PRINCIPAL GROUPS OF PLANTS. 



spray of its branches gives it a delicate beauty. Although its lum- 

 ber is not very strong nor durable, it is extensively used. The 

 bark is also used to an enormous extent in the manufacture of 

 heavy leather. In recent years many tanneries have been built in 

 the hemlock districts so as to be near the supply of bark. For 

 the finer grades of leather the hemlock bark is mixed with that of 



"Pfltrohua 



\rar auatnaca 



F> u o u r9 



1 monTana 



FIG. 74. Cross-sections of leaves of six different species of Pinus, showing in the 

 diagrams the variation in the shapes of the cross-sections, with the distribution of the oil 

 reservoirs, and beneath each an enlarged view of the epidermal layer and underlying tissues. 

 From "Minnesota Trees and Shrubs." 



the oak, in order to avoid the reddish color produced by the 

 former. V. 



Another important genus is Juniperus. The red cedar (/. vir- 

 giniana, also called Sabina virginiana} or savin is a tree producing 

 valuable fine-grained soft and durable wood which is used exten- 

 sively in making chests, pails, posts, etc. It is interesting to note 

 that this tree, which is frequently planted to form windbreaks, 

 develops the fungus Gymnosporangium in the form of cedar 

 apples, which in the secidial stage produce the leaf rust of apple. 



