PREVENTIVE AND COMBATIVE MEASURES. 75 



needle-rust). The exclusion of the aspen from the neighbour- 

 hood of pine plantations is advisable as a means of limiting 

 the pine-disease, and is now being recommended in forestry. 



Still another example is Puccinia graminis the rust of 

 wheat and its Aecidium on the barberry. This is, in all 

 probability, able to reproduce itself by means of uredospores on 

 wild grasses, and to retain its position without the barberry, yet 

 the latter doubtless tends to distribute the disease, and its 

 removal minimises the risks of attack. 



An investigation of the heteroecious rust-fungi will easily 

 furnish many examples of the same kind, and lead to the con- 

 clusion that Euphorbia cyparissias, for example, should be 

 exterminated near fields of peas or other Leguminosae because 

 of Vromyces pisi, and U. striatus. 



III. Avoidance or removal of conditions which favour infection. 



Various examples of this have already been given when 

 the conditions disposing plants to disease were under considera- 

 tion in our last chapter. 



(1) The most important measures of this class are those 

 directed against infection through wounds. This may be 

 attained by avoiding any unnecessary wounding of woody plants, 

 and the immediate treatment of any wounds rendered necessary 

 in pruning or other operations. 



When the stems of woody plants are injured, the first step 

 towards healing the wound proceeds from the tree itself. 

 Conifers containing resin have in it a very ready agent im- 

 mediately available ; the resin escapes from its ducts and soon 

 hardens into a crust on exposure to air. In the case of non- 

 resinous conifers and of broad-leaved trees, the first steps 

 towards healing are less obvious, but it has been found that a 

 healing tissue immediately begins to form on wounded surfaces.^ 

 It consists of a parenchyma, the formation of which is induced 

 apparently by atmospheric air penetrating into the wood, and 



' V. Tubeuf, " Ueber normale u. pathogene Kernbildung d. Holzpflanzen 

 u. d. Behandlung v. Wunden derselben, Zeitschrift f. Forst.-u. Jagd.-wesen, 1889. 

 Contains Bibliography of allied papers. 



R. Hartig, Diseases of Trees, English Edition, 1894. 



Gaunersdorfer, Sitzungsher. d. k. Akad. d. Wissenschaft, Vienna, ^881. 



Boehm, " Ueber die Function d. veget. Gefasse," Botan. Zeitung, 1879. 



