RELATIONSHIPS OF THE OAKS. 171 



oaks now existing that a common origin is also sug- 

 gested, and similar leaves have been discovered in Ter- 

 tiary deposits in Northwest America. If all the evi- 

 dence is put together, we may conclude with Asa Gray 

 that " the probable genealogy of Q. Robur, traceable in 

 Europe up to the commencement of the present epoch, 

 looks eastward and far into the past on far-distant 

 shores." 



Many of the oaks yield products which are made use 

 of in the arts, apart from their timber, the most valu- 

 able of which comes from our European oak, the white 

 oaks of North America, and one or two Himalayan 

 species. In several countries oaks are grown for the 

 sake of the bark, cups, etc., as a tanning material, and 

 these even form important articles of export. Quer- 

 citron, a yellow dye and tanning material, is obtained 

 from Q. tinctoria in North America. 



Cork, as used for bottling and other purposes, is 

 obtained in Spain, the south of France, and in Algiers, 

 from the thick periderm of Q. Siiber. 



Q. infectoria yields the chief galls of commerce. 

 They are caused by the punctures of Cynips gallw tinc- 

 torice, and are used for making ink and for dyeing. In 

 these and similar galls the value depends on the pres- 

 ence of relatively large quantities of tannic and gallic 

 acids which they contain. 



