Ironside. — -Anatomical Structure of N .Z. Piperaceae. 



341 



The cambium shows especially well the tangential divisions for wood 

 and bast formation. The cambial or merismatic region is continuous 

 round the stem in the outer ring, though it is only where the cambial layer 

 passes through the bundle that it gives rise to wood and bast. Between 

 the bundles the tangential divisions increase to the medullary ray. In no 

 instance are secondary bundles found between the primary bundles. 



The wood, as is seen in longitudinal sections, shows pitted and scalari- 

 form vessels and pitted wood-fibres, besides annular and spiral elements 

 of the protoxylem. Some of the bundles show the primary elements of 

 the wood to be distinctly scattered ; longitudinal sections and oblique 



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Fig. 1. Outer tissues, young stem : a. epidermis ; b, collenchyma ; c, sclerenehyina - 



X 150. M. excels ipn. 

 Fig. 2. Portion of sclerenehyina band : a, sclerenehyina ; b, medulla at break in rinsi 



passing into medullary raj r . x 44. M. excelsum. 

 Fig. 3. Inner portion of bundle from outer ring, showing scattered primary xylem : 



a, secondary xylem ; b, wood-fibres ; c, c', protoxylem to sides and to inside 



of bundle ; d, boundary of sclerenchymatous band. X 44, M . excelsum. 

 Fig. 4. Bundle from stem : a, phloem ; b, cambial region ; c, xylem. x 150. Pepe- 



romia Endlicheri. 



transverse sections show the protoxylem in many cases to be endarch, 

 but there are frequently scattered elements to the sides of the bundle as 

 well (see fig. 3). The question is. Is this a trace of centripetal wood- 

 formation ? 



In the medullary bundles secondary-wood formation does not take 

 place to the same extent as in the outer ring. 



On the inside of the wood, in the bundles of both rings, are a fair 

 number of parenchymatous elements. At first sight they look like internal 

 phloem, but on examination of young stems, and especially of those bundles 

 of the inner ring where there are very few lignified elements, it is seen that 

 these are only parenchymatous elements. Some bundles show only 6 to 8 

 lignified elements in a mass of tissue staining deeply with haematin. As 

 more wood is formed, more of this becomes lignified also, but a little to 

 the inside always remains unlignified. In bundles where the primary 

 xylem is scattered, the elements are scattered through parenchymatous 

 elements. 



The medullary rays are very wide, sometimes 5 to 6, often 10 to 12 

 elements wide, parenchymatous mainly : in older stems slightly lignified. 



