AS AIDS IN THE DETERMINATION OF SPECIES. 



17 



hypodermal layer, noted in globulus, is decidedly absent in many, bnt indications of 

 it in some, such as grossa. In ficifolia there is a decided and well-marked hypoderma, 

 usually consisting of two layers of cells. The cortex may be compared as to the 

 number and size of cortical cavities in it. This is shown in Table I., so that it is 

 unnecessary to dwell further upon it. In one case at least (E. tetraptera), the 

 cortical cells are continued into the organic centre of the section by means of two 

 narrow and adjoining lines of cells. 



The hard bast is more or less dense in different sections, forming usually an 

 interrupted or broken ring. 



The soft bast, on the other hand, usually surrounds the wood completely, but it 

 is sometimes discontinuous. 



The wood-curve may be continuous or discontinuous, with a thinner or thicker 

 body, and longer or shorter horns, but, on the whole, so varied in character as to 

 serve in many cases as one of the characteristic features of the section. 



Broken curves occur in all the species — in some they are the rule, as in lehmanni, 

 in many they are exceptional, as in globulus, which was the most constant in its 

 features of all the types investigated. 



This will be the most appropriate place to compare the essential features in the 

 transverse section of a young stem and a young leaf of E. globulus. The leaf being 

 sessile, it was necessary to take the base of the midrib instead (Figs. 8 and 9). 



