AS AIDS IN THE DETERMINATION OF SPECIES. 



45 



Hindmarsh. They constitute large portions of the " Mallee Scrub," and along with 

 E. incrassata, LabiUardiere, and its smaller variety, E. dumosa, are the only species 

 of Eucalypt with the above Australian distribution, according to present knowledge. 



They are all shrubby, or at most somewhat arboreous, with leaves equally green 

 on both sides. 



Only a few sections were made of itnchiata and oleosa, but there is a decided 

 difference between the wood-curve of each, so that they are thereby easily 

 distinguished. (Figs. 41 and 42.) 



It is believed that the foregoing descriptions, with the accompanying drawings, 

 when taken in conjunction with external leaf characters, will materially aid m the 

 discrimination of the investigated species. Various specimens have been taken at 

 random and tested, and referred to their respective types. 



As a very instructive example of this, no better could be given than the 

 following :— One of us had a few leaves sent without any information as to then- 

 nature o*^' locality, and sections were made in due course, and the following notes :— 

 " The leaf looks like that of citriodora, but the characteristic odour is wanting. 

 The first section shows two canals, and fairly numerous cortical cavities ; outline 

 somewhat like citriodora. A second section from another petiole has very irregular 

 outline, but otherwise agrees with the first. A third section was cut from another 

 petiole, which was still more like citriodora, a third canal tube appearing as in one 

 of the type specimens of that species. I conclude that this specimen is either a 

 variety of citriodora, or another very closely allied species." The leaf belonged to 

 E.maculata, thus justifying, from quite an independent source, Baron von Mueller's 

 determination of citriodora as a variety of that species. 



And, in the practical test to be presently described, the utility of the method 

 will be more fully shown. But, apart altogether from the practical utility of this 

 investigation, there is new light thrown on the histology of this important genus. 



M. L. Petit* has described the structure of the petiole of Dicotyledons in 48 

 families, including 300 genera and nearly 500 species, one of the families being the 

 Myrtaceee, to which the Eucalypts belong. 



He found the form of the petiole to be always convex below and concave on the 

 upper face, whereas our observations show, in the twisted petioles of Eucalypts, that 

 while this is often the case it is not invariably so. Thus, in obcordata and alpina the 

 upper surface of the section is more convex than the lower, and in grossa and rostrata 

 the section is round. The lower surface is generally convex and the upper fiat. 



' Mem. Sci. Phys. and Nat. Bordeaux III. (1887). 



