76 PREVENTIVE AND COMBATIVE MEASURES. 



its object probably is to restore the same condition of gaseous 

 pressure inside the tree as existed previous to the injury. A 

 number of woody plants, for example, Robinia and Quercus, 

 which normally form tyloses in their heart- wood or sap-wood, 

 <1" the same on wounded surfaces, and thereby stop up all 

 the cut vessels. 1 



The formation of tyloses is due to sac-like ingrowths into the 

 vessels from adjoining parenchyma, and can only take place 

 where rapid growth of the closing membrane of pits or the thin 

 portions of the wall of annular or spiral vessels occurs. Tyloses- 

 f'trmation takes place in normal heart- wood, and also in the 

 sap-wood of many kinds of trees, except in the very youngest 

 water-conducting year-rings. It also occurs in leaf-scars at the 

 the time of normal defoliation. 2 Species of trees in which 

 tyloses are not normally produced in the heart-wood, but in 

 which the vessels of that region become filled with resinous 

 secretions, use these substances as healing agents in the case of 

 leaf-fall or wounds to the wood. For these reasons it is quite 

 correct to designate these preliminary steps towards wound- 

 closure as a pathogenic formation of duramen, and the tissue 

 derived from the process as wound-duramen. Similarly a corky 

 tissue wound-cork may be formed in consequence of wounds 

 to the bark or as an accompaniment of certain diseases. I 

 have repeatedly observed that the normal duramen is preyed on 

 for nutriment by many wound-parasites, and also that this 

 wound-duramen is not sufficient to keep out germinating spores 

 of the wound-parasites. It cannot therefore be designated a 

 protective wood, nor are the artificial methods of closing wounds 

 so superfluous as some would have us believe. 3 



Frank says : " The use of all such artificial means of healing wounds 

 is thus only necessary in serious cases, in which, in consequence of delay 

 in the healing-process, decay would be inevitable without some septate 

 u' nt. Smaller wounds, and particularly cut surfaces of twigs or thinner 

 branches, are, by the natural formation of protective wood accompanying 

 very wound of the wood, sufficiently protected for the few years the 



1 Molisch, "Zur Kenntniss d. Thyllen," Akad. d. Wissenschctft, Vienna, 1888; 

 \Vieler, Biolog. Centralblatt, 1893. 



- Staby, " Ueber Verschluss d. Blattnarben nach Abfall d. Blatter," Flora, 

 1886. 



:: I'nii'l, 1'ruiijxlxiin'x Jahrimrh, 1888. 

 !' mine, Landurirthschaftl. Jahrbuch, 1885. 



, Die Krankheiten d. Pflanzen, 1894, p. 153. 



