4d(> OIL-GLANDS IN THE BARKS OF CERTAIN EUCALYPTS, 



E. viminalis Labill. "Manna" or "White Gum." 



Possesses a bark which is typically smooth, decorticating in long strips and 

 therefore known sometimes as "Ribbony Gum." At other times it is rough and 

 persistent on the lower portion of the tree. The oil-glands are, in the older 

 bark, comparatively small, about O.li mm. x 0.09 mm. The bark was not fresh 

 and the contents had become dark and granular. In 100% alcohol, however, 

 they went into solution with the exception of a small dark cellular mass. 



Alkannin showed numerous small oil droplets but the larger ones were too 

 dark in colour to stain definitely. In smaller twigs the glands were not numerous 

 in some of the sections examined. The glands were typically compressed radially 

 and approach the cambium more closely in the broader medullary rays. The 

 phellogen evidently develops at a considerable distance below the epidermis, and 

 subsequently develops at some distance below the abscission layers, thus giving 

 rise to the process of decortication. The bark thus removed is more or less per- 

 sistent on the trunk. Fibre zones are arranged comparatively regularly through- 

 out the secondary bast. 



A seedling of E. Macarthuri, 2 feet 6 inches in height, was examined and 

 oil-glands were found in the outer portion of the stem nodule which was about 

 1 inch in diameter. The glands were small, and not numerous, averaging 0.07 

 mm. in diameter, with pale yellow contents. Definite oil-glands were also fovmd 

 in the phloem of roots 4 mm. in diameter. They were usually small, but measured 

 up to 0.12 mm. in diameter, and were evidently of a secondary character, as 

 they were quite numerous and were distributed from the outer limits of the 

 phloem to the periderm. In roots of only 1 mm. diameter, oil-glands were also 

 prominent but were not found in roots smaU-er than this. The odour of geranyl- 

 aeetate was very pronounced when the roots were cut. 



A seedling of E. globulus, a species in which oil-glands do not occur in the 

 bark, was also examined. Small oil-glands were found in the cortical tissue of 

 young stems and several, measuring up to 0.15 mm., occurred in the stem nodule. 

 No glands were found in the roots. 



A seedling of E. piperita showed the usual cortical oil-glands in the stem 

 but none were found in the swollen stem base, though they probably occur there, 

 or in the roots. 



The occurrence of oil-glands in the bark, roots, leaves, etc., seems to indicate 

 that the function is one of protection against insect or f ungnis attack rather than 

 as food storage organs or for the reduction of transpiration, as suggested by 

 Tyndall. Although Euealypts are by no means immune from insect attack, 

 particularly the leaves in some species, yet this does not rule out the protective 

 nature of the oil, the bactericidal value of which is well known. If the glands 

 were used as a storage tissue, then it is evident that by the shedding of the bark 

 and leaves the plant would lose a considerable amount of valuable reserve 

 material. In the majority of Euealypts there is an entire absence of what might 

 be termed a discharge mechanism in connection with the leaf oil-glands and it is 

 obvious that there can be nothing of this nature in the deep-seated oil-glands of 

 the secondary phloem of stem and root. For this reason it does not seem possible 

 that Tyndall's reduction of transpiration theory can apply in this case, though 

 he has shown that a medium of an essential oil does absorb radiant heat. 



Oil-glands occur in the great majority of species of Eucalyptus in the cor- 

 tical tissues of the very young stems before the development of the periderm, 

 and well developed oil-glands are also found in leaves while less than 1 mm. in 

 diameter and still in bud. These 'facts seem to indicate that the presence of the 

 essential oil is primarily for protective reasons. 



