430 OIL-GLANDS IX THE BAEKS OP CEETAIIT EUCALYPTS^ 



Although these glands are only of specific importance, yet they are valuable 

 taxonomieally in the particular species in which they occur and, since the second- 

 ary-development can usually be recognised in small stems such as are commonly 

 found among herbarium material, their presence may be used as an aid to 

 identification. 



Most of the detailed investigation of the bark oil-glands was carried out on 

 E. Macarthuri, as their arrangement and distribution in this case is apparently 

 t3rpical of the other species. 



In almost all Euealypts which have been examined in this work, oil-glands 

 are a common feature in the cortical tissues of the young stems, especially before 

 the development of the periderm. In the young stems the glands are close to 

 the epidermis, but, although they are often numerous, there is apparently no in- 

 crease in their number and, even in the more mature stems, they still form a 

 ring in the cortex. There is also no sign of glands in the secondary phloem 

 and they may be regarded as purely cortical in their origin and distribution. In 

 the barks of older trees there is also no trace of glands and naturally the original 

 cortical glands have long since ceased to function. 



In E. Macarthuri, however, an examination of the "bark" of a twig an inch 

 in diameter, or of the bark of a tree several feet in diameter, shows an enormous 

 incresise in the number of glands, and it is evident that their development must 

 be of a secondary character. In tracing this secondary increase, an examination 

 was first made of the young twig-s of E. Macarthuri about 1.0 mm. in diameter. 

 In many eases no oil-glands were found at all, in a few sections (PL xlviii., fig, 

 1) glands 0.10 mm. to 0.15 mm. in diameter were present in the cortex, flattened 

 with the longer axis tangentially directed, and close to the epidermal layer. The 

 phellogen was not continuous, but along portion of the stem showed the early 

 cells of the periderm, though the epidermal layer had not yet broken away. 



The two oil-glands seen in the figure correspond in mode of occurrence and 

 position with those commonly found in the cortical tissues of young Eucalyptus 

 stems, as already pointed out. The outer limit of the primai-y phloem is limited 

 by a broken ring of small groups of mechanical fibres, as pointed out by Solereder 

 (1908) as a characteristic of the Myrtaceae. Slightly thicker stems, about 2.0 

 mm. in diameter, showed no oil-glands in the cortex in the particular sections 

 examined, though it is evident that some must occur. At this stage, however, 

 there is apparently no increase in their number, although the epidermis had 

 commenced to strip off — a result of the outward pressure of the periderm. The 

 phloem was about 0.25 mm. in thickness. In stems 6.0 mm. in diameter, several 

 glands were found measuring .09 mm. x 0.45 mm., almost on the border-line be- 

 tween the phloem and the cortex. They were few in number and in many cases 

 were found to occupy a position in the medullary rays. A few are evidently 

 secondary in character. The periderm was about 0.075 mm. in thickness, the 

 secondary phloem 0.70 mm., the cortex 0.10 mm. 



A section of a 6 mm. stem (PI. xlviii., fig. 2) is shown in which there is 

 almost an entire absence of oil-glands, though two are seen in the top corners of 

 the illustration. It is apparent that very little secondary development has yet 

 occurred, since these glands are probably of a primary character and correspond 

 in origin with those in Plate xlviii., fig. 1. Slightly older stems, with a thickness 

 of almost 10 mm., showed numerous oil-glands in section. These were flattened, 

 with the longer axis tangentially directed, the larger ones being within 0.3 mm. 

 of the outer periderm layers, the latter being quite thin (0.075 mm.), and show- 

 ing signs of flaking off. The glands were often 0.22 mm. x 0.10 mm. in size. 

 Numerous somewhat smaller glands were also found towards the periphery. 



