r 



101 ] 



VII. 



TYLOSES IN THE BRACKEN FERN 

 (PTEBIS AQUILINA, Linn.). 



By T. JOHNSON, D.Sc, F.L.S., Professor of Botany in the Royal 

 College of Science, and Keeper of the Botanical Collections, 

 National Museum, Dublin. 



(Plate VI.) 



[Read, May 19 ; Received for Publication, May 26 ; Published, August 17, 1903.] 



A general feature in the stem of some woody plants is, that as 

 the water-conducting sap-wood becomes converted into the hard, 

 durable, non-conducting heart-wood, the cavities of the wood- 

 tubes or xylem vessels become more or less blocked up by masses 

 of large, bladder-like, thin-walled cells which have entered the 

 cavity of the vessel by the bulging in, through the pits of the 

 vessels, of the surrounding xylem parenchyma, at one or more 

 points. Each intrusion is called a tylose. Tyloses are well seen 

 in the stem of the False Acacia tree, and of the Yine. Recently 

 Delacroix noted that the stem of the common potato plant, which 

 is ordinarily free from tyloses, shows them invariably when the 

 potato plant is suffering from the microbe form of the disease 

 commonly called " yellow blight" — an observation I have myself 

 been able to confirm. 



This pathological appearance of tyloses is in keeping with 

 their production in the stems of trees suffering from wounds, 

 artificial or otherwise. Here the interference with the flow of sap, 

 or other disturbance in the plant economy, may be accompanied 

 by the more or less complete plugging up of the cavities of the 

 xylem vessels, in part by wound gum, and in part by tyloses. 



Recently, in the usual course of inspection of the microscopic 

 preparations made by the students in the botanical class in the 

 Royal College of Science, I was struck by the appearance of the 

 xylem in one of the preparations. One of the tracheides, as the 



SCIENT. PE.OC. K.D.S., VOL. X., PART I. K 



