250 DESCRIPTIVE CATALOGUE 



of elongated pitied elements and excessively thickened elements, 

 with a small quantity of parenchyma. The sequence of the 

 various elements is remarkably uniform : one thick- walled 

 element alternates with two thinner-walled elements on the 

 same radius (text-fig. 73, and Pis. XXIV & XXV). At 

 the corner between four adjacent thin-walled elements the 

 small space is sometimes filled by small parenchyma-cells, 

 which sometimes stretch tangentially between the pairs of 

 pitted elements. Xo other parenchyma seems to be normally 

 present, though near the cork-bauds the cells arc sometimes 

 irregular. 



The regular alternation of thick- and thin-walled elements 

 results in a definite tangential zoning of the structure, which 

 can be seen in 1M. XXV, fig. 1. 



The medullary rat/s, which penetrate the whole tissue, are 

 wide and conspicuous, principally 2-4, up to 7 elements distant, 

 and entirely uniseriate. They are fairly high in tangential 

 section, running from about 4 up to 30 cells in vertical 

 sequence. 



In transverse section, as well as in the cut surface of the 

 block, the naked eye readily detects what appear to be well- 

 marked and regular annual rings or growth-tone*. These arc 

 roughly equidistant, and run nearly straight and parallel with 

 the longer diameter of the section. I'nder the microscope the 

 apparent " growth-rings " are less conspicuous, but are recog- 

 nisable as bands of two or three very narrow cells. While in 

 transverse section these narrow elements are not unlike the 

 narrow elements common in autumn tissues, in longitudinal 

 section they are short and have every characteristic of cork-cells 

 (PI. XXIV, fig. l,n.). 



There appears to be no indication of true growth-rings in the 

 tissues. 



While the bands of narrow cork-elements are arranged so 

 that to the naked eye they appear very regular, their course is 

 less so tinder the microscope, where irregular loops etc. can be 

 seen, locally giving rise to small groups of irregular cells. 



DETAILS OF ELEMENTS. The tissue consists of three kinds 

 of elements : (a) excessively thickened, elongated elements; 

 (/>) thinner-walled elements (sieve-tubes ?) with large pits ; 

 (<) small parenchyma-cells. 



