THE TISSUES OF THE STEM 



61 



ventilation, and of interchange of materials in a radial direction 

 through the vascular column. 



The complete ring of cambium is capable of indefinite activity of 

 growth and cell-division. Cells produced on its inner margin develop 

 as wood-vessels, wood-fibres, or wood-parenchyma by changes such 

 as those above described, and they form additions to the internal 

 column of wood. Cells produced on the outer margin of the cambial 

 zone develop as sieve-tubes, bast-fibres, or bast-parenchyma, by 



1 



sm 



Fig. 39. 



Medullary rays of the Pine (em, tm, sm), seen in a radial section through the 

 cambial region (c). Phloem to the left, xylem to the right. ( x 240.) Strasburger. 



changes such as those above described, and they form additions to 

 the external zone of phloem or bast. The internal additions are the 

 more active, so that the woody column grows more rapidly than the 

 bast. This is suggested by Fig. 37, C, and the proportions are actually 

 shown for the Lime in Fig. 40. The relations of the wood and 

 bast to the cambium, and that irregularity of arrangement of the 

 several constituents which is usual for Dicotyledons, are suggested 

 in detail in Fig. 41. The chief bulk of the wood and bast consists 

 of tissue-elements elongated in a vertical direction. This structure 

 would make radial interchange a difficulty were it not for the fact 

 that the medullary rays penetrate radially inwards and outwards 



