480 



OCC'UKBEXCE OF OIL DUCTS IN CERTAIN EUCALYPTS AND ANGOPHORAS. 



leaves, with two well-developed ducts in each; Text-fig. li c, still nearer the base, 

 shows the linking- up on one side of one of the outer leaves witli the stem, 

 and it will be noticed that the duets on this side have disappeared. In Text- 

 fig. 6 d the left hand petiole has almost fused with the stem but, although yet 



Text-fig. 6. — Transverse sections of a leaf bud (a) and (b), and stem (e) 

 and (f), with intermediate sections (c) and (d), showing the junction of the 

 almost opposite leaves with the axis, and the arrangement of the central ducts 

 which are not continuous at this point. 



distinct, tlie ducts in this ca,se are also wanting. It will l)e seen that although 

 almost opposite, the leaves show a tendency to become alternate, a characteristic 

 of the normal growth in this species. Text-lig. (i e sliows the complete linking 

 tip of both outer leaves with the stem, and the reappearance on the right of 

 two ducts in a position in the stem corresponding to that occupied whilst in 

 the petiole. In Text-fig. 6 / the ducts have also reappeai'ed on the left hand 

 side, and it is a striking feature that tliree of the original st*m ducts liaxe 

 disappeared, and the fourth is iiiucli reduced in size. This reduction in iiuinbcr 

 of the ihicts accounts tor the usual ajiiiearance of four in tlie stem, although 

 as already pointed out this number is ol'ten exceeded. 



The presence of a delinite epithelium within the cavity is not always 

 characteristic of these ducts. In many cases an epithcHal Hning doulitless 

 exists (Text-fig. 1), and in a number of sections tlie cavity is almost blocked 

 by rows of more or less concentrically arranged thin-walled cells. In other 

 cases, particularly in older stems and leaves, there is no trace of any definite 

 epithelial layer, which jn'obalily undergoes disintegrati«m. 



Conteilta of Si'crclDrii Piicln. 



In all those species of which fresh material could be obtnined the contents 

 of the secretory ducts were examined iti situ. They were found to be yellowish 

 in colour as a rule, though sometimes almost colourless, and usually granular 

 in appearance under the microscope, owing to a number of small globules be- 

 ing present. Treatment with increasing strength of alcohol had little or no 

 effect until (10% was reached. Here contraction occurred, the contents usually 

 going into final solution in 0.5-1 009^ alcohol. This sdlubility of the contents of 

 the duct is in accm-d with the results nbtained for the oil in the mesophyll ghinds 

 of E. corymhosa. 



