than to the phloem, so that while the former may 



