BY M. B. WELCn. 431 



I 



averaging about 0.12 mm. in length, and all showing the characteristic tangential 

 flattening. The total thickness from cambium and phellogen averaged 1.5 mm. 

 The glands are distributed usually along the broader medullary rays, the smaller 

 cavities being nearer the cambium. Glands measuring up to 0.12 mm. in diameter 

 were found within 0.45 mm. of the cambium, and at a distance of 2 mm. from 

 the latter averaged 0.015 mm. in size. 



Towards the cortex the smaller medullary raj's become; irregular and bent, 

 losing their characteristic straightness. The meduUary rays usually consist of a 

 single row of cells, in transverse section, and seldom broaden out imtil at least 

 half way to the cortex. Wherever the oil-glands occur the parenchymatous cells 

 of the ray have their longer axes directed tangentially, and the gland is situated 

 in the fan-like area formed by the widening at that point. In many cases the 

 ray is ag-ain contracted to a single row of cells. Plate xlviii., fig. 3. shows a 

 transverse section of a 10 mm. stem in which the distribution of the oil-glands 

 in the meduUary rays is apparent. There are numerous rl.cmbohedial crystals 

 (presumably calcium oxalate) distributed throughout the phloem, sometimes ar- 

 ranged in concentric rings, and numerous small areas of immature bast-fibres are 

 developing. The oil-glands are always surrounded with several rows of paren- 

 chjTnatous cells and, in the more mature cavities, are found the remains of what 

 are apparently disintegrated epithelial layers. 



In a transverse section of the bark which, even in very young stems, flakes 

 off in thin layers, it is evident that the meristematie cells are cutting oft:, on the 

 outside, absciss-layers of usually a single row of thin-walled unsuberised cells 

 separating the zones of one, two or three layers of normal cork-eeUs. The 

 thickness of the persistent corky tissue is surprisingly small, usually only a few 

 cells in thickness even in the smaller branches. It is, however, a common oc- 

 currence to find evidence of the development of a new phellogen nearer the cam- 

 bium and by the development of the periderm the outer tissues, including oil- 

 glands, ai-e cut off from the inner portion and eventually thrown off. 



A transverse section of a 16 mm. stem (PI. xlviii., fig. 4), as would be ex- 

 pected, shows a much greater development of the secondary oil-glands. 



These are often in contact and are, as usual, flattened so that their tan- 

 gential and vertical axes are greatest. 



A portion of the phloem and cortical tissues of a 45 mm. stem is shown in 

 transverse section in Plate xlviii., fig. 5. The enormous development of the oil- 

 glands in the secondary bast is apparent, and they are distributed chiefly in the 

 broader fan-shaped rays though glands are seen to occur also in the smaller rays. 

 Glands measuring 0.23 mm. and 0.11 mm. are common. The total width of the 

 secondary phloem is 3.5 mm. It is evident that the glands are not developed, 

 except at some distance from the cambium, and by far the greater development 

 and number occur in the outer half of the secondary tissues. 



In radial section (PL xlviii., fig. 6), the glands are found to be approximately 

 circular in section and it is evident that there is no trace of elongation into any- 

 thing resembling a duct. The glands occur in irreg-ular groupings, often two or 

 more in contact. The loose parenchymatous tissue in which they occur is ir- 

 regularly divided by bast-fibres and parenchyma-elements of the secondary phloem, 

 since, as has been pointed out, the medullary rays are bent from their usual 

 radial direction in this region. 



It will be seen from the section that towards the phellogen the glands are 

 comparatively small, reaching their maximum size nearer the cambium. The 

 great increase in the number of glands in the cortical tissues, as well as in the 

 fan-shaped medullary rays, indicates that they are developed in any position, 



