22 THE TRANSVERSE SECTIONS OF PETIOLES OF EUCALYPTS 



petiole, the epidermis, cortical cavities, central canals, and vessels, and in using the 

 expressions "large," "small," or "moderate in size," it all depends upon what is 

 considered an average size. 



While it is not advisable to lay down a hard and fast line, it is well to have a 

 general understanding of the meaning of the relative terms. 



It may be stated generally that the average section is reckoned as being about 1 

 line X 1 line or f line, and above or below that will be large or small respectively. 



The size of the section seems to be rather a constant and trustworthy feature. 

 The absolute size is first given, then the relative breadth and thickness. 



The thickness of the epidermis only varies within certain limits, and cannot 

 often be used as a characteristic, but when it is thicker than about i m., or thinner 

 than ^ijj in., it may be regarded as above or below the average. The cortical cavities 

 and central canals maybe considered large up to ^ in., moderately large up to 

 2^ in., and small beyond that. The size of the largest in each species only is 

 given. 



The vessels have a wide range of variation. In settling the average size it is 

 largely a matter of judgment, and the largest size given in each case has to be taken 

 into account. Eoughly, when the largest size is ji^ in. or more, and the average not 

 too low, the vessels might be described as "large," while at ^ in,, or less, and a 

 corresponding average, they would be " small," and the intermediate would be 

 "moderately large," or medium-sized. 



Of course the standard of measurement is confined to the petioles of Eucalypts, 

 and has no reference to the absolute sizes of the parts mentioned m any other 

 plants. 



The size of the cavities and canals varies in different parts of the petiole, and 

 likewise the width of the vessels, so that too much importance must not be attached 

 to the actual measurements given. The general results of these measurements may 

 be given here. 



Size of section varies in breadth from I to IJ lines [tetraptera), and in thickness 

 from \ line {stuartiana) to 1^ lines {tetraptera). 



Epidermis varies in thickness from jig in. {viminalis and stuartiana) to ~ in. 

 [tetraptera). A very common thickness is ^ij- — -m^"^- 



Cortical cavities vary from gij in. [diversicolor) to ^iu. {globulus), reckoning only 

 the largest in each species. 



