302 Annual Report Agricultural Experiment Station 



often be seen in the fissures. Affected trees rarely live longer than 

 two or three years and serve to spread the disease. It is recom- 

 mended that persons who desire cottonwood or poplar trees plant 

 the native cottonwoods, since these are more resistant to the canker 

 disease than introduced species like the Carolina poplar. There 

 are several species of native cottonwood in Arizona which thrive 

 at our various altitudes. 



SCIENTIFIC 



The work on the herbarium, which claimed so large an amount 

 of time last year, was completed early during the present year. 

 The University plant collections now number 74,000 sheets. Our 

 plant collections are complete enough now to enable one to work 

 to advantage, both on the native and cultivated plants. During the 

 year a collection of biological literature, numbering 2,500 pamphlets 

 and separates, has been classified and arranged systematically for 

 convenience in work in botany. Many of these books and pam- 

 phlets were presented to the Botanical Department by the Depart- 

 ment of Botanical Research of the Carnegie Institution, Tucson, 

 Arizona. Others have been secured thru exchange of botanical 

 material, including plant specimens. 



J. J. Thornber, 



Botanist. 



