60 THE TRANSVERSE SECTIONS OF PETIOLES OF EUCALYPTS 



To take just another striking example. E. citriodora is regarded on general 

 grounds as a variety of E. iimculata, and from the transverse sections of the petioles 

 of both the same conclusion might be drawn. It is a suggestive fact that the 

 resemblances and differences are mainly of a parallel kind with those of E.fici folia 

 and E. calophylla. But the differences are evidently less in degree, or rather the 

 resemblances are closer, for they generally agree in thickness of epidermis, size, and 

 number of cortical cavities, and size of central canals. But the vessels are about 

 twice the size of those of maculata, while the wood itself is relatively much more 

 feebly developed. 



On the whole the study of the transverse sections of petioles of well-developed 

 and fully-formed leaves of Eucalypts has led us to the following conclusions : — 



1. Fresh species may be identified by means of such sections, combined with 

 external characters of the leaf. The aggregate characters of the section are employed 

 but chiefly those derived from the following parts: — Epidermis, hard bast, wood with 

 its vessels, cortical cavities, and central canals. 



2. Herbarium material, by appropriate treatment, may likewise be used. 



3. The size and shape of the transverse section is often characteristic. 



4. The relative thinness or thickness of the epidermis is a more or less 

 constant feature, and it is often indicative of the habitat of a plant, say of a desert 

 species. 



5. The number, size, and arrangement of cortical cavities are very variable, but 

 their relatively large size in some species is such a striking feature that it may be 

 used for purposes of discrimination. 



6. The central canals are so decided and distinct, and occur in such a limited 

 number (at least of thirty-two species and varieties examined), that they might con- 

 veniently be used as a diagnostic character for a section of the Eucalypts. 



7. Crystals are always present, more or less, but they form too precarious a 

 feature to be relied upon. 



8. The hard bast, whether dense or scanty, seems to be generally characteristic. 



9. The wood-curve is likewise a characteristic feature, with the relative number, 

 size, and arrangement of vessels ; but m some few instances it is too variable to be 

 much relied on. 



10. The broken nature of the wood-curve, as found in some species, cannot be 

 used as a distinctive character, like the invariably unbroken curve in others, because on 

 fuller investigation unbroken curves are found in all. 



