77 



CHAPTER III. -STRUCTURE AND DEVELOPMENT OF 

 PLANT MEMBERS. 



72. If the young stem of a s, tem of 



Gymnosperm, or Dicotyledon, is* cut across after the colla- pfrmg 03 " 

 teral vascular bundles have become differentiated, the latter and Dicoty-- 

 will be found to be in a circle around the stem, the xylem of Ied ns. 

 each bundle being nearest the centre of the stem and the 

 phloem nearest the circumference. The fundamental tissue 

 occupying the centre of the stem, inside the circle of vascular 

 bundles, forms the pith, while that between the bundles con-^ " ary 

 stitutes the primary medullary rays. In perennial stems, 

 and sometimes also in annual stems, after the differentiation 

 of the primary vascular bundles, a further process sets in 

 termed secondary growth in thickness. This is accomplished 

 by the activity of the cambium of the vascular bundles which 

 is a layer of elongated, delicate-walled cells, full of protoplasm, 

 situated between the xylem and phloem, and by the forma- 

 tion and growth of a similar thin layer of cambium in the 

 primary medullary rays between the bundles, which, uniting 

 with the cambium of the bundles, forms a complete ring 

 around the stem. From this cambium new cells are 

 continually being formed, through the division of the 



tt 



cambium cells by tangential walls, both on the inside and 

 outside of the ring, towards the centre and circumference 

 of the stem respectively ; the former develop into xylem and 

 the latter into phloem elements. The primary medullary rays 

 are thus broken up by the formation of new xylem and phloem 

 elements from the cambium between the bundles. These 

 elements, however, follow a more or less sinuous course longi- 

 tudinally leaving a net-work of narrow elongated openings 

 which are filled by rows of cells of fundamental tissue, chiefly 

 parenchyma, running horizontally from the centre of the stem 

 towards the circumference. These cells constitute the second- 

 ary medullary rays and as the cambium cells opposite these 

 rays continually form new medullary ray cells opposite thorn, 

 both on the inside and outside of the cambium ring, a medul- 

 lary ray, once started, is kept open as the stem increases in 

 thickness and extends both through xylem and phloem elements. 

 As the circumference of the stem gradually enlarges, now 

 medullary rays originate from the cambium between those al- 

 readv formed. By mean of the medullary ravs communication 



tf w , 



