152 Transactions. — Botany. 



the ordinary cortex of ground parenchyma ; the pith consists 

 of large, thni-walled cells. 



In an older stem, of a year's growth, there is a develop- 

 ment of cork, rectangular cells arranged regularly in rows, 

 with brown walls slightly thickened arising from the phellogen 

 layer (fig. IV., ph.). At irregular intervals the cork splits, 

 and leaves crevices through its whole thickness. There are 

 no lenticels of the ordinary form. The cortex is a fairly wide 

 band of colourless, thin-walled cells,, more or less circular ; 

 they contain no starch, and no visible contents other than a 

 clearly defined nucleus. The resin-passage, abutting on the 

 phloem, is very well marked, and is surrounded by small cells 

 with granular contents and thin walls. The process of their 

 formation is seen to be this : A large granular cell, bordering 

 on the vascular cylinder, divides, and the daughter cells 

 divide again, the result being four granular cells, which go 

 on dividing, and tend to separate round an elliptic cavity 

 (fig. IV., A, B, C, D). 



The phloem of the young stem is seen to consist wholly of 

 thin-walled elements, sieve-tubes, companion-cells, and paren- 

 chyma ; but at this stage sclerenchyma appears in masses at 

 its outer edge, beneath the resin-passages (fig. IV., scL), with 

 very narrow lumina. 



At first two or three cells immediately interior to the 

 cavity appear to have slightly thickened walls. These gradu- 

 ally become thicker and thicker, and the neighbouring cells 

 show the same process ; and when it is complete they form 

 an elliptic mass of an indefinite number of cells. When 

 separated from the surrounding tissue, they are seen to be 

 of the same size and form as the cells of the phloem paren- 

 chyma, and tlieir thick walls are slightly pitted. 



The sieve-tubes, with their granular contents, and accom- 

 panied by compcxnion -cells, are found only in the inner portion 

 of the phloem; the outer portion consists of large -celled 

 phloem parenchyma, thin-walled, elongated, with rounded 

 ends closely dove-tailing. 



The medullary rays in the phloem are much like the 

 parenchymatous cells, and are several cells wide (fig. IV., 

 m.r.). 



The cambium consists, as usual, of a layer of thin-walled, 

 rectangular cells. In this stage interfascicular cambium has 

 appeared between the bundles, forming a closed ring of 

 vascular tissue ; but the primary medullary rays still consist 

 of several layers of cells, which are slightly thickened, with 

 pitted walls (fig. V., B), in the xylem ; the cells are longi- 

 tudinally elongated, with square ends. 



The xylem consists of vessels, tracheides, and fibres. The 

 secondarv vessels have wider lumina than the other cells ; 



